Transition Elements · تدريب
٥٠ سؤال · كل إجابة مشروحة
What is the wavelength of a wave?
كل الأسئلة بإجاباتها · Transition Elements
What is the wavelength of a wave?
- D. The time one wave takes to pass a point
- C. The number of waves that pass a point each second
- A. The distance between two neighbouring crests
- B. The height of a crest above the middle line
الإجابة: The distance between two neighbouring crests
Wavelength is the distance between two neighbouring crests (or two neighbouring troughs): the length of one full wave. Its unit is a unit of length, such as the metre or the nanometre (nm). The height of a crest is the amplitude, not the wavelength. The number of waves that pass a point each second is the frequency. Light travels as waves that carry energy, and light waves differ in wavelength and frequency.
Two light waves travel at the same speed. Wave P has a wavelength of 450 nm and wave Q has a wavelength of 650 nm. Which wave has the higher frequency?
- D. We cannot tell without knowing how bright each light is
- B. Wave Q
- A. Wave P
- C. Both have the same frequency, because they have the same speed
الإجابة: Wave P
Both waves move at the same speed, so in one second the same length of wave train passes a point. Shorter waves fit more crests into that length, so more of them pass each second. P has the shorter wavelength (450 nm < 650 nm), so P has the higher frequency. For light: the shorter the wavelength, the higher the frequency. Brightness does not decide the frequency.
Which of the following rays has the highest penetrating power?
- B. Ultraviolet rays
- D. Infrared rays
- C. X-rays
- A. Gamma rays
الإجابة: Gamma rays
The electromagnetic family in order of increasing energy: radio waves, microwaves, infrared, visible light, ultraviolet (UV), X-rays, gamma rays. As the wavelength gets shorter, the energy increases. At the high-energy end, the penetrating power also rises from UV to X-rays to gamma rays. So gamma rays have the highest penetrating power. Chapter 1 uses this: cobalt-60 produces gamma rays, which have high penetrating power. So they are used to detect cracks and welding connections in industrial products.
Four rays W, X, Y and Z belong to the electromagnetic family. Their wavelengths are: W: 500 nm X: 0.01 nm Y: 10 000 nm Z: 200 nm Arrange them in ascending order of energy (lowest energy first).
الإجابة: Ray Y (10 000 nm) → Ray W (500 nm) → Ray Z (200 nm) → Ray X (0.01 nm)
Rule: in the electromagnetic family, the shorter the wavelength, the higher the energy. So arrange from the longest wavelength to the shortest: Y (10 000 nm) < W (500 nm) < Z (200 nm) < X (0.01 nm). Y is infrared, W is visible light, Z is ultraviolet and X is in the X-ray range. Common mistake: putting the biggest wavelength number last. A bigger wavelength means LESS energy, not more.
Titanium dioxide, TiO₂, can also be named titanium (IV) oxide. What does the Roman numeral (IV) in this name tell you?
- C. The formula contains 4 titanium atoms
- D. Oxygen has a valency of 4 in this compound
- B. The formula contains 4 oxygen atoms
- A. The valency of titanium in this compound is 4
الإجابة: The valency of titanium in this compound is 4
A Roman numeral after a metal's name gives the valency of the metal in that compound. In titanium (IV) oxide, Ti has valency 4 and O keeps its usual valency of 2. Swap the valencies: Ti₂O₄. Simplify by 2: TiO₂. So the numeral is not a count of atoms: TiO₂ has 1 Ti atom and 2 O atoms. Some elements have more than one valency, which is why the numeral is needed. TiO₂ is the compound used in sun protection cosmetics (Ch.1 p.5).
Which is the correct formula of iron (III) sulphate? (The sulphate group, SO₄, has valency 2.)
- C. FeSO₄
- B. Fe₃(SO₄)₂
- D. Fe₂SO₄
- A. Fe₂(SO₄)₃
الإجابة: Fe₂(SO₄)₃
Steps: (1) symbols: Fe and SO₄. (2) valencies: Fe = 3 (from the Roman numeral III), SO₄ = 2. (3) swap: 2 Fe and 3 SO₄ groups. (4) 2 and 3 cannot be simplified. The group is taken 3 times, so it goes in brackets: Fe₂(SO₄)₃. Check: 2 × 3 = 6 and 3 × 2 = 6, so the valencies balance.
Look at the balanced equation 3Fe + 2O2 → Fe3O4. How many oxygen atoms are on the left side?
- B. 2
- C. 3
- A. 4
الإجابة: 4
A coefficient multiplies the whole formula after it; a subscript counts the atoms inside one molecule. 2O₂ means 2 molecules × 2 O atoms = 4 O atoms. The right side, Fe₃O₄, also has 4 O atoms, and Fe is 3 on each side, so the equation is balanced. Fe₃O₄ is the magnetic iron oxide you meet later in Chapter 1.
Which coefficients balance this equation? Fe2O3 + CO → Fe + CO2 (in the order Fe₂O₃, CO, Fe, CO₂)
- D. 2, 3, 4, 3
- A. 1, 3, 2, 3
- B. 1, 1, 2, 1
- C. 1, 2, 2, 2
الإجابة: 1, 3, 2, 3
Change coefficients only, never subscripts. Fe: Fe₂O₃ has 2 Fe, so put 2 before Fe. O: each CO takes one O and becomes CO₂. Fe₂O₃ has 3 O, so 3 CO are needed, giving 3 CO₂. Check every atom: Fe 2 = 2; C 3 = 3; O 3 + 3 = 6 on the left and 3 × 2 = 6 on the right. Fe2O3 + 3CO → 2Fe + 3CO2.
Which statement describes a catalyst?
- C. It increases the amount of product made from the same reactants.
- A. It speeds up a chemical reaction and is not used up by it.
- B. It speeds up a chemical reaction and is used up as the reaction goes on.
- D. It is one of the reactants, so it is written on the left of the arrow.
الإجابة: It speeds up a chemical reaction and is not used up by it.
A catalyst speeds up a chemical reaction without being used up: its mass and chemical nature are the same at the end. It does not change the amount of product; it only helps the reaction reach it faster. In an equation, the catalyst is written over the arrow, not among the reactants.
A reaction gives off a gas. A student runs it twice with the same amounts of reactants. Without powder X: 50 mL of gas form in 10 minutes. With 1.0 g of powder X: 50 mL of gas form in 1 minute. At the end, she dries and weighs X: its mass is still 1.0 g. What is X in this reaction?
- B. A reactant, because it changed the speed of the reaction
- C. A product of the reaction
- D. A substance with no effect on the reaction
- A. A catalyst
الإجابة: A catalyst
Two pieces of data decide. (1) With X, the same volume of gas forms 10 times faster, so X speeds up the reaction. (2) X's mass is the same at the end, so X is not used up. A substance that speeds up a reaction without being used up is a catalyst. It is written over the arrow in the equation, not among the reactants.
Which set of properties belongs to most metals?
- B. Shiny, poor conductors of heat and electricity, brittle
- A. Shiny, good conductors of heat and electricity, malleable
- D. Dull, poor conductors of heat and electricity, malleable
- C. Dull, good conductors of heat and electricity, brittle
الإجابة: Shiny, good conductors of heat and electricity, malleable
Most metals are shiny, good conductors of heat and electricity, malleable (can be hammered into sheets) and ductile (can be drawn into wires), and mostly solid with high melting points. Most non-metals are dull, poor conductors and brittle when solid. Note the word 'most'. Manganese, for example, is a brittle metal. That is why Chapter 1 says it is never used as a pure metal.
A company makes the bottoms of cooking pans. The pan bottom must pass heat quickly from the flame to the food. Which property of the metal matters most for this job?
- B. It is ductile
- C. It is shiny
- D. It has a high melting point
- A. It is a good conductor of heat
الإجابة: It is a good conductor of heat
Match the use to the property it needs. The pan's job is to carry heat from the flame to the food, so the key property is being a good conductor of heat. A high melting point is also useful (the pan must not melt), but it does not make heat pass quickly. Ductile and shiny do not help this job. Chapter 1 uses the same thinking: copper is used in electric cables because it is a good conductor of electricity.
Which of the following substances is an alkali?
- B. HNO3
- C. KNO3
- A. KOH
- D. Cu(OH)2
الإجابة: KOH
An alkali is a base that dissolves in water, giving hydroxide ions (OH-). KOH (potassium hydroxide) contains the OH group and dissolves in water, so it is an alkali. Its formula has an H, but it gives OH- ions, not H+ ions. HNO3 is an acid: it gives H+ ions in water. Cu(OH)2 is a base, but it does not dissolve in water, so it is not an alkali. KNO3 is a salt: potassium replaced the hydrogen of nitric acid.
Potassium hydroxide reacts with dilute sulphuric acid. What is the name of the salt formed?
- D. Potassium nitrate
- C. Potassium chloride
- B. Potassium sulphide
- A. Potassium sulphate
الإجابة: Potassium sulphate
Acid + base → salt + water. The salt takes its metal from the base (potassium) and its ending from the acid: sulphuric acid (H₂SO₄) gives sulphates. 2KOH + H2SO4 → K2SO4 + 2H2O. Careful: a sulphate (SO₄: sulphur and oxygen) is not a sulphide (sulphur only). Chapter 1 uses both: zinc sulphide, ZnS (X-ray screens), and copper (II) sulphate, CuSO₄ (insecticide and fungicide).
Which of the following oxides is an acidic oxide?
- B. NiO
- D. BaO
- A. NO2
- C. CoO
الإجابة: NO2
Most non-metal oxides are acidic oxides: they dissolve in water to give acids. Most metal oxides are basic oxides: they react with acids to give a salt and water. NO2 is an oxide of nitrogen (N), a non-metal, so it is acidic. NiO (nickel), CoO (cobalt) and BaO (barium) are oxides of metals, so they are basic. Read symbols carefully: N is nitrogen but Ni is nickel; Co is cobalt, not carbon + oxygen.
Nickel (II) oxide, NiO, is a basic oxide. What forms when it reacts with dilute sulphuric acid?
- B. Nickel (II) sulphate and hydrogen gas
- C. Nickel metal and water
- A. Nickel (II) sulphate and water
- D. No reaction, because a metal oxide reacts only with water
الإجابة: Nickel (II) sulphate and water
A basic oxide + an acid → a salt + water (no gas). NiO + H2SO4 → NiSO4 + H2O. The salt takes the metal from the oxide and the ending 'sulphate' from sulphuric acid. The O of the oxide and the H of the acid form water. Hydrogen gas forms when an active metal itself (not its oxide) reacts with a dilute acid.
An iron anchor on a ship at sea rusts quickly. An iron statue in a very dry desert rusts very slowly. What is the best explanation?
- D. The desert heat protects iron from rusting.
- B. Desert air has no oxygen, so the statue cannot react with it.
- A. At sea, iron meets oxygen and moisture together; desert air has very little moisture.
- C. Water alone causes rusting, so oxygen plays no part.
الإجابة: At sea, iron meets oxygen and moisture together; desert air has very little moisture.
Rusting is the corrosion of iron. It needs oxygen and moisture (water) together. Sea air and sea spray give the anchor both. Desert air contains oxygen, but very little moisture, so the statue rusts very slowly.
An iron water tank on a roof is covered with a layer of red-brown rust. The owner says: "Good, this rust layer now protects the tank, just like a coat of paint." Which reply is correct?
- B. Yes. Rust is an oxide, and every oxide layer stops oxygen from reaching a metal.
- C. Yes. Rust is less reactive than iron, so the tank is now safe.
- D. No. But the only problem with rust is that it looks ugly.
- A. No. Rust is porous and flakes off, so oxygen and moisture still reach the iron below.
الإجابة: No. Rust is porous and flakes off, so oxygen and moisture still reach the iron below.
Rust (hydrated iron (III) oxide) is porous and flakes off. Oxygen and moisture pass through it and reach fresh iron below, so rusting continues until the iron is eaten away. Paint is different: it is a continuous barrier that keeps air and moisture away. Keep this contrast for Chapter 1: the oxide layer on chromium is non-porous, so it does protect the metal.
Which of the following changes is a reduction?
- C. CO → CO₂
- D. H₂S → S
- A. Fe₂O₃ → Fe
- B. Fe → Fe₃O₄
الإجابة: Fe₂O₃ → Fe
Reduction = loss of oxygen (or gain of hydrogen). Oxidation = gain of oxygen (or loss of hydrogen). Fe₂O₃ → Fe: loses oxygen, so it is reduction. Fe → Fe₃O₄ and CO → CO₂: gain oxygen, so they are oxidation. H₂S → S: loses hydrogen, so it is oxidation.
Burning magnesium keeps burning in carbon dioxide gas: 2Mg + CO2 → 2MgO + C. Which substance is the oxidizing agent?
- D. C
- A. CO₂
- C. MgO
- B. Mg
الإجابة: CO₂
Mg gains oxygen (Mg → MgO): Mg is oxidized, so Mg is the reducing agent. CO₂ loses oxygen (CO₂ → C): CO₂ is reduced. The substance that supplies oxygen to another substance is the oxidizing agent, so CO₂ is the oxidizing agent. Remember: the oxidizing agent is itself reduced. Chapter 1 calls K₂Cr₂O₇ and MnO₂ oxidizing agents for the same reason.
Which of the following metals does NOT displace hydrogen from dilute hydrochloric acid?
- B. Zinc
- C. Iron
- A. Copper
- D. Magnesium
الإجابة: Copper
In the activity series (K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb, (H), Cu, Hg, Ag, Pt, Au), metals above hydrogen displace it from dilute acids, and metals below hydrogen do not. Copper is below hydrogen, so it gives no hydrogen with dilute HCl. This matches Chapter 1: copper has limited chemical activity, while scandium is highly active.
Part of the activity series (most active first): Mg > Zn > Fe > (H) > Cu > Ag. A strip of zinc is put in copper (II) sulphate solution. What happens?
- A. Zinc displaces copper: copper metal forms on the strip and zinc sulphate forms in solution.
- C. Copper displaces zinc from the strip, and the strip dissolves.
- D. Hydrogen gas is given off, because zinc is above hydrogen.
- B. No reaction, because a metal does not react with a salt solution.
الإجابة: Zinc displaces copper: copper metal forms on the strip and zinc sulphate forms in solution.
A more active metal displaces a less active metal from the solution of its salt. Zn is above Cu in the series, so zinc displaces copper: Zn + CuSO4 → ZnSO4 + Cu. Red-brown copper metal coats the zinc strip. Copper cannot displace zinc, because copper is less active. No hydrogen forms, because there is no acid.
Cobalt-59 and cobalt-60 are two isotopes of cobalt (₂₇Co). How are their atoms different?
- B. A cobalt-60 atom has one more proton.
- A. A cobalt-60 atom has one more neutron.
- D. Cobalt-60 has a different atomic number, 28.
- C. A cobalt-60 atom has one more electron.
الإجابة: A cobalt-60 atom has one more neutron.
Isotopes are atoms of the same element: the same atomic number (the same number of protons, here 27) but different mass numbers. Mass number = protons + neutrons. Cobalt-59: 59 − 27 = 32 neutrons. Cobalt-60: 60 − 27 = 33 neutrons. So a cobalt-60 atom has one more neutron. Each neutral atom still has 27 electrons.
Copper has two stable isotopes: copper-63 (abundance 69%) and copper-65 (abundance 31%). Take the mass of each isotope as equal to its mass number. Calculate the average atomic mass of copper, to one decimal place.
الإجابة: 63.6 ± 0.05 u
Average atomic mass = the sum of (mass × abundance) for each isotope. (63 × 69 + 65 × 31) / 100 = (4347 + 2015) / 100 = 6362 / 100 = 63.62 ≈ 63.6 u. Check: the answer lies between 63 and 65, and closer to 63, because copper-63 is the more abundant isotope. Common mistake: taking the simple mean, (63 + 65) / 2 = 64, which ignores the abundances. The book's table gives copper's atomic mass as 63.5, because the real isotope masses and abundances differ slightly from the round values used here.
Which statement describes gamma rays?
- B. Positively charged particles that a sheet of paper can stop
- C. Low-energy rays, weaker than visible light
- A. High-energy electromagnetic rays with high penetrating power
- D. Rays that make every object they hit radioactive
الإجابة: High-energy electromagnetic rays with high penetrating power
Some isotopes have unstable nuclei that decay and give out radiation. Gamma rays are electromagnetic rays, like light but with a much shorter wavelength. They have very high energy, no charge and high penetrating power. An object exposed to gamma rays does not become radioactive: the rays pass through it, and no radioactive substance is added to it.
A beam of gamma rays is aimed at three materials, one at a time: a sheet of paper, an aluminum sheet 5 mm thick, and a thick block of lead. Which result is expected?
- C. The aluminum sheet stops the rays completely.
- D. The rays pass through all three materials equally.
- B. The sheet of paper stops the rays.
- A. The rays pass through the paper and the aluminum; the thick lead block greatly reduces them.
الإجابة: The rays pass through the paper and the aluminum; the thick lead block greatly reduces them.
Gamma rays have high penetrating power: they pass easily through paper and through a few millimetres of aluminum. Only thick, dense materials, such as lead or thick concrete, reduce them strongly. This is why cobalt-60 gamma rays can check thick industrial products and welding connections for hidden cracks (Ch.1 p.7).
Choose the correct answer: A little scandium is added to aluminum in the manufacture of MiG fighter jets, because the alloy formed is ...
- B. heavy and as dense as steel
- A. light and very hard
- C. inert, so the body does not reject it
- D. able to give light that looks like sunlight
الإجابة: light and very hard
Book (p.5): adding a little amount of scandium to aluminum gives a light and very hard alloy, used in the manufacture of MiG fighter jets. Aluminum gives lightness; a little scandium makes it very hard. Note: only a little scandium is added; the jet is not made of pure scandium. Scandium's second use: it is added to mercury vapour lamps to produce high-quality light that looks like sunlight, so it is used in TV photography at night. 'Inert, not rejected by the body' is the reason for titanium in dental implants.
Choose the correct answer: Vanadium pentoxide (V₂O₅) is used ...
- B. as a strong oxidizing agent in the dry cell
- D. in the manufacture of car springs
- C. in sun protection cosmetics to prevent the effect of UV
- A. as a catalyst and as a dye in the ceramics and glass industry
الإجابة: as a catalyst and as a dye in the ceramics and glass industry
Book (p.6): vanadium pentoxide is used as dyes in the manufacture of ceramics and in the glass industry, as a catalyst, and in the manufacture of strong magnetic conductors. (Its best-known catalytic use is in the preparation of sulphuric acid by the contact method, p.15.) Vanadium metal is different: a small portion added to steel gives an alloy of high hardness that resists corrosion, used in car springs. MnO₂ is the oxidizing agent in the dry cell; TiO₂ is used in sunscreen.
Give reasons: A chromium-plated tap stays shiny for years in the moist air of a bathroom, although chromium is a chemically active metal.
الإجابة: Key points: (1) Chromium is chemically active, so it reacts with the oxygen of air and an oxide forms on its surface. (2) The molecular volume of the produced oxide is larger than that of the metal atoms it formed from. (3) So the oxide forms a continuous, non-porous layer over the surface. (4) This layer prevents the continued interaction of the metal with the oxygen of air, so chromium resists the effect of the atmosphere and the tap stays shiny.
Model answer: Chromium is a chemically active metal, so it does react with the oxygen of air. But the oxide formed on its surface has a molecular volume larger than that of the metal atoms it came from. The oxide therefore covers the whole surface as a non-porous layer. This layer prevents the continued interaction of chromium with the oxygen of air, so the metal under it is protected and stays shiny (book p.6). Words your exam answer must contain: active metal, oxide layer, molecular volume larger than the metal atoms, non-porous, prevents continued interaction with oxygen. Common mistakes: (1) 'chromium is an inactive metal' (the book says it is active); (2) 'any oxide layer protects' (rust on iron is porous and flakes off, so iron keeps rusting).
Material Property P brittle; breaks when hammered Q harder than steel R resists corrosion Three manganese materials (descriptions from the book's reasons) The table describes three materials P, Q and R that contain manganese. Which choice gives the material for a railway track and the material for soft-drink cans?
- D. Track: Q; cans: Q
- B. Track: P; cans: R
- C. Track: R; cans: Q
- A. Track: Q; cans: R
الإجابة: Track: Q; cans: R
P is brittle, like pure manganese. So manganese is not used as a pure metal; it is always used as alloys or compounds (book p.6). Q matches ferromanganese alloy. It is used in railway track, because it is harder than steel. R matches aluminum and manganese alloys. They are used in the manufacture of soft-drink cans, because they resist corrosion. So: track Q, cans R. Each use is chosen by a property: hardness for track, resisting corrosion for cans.
Choose the correct answer: Iron is used as a catalyst in the Fischer-Tropsch method. Its role there is to ...
- A. speed up the change of water gas into fuel, without being used up
- D. supply hydrogen that combines with carbon monoxide
- C. be used up, so fresh iron is added for each batch
- B. react with water vapour and turn it into fuel
الإجابة: speed up the change of water gas into fuel, without being used up
Book (pp.6-7): iron is used as a catalyst in the manufacture of ammonia by the Haber-Bosch method, and in the conversion of water gas (a mixture of hydrogen and carbon monoxide) to a fuel by the Fischer-Tropsch method. A catalyst speeds up the reaction and is not used up. Water gas is not water vapour: it is a mixture of CO and H2.
Choose the statement that gives a use of nickel with its correct reason, according to the book.
- D. Nickel plating is used to make other metals lighter.
- C. Nickel-cadmium batteries are used, because they cannot be recharged and so last longer.
- A. Nickel-chromium alloys are used in heating coils, because they resist corrosion at high temperature.
- B. Nickel-chromium alloys are used in heating coils, because they are the best conductors of electricity.
الإجابة: Nickel-chromium alloys are used in heating coils, because they resist corrosion at high temperature.
Book (p.7): nickel is used in the nickel-cadmium battery, which can be recharged. With steel it forms alloys that are hard and resist rust and the effect of acids. Nickel-chromium alloys are used in heating coils and electric furnaces because they resist corrosion at high temperature. Nickel plating protects other metals from oxidation and rust and gives them a beautiful appearance. Finely divided nickel is a catalyst in the hydrogenation of oils. Common mistake: 'Ni-Cr coils are used because they conduct well'. The book's reason is resistance to corrosion at high temperature.
Choose the correct statement about copper.
- B. Bronze is an alloy of copper and zinc.
- D. Fehling solution, a copper compound, changes from orange to blue when it detects glucose.
- A. Copper is used in electric cables because it is a good conductor of electricity.
- C. Copper is used to galvanize iron to protect it from rusting.
الإجابة: Copper is used in electric cables because it is a good conductor of electricity.
Book (p.7): copper is a good conductor of electricity. So it is used in electric cables and in the coins industry. Copper is the first discovered metal. Copper-tin alloys are known as bronze; zinc is not part of bronze. Copper (II) sulphate, CuSO₄, is used as an insecticide and fungicide in the purification of water. Fehling solution (one of copper compounds) detects glucose: its blue colour changes to orange. Galvanizing other metals to protect them from rusting uses zinc, not copper.
Metal Rigidity Behaviour in the human body Density P very hard reacts slowly with body fluids; its ions are toxic high Q as rigid as steel inert; the body does not reject it less than steel R soft inert; the body does not reject it low Three metals considered for a dental implant (data for illustration) A dentist needs a metal for a dental implant that will stay in the jawbone for many years. The table describes three metals P, Q and R. Using the book's reasoning for titanium, which metal is the best choice?
- C. R, because it is inert and the lightest
- B. P, because it is the hardest of the three
- D. P, because it reacts with body fluids and bonds to bone
- A. Q, because it is inert and rigid
الإجابة: Q, because it is inert and rigid
The book (p.5) says titanium is used in dental implants and artificial joints because it is an inert substance, so the body does not eject (reject) it and it does not cause any type of poisoning. Titanium is also as rigid as steel but less dense. Metal Q has exactly these properties, so it is the best choice. P is very hard, but it reacts with body fluids and releases toxic ions, so it fails the 'no poisoning' condition. R is inert, but soft, so it cannot carry the force of chewing. Common mistake: choosing by hardness alone. Rigidity matters, but the book's reason for implants is that titanium is inert.
Choose the correct answer: Which of the following is a use of chromium metal itself, not of one of its compounds?
- C. Acting as an oxidizing agent
- B. Manufacture of dyes
- D. Manufacture of car springs
- A. Plating other metals
الإجابة: Plating other metals
Book (p.6): chromium (the metal) is used in metal plating and leather tanning. Its compounds: chromium (III) oxide, Cr₂O₃, is used in the manufacture of dyes, and potassium dichromate, K₂Cr₂O₇, is used as an oxidizing agent. Car springs are made of steel with a little vanadium. Chromium plating works because chromium resists the effect of the atmosphere: a non-porous oxide layer covers it.
Choose the correct answer: The oxide layer formed on the surface of chromium protects the metal, because ...
- B. the molecular volume of the oxide is smaller than that of the metal atoms, so the layer is packed tightly
- C. the oxide layer is porous, so air passes through it without reacting
- D. chromium is an inactive metal, so its oxide forms very slowly
- A. the molecular volume of the oxide is larger than that of the metal atoms, so the layer is non-porous
الإجابة: the molecular volume of the oxide is larger than that of the metal atoms, so the layer is non-porous
Book (p.6): chromium is a chemically active metal, but it resists the effect of atmospheric air, because the molecular volume of the oxide formed on its surface is larger than that of the metal atoms. So the oxide forms a non-porous layer that prevents the continued interaction with the oxygen of air. A smaller oxide would leave gaps; a porous layer (like rust) lets air and moisture reach the metal.
Choose the correct answer: In the dry cell, manganese dioxide (MnO₂) acts as ...
- A. a strong oxidizing agent
- B. a catalyst
- D. an antiseptic substance
- C. a reducing agent
الإجابة: a strong oxidizing agent
Book (p.6): manganese dioxide, MnO₂, acts as a strong oxidizing agent used in dry cell. Potassium permanganate, KMnO₄, is the antiseptic substance. Manganese II sulphate, MnSO₄, is the fungicide. You may have met MnO₂ as a catalyst in the decomposition of hydrogen peroxide. That is a different reaction. In the dry cell, the book gives it the role of an oxidizing agent.
Choose the correct answer: Cobalt-60 is used in food preserving and in the treatment of tumours, because it ...
- C. produces ultraviolet rays, which cannot pass through matter
- A. produces gamma rays, which have high penetrating power
- B. can be magnetized, like iron
- D. is a stable isotope that never changes
الإجابة: produces gamma rays, which have high penetrating power
Book (p.7): cobalt has twelve radioactive isotopes; cobalt-60 is a very important one because it produces gamma rays, which have high penetrating power. So it is used in food preserving, to check the quality of industrial products, to detect cracks and welding connections, and in medicine for the diagnosis and treatment of tumours. Cobalt's magnetic property is a different use: like iron, it can be magnetized, so it is used to make magnets.
Form of nickel Time for hydrogenation Mass of nickel at the end one lump 60 min 5.0 g finely divided powder 5 min 5.0 g Hydrogenation of an oil with nickel (data for illustration) Two experiments hydrogenate the same amount of an oil, using the same mass of nickel as a catalyst. The results are shown in the table. Which conclusion is correct?
- C. Nickel is a reactant here, because changing its form changed the time
- A. The powder is faster because more of its surface meets the reactants; it is not used up
- D. The lump is better, because the nickel stays in one piece
- B. The powder is faster because it reacts with the oil and is used up faster
الإجابة: The powder is faster because more of its surface meets the reactants; it is not used up
Analyze the two columns. (1) Mass at the end: 5.0 g in both experiments. So nickel is not used up: it is a catalyst, not a reactant. (2) Time: the powder takes 5 min, the lump takes 60 min. With the same mass, the powder has a much larger surface area. The reactants meet the catalyst at its surface. More surface lets more reactant molecules react at the same time, so the reaction is faster. This is why the book uses finely divided nickel as a catalyst in hydrogenation processes of oil (p.7).
Element X belongs to the first transition series. X is used in galvanizing other metals to protect them from rusting. Its oxide is used in the manufacture of paints, rubber and cosmetics, and its sulphide in X-ray screens. Which is the electronic configuration of X? (Atomic numbers: Cr = 24, Ni = 28, Cu = 29, Zn = 30)
- B. [Ar]\,4s¹\,3d¹⁰
- D. [Ar]\,4s¹\,3d⁵
- A. [Ar]\,4s²\,3d¹⁰
- C. [Ar]\,4s²\,3d⁸
الإجابة: [Ar]\,4s²\,3d¹⁰
Step 1: identify X from its uses (book pp.7-8). X is used in galvanizing other metals to protect them from rusting. Zinc oxide, ZnO, is used in paints, rubber and cosmetics. Zinc sulphide, ZnS, is used in illuminating paints and X-ray screens. So X is zinc, atomic number 30. Step 2: write its configuration. After argon (18 electrons), 12 electrons remain. Two fill 4s, then ten fill 3d: [Ar]\,4s²\,3d¹⁰. Nickel and chromium also coat other metals; copper is the element next to zinc, used in electric cables. [Ar]\,4s¹\,3d¹⁰ is copper. Copper's real configuration differs from the expected one; you will study why later in this chapter. [Ar]\,4s²\,3d⁸ is nickel, used for plating other metals. [Ar]\,4s¹\,3d⁵ is chromium, used in metal plating.
Match each material in column (A) with its use in column (B).
الإجابة: Scandium added to mercury vapour lamps → Light like sunlight for TV photography at night · Titanium-aluminum alloys → Aircraft and space shuttles · Titanium metal → Dental implants and artificial joints · Titanium dioxide (TiO2) nanoparticles → Sun protection cosmetics
Book (p.5): (1) Scandium is added to mercury vapour lamps to produce light with high quality like sunlight. So it is used in TV photography at night. (2) Titanium-aluminum alloys are used in the manufacture of aircraft and space shuttles. Titanium maintains its durability at high temperatures, while the hardness of aluminum decreases. (3) Titanium metal is used in dental implants and artificial joints. It is inert, so the body does not reject it and it causes no poisoning. (4) TiO2 nanoparticles are used in sun protection cosmetics. They prevent the effect of UV on the skin. Common confusion: titanium metal and titanium dioxide. Sunscreen contains the compound TiO2, not the metal. Also keep apart the scandium-aluminum alloy, which is used in MiG fighter jets.
Potassium dichromate (K₂Cr₂O₇) is used as an oxidizing agent. In a reaction, it changes a substance Y into YO (Y gains oxygen). Which choice describes both changes correctly?
- D. Y is reduced; K₂Cr₂O₇ is reduced too
- A. Y is oxidized; K₂Cr₂O₇ is reduced
- C. Y is oxidized; K₂Cr₂O₇ is oxidized too
- B. Y is reduced; K₂Cr₂O₇ is oxidized
الإجابة: Y is oxidized; K₂Cr₂O₇ is reduced
Step 1: Y → YO. Y gains oxygen, so Y is oxidized. Step 2: the oxygen came from K₂Cr₂O₇. The oxidizing agent supplies oxygen (it oxidizes the other substance), so it is itself reduced. Oxidation and reduction always happen together. That is the meaning of the book's phrase 'potassium dichromate is used as an oxidizing agent' (p.6). The same idea explains MnO₂, a strong oxidizing agent in the dry cell.
Consider the following statements about chromium in air: (I) Chromium resists the effect of the atmosphere because it does not react with oxygen. (II) The oxide layer on the surface of chromium is non-porous. (III) The oxide layer stops the continued interaction of chromium with the oxygen of air. Which of these statements is/are correct?
- D. II only
- A. II and III only
- C. I, II and III
- B. I and II only
الإجابة: II and III only
(I) Wrong: the book says chromium is a chemically active metal. It does react with oxygen; that reaction is what forms the oxide layer. (II) Correct: the molecular volume of the oxide is larger than that of the metal atoms, so the layer is non-porous. (III) Correct: this non-porous layer prevents the continued interaction of the metal with the oxygen of air. Answer: II and III only. Key idea: chromium resists the atmosphere because of its oxide layer, not because it is unreactive.
Match each manganese compound or alloy in column (A) with its use in column (B).
الإجابة: Manganese dioxide (MnO₂) → Strong oxidizing agent in the dry cell · Potassium permanganate (KMnO₄) → Antiseptic substance · Manganese (II) sulphate (MnSO₄) → Fungicide · Ferromanganese alloy → Railway tracks · Aluminum-manganese alloy → Soft-drink cans
Book (p.6): MnO₂ → strong oxidizing agent in the dry cell; KMnO₄ → antiseptic substance; MnSO₄ → fungicide; ferromanganese → railway tracks (harder than steel); aluminum-manganese alloy → soft-drink cans (resists corrosion). Manganese itself is brittle, so it is only used in alloys and compounds. Common confusion: swapping KMnO₄ (antiseptic) and MnSO₄ (fungicide). Tip: '-sulphate' goes with 'fungicide', just like copper (II) sulphate, which is also a fungicide.
A factory wants to check thick steel pipes for hidden cracks in their welding connections. A food company wants to preserve packed spices. Both plan to use cobalt-60. Consider the following statements: (I) Both uses depend on the gamma rays of cobalt-60, which can pass through thick materials. (II) The spices become radioactive after the treatment, so they cannot be eaten. (III) Cobalt-59 could replace cobalt-60, because it is an isotope of the same element. Which of these statements is/are correct?
- A. I only
- D. I, II and III
- C. I and III only
- B. I and II only
الإجابة: I only
(I) Correct: the book (p.7) says cobalt-60 produces gamma rays with high penetrating power, used in food preserving and to detect cracks and welding connections. (II) Wrong: gamma rays pass through the spices and kill microbes; no radioactive substance is added, so the spices do not become radioactive. (III) Wrong: isotopes have the same number of protons but different numbers of neutrons, so their nuclei differ. Cobalt-59 is the stable isotope of cobalt and gives out no gamma rays; cobalt-60 has an unstable nucleus. Answer: I only.
Match each nickel material in column (A) with its use or property in column (B).
الإجابة: Nickel-cadmium battery → Can be recharged · Nickel-steel alloys → Hard; resist rust and the effect of acids · Nickel-chromium alloys → Heating coils and electric furnaces · Nickel plating → Protects metals from oxidation and rust; beautiful appearance · Finely divided nickel → Catalyst in the hydrogenation of oils
Book (p.7): the nickel-cadmium battery can be recharged; nickel-steel alloys are hard and resist rust and the effect of acids; nickel-chromium alloys are used in heating coils and electric furnaces because they resist corrosion at high temperature; nickel plating protects other metals from oxidation and rust and gives them a beautiful appearance; finely divided nickel is a catalyst in the hydrogenation of oils. Common confusion: nickel-steel (hard, resists acids) with nickel-chromium (resists corrosion at high temperature, for heating).
Match each copper or zinc material in column (A) with its use or name in column (B).
الإجابة: Copper (II) sulphate (CuSO₄) → Insecticide, fungicide and water purification · Fehling solution → Detecting glucose · Zinc oxide (ZnO) → Paints, rubber and cosmetics · Zinc sulphide (ZnS) → Illuminating paints and X-ray screens · Copper-tin alloy → Bronze
Book (pp.7-8): copper (II) sulphate → insecticide and fungicide, and in the purification of water; Fehling solution (one of the copper compounds) → detecting glucose (its colour changes from blue to orange); zinc oxide → paints, rubber and cosmetics; zinc sulphide → illuminating paints and X-ray screens; copper-tin alloys → bronze. Common confusions: bronze is copper + tin (not zinc); CuSO₄ is not the glucose test, Fehling solution is; ZnO and ZnS are easy to swap: the sulphide is the one that glows (illuminating paints, X-ray screens).
Time (weeks) Sheet X (mg) Sheet Y (mg) 0 0 0 1 4 3 2 5 6 3 5 9 4 5 12 5 5 15 6 5 18 7 5 21 8 5 24 Total gain in mass of two metal sheets in moist air, flakes included (data for illustration) Two metal sheets, X and Y, with the same surface area, are left in moist air. The oxide layer on X stays attached. The layer on Y cracks and flakes off; the flakes are collected and weighed with the sheet. The graph shows the total gain in mass of each sheet since week 0 (oxygen joining the metal). Which conclusion is correct?
- B. X is a less active metal than Y, because its mass stopped rising
- D. Both oxide layers protect; Y's layer just works more slowly
- C. Y is like chromium, because it keeps reacting with oxygen
- A. X is like chromium: its nonporous oxide layer stops further reaction
الإجابة: X is like chromium: its nonporous oxide layer stops further reaction
Read the graph in two parts. (1) Week 0 to week 1: X gains 4 mg, more than Y's 3 mg. So X is an active metal that reacts with oxygen. (2) From week 2 on, X's total gain stays at 5 mg. So the reaction has stopped: a protective layer covers X. Its layer stays attached, so it is nonporous. This is chromium's behaviour (book p.6). The molecular volume of its oxide is larger than that of the metal atoms. So the nonporous oxide layer prevents the continued interaction with oxygen of air. Y's total gain rises by 3 mg every week, and its layer flakes off. Its layer is porous, so fresh metal keeps meeting oxygen and moisture, like iron with rust. Common mistake: 'X stopped reacting, so X is less active.' The fast start shows X is active. The layer protects it, not low activity.
Arrange the following elements as they appear in the first transition series, from left to right: nickel (Ni), titanium (Ti), copper (Cu), manganese (Mn), vanadium (V).
الإجابة: titanium (Ti) → vanadium (V) → manganese (Mn) → nickel (Ni) → copper (Cu)
The first transition series lies in the fourth period, after calcium: ₂₁Sc, ₂₂Ti, ₂₃V, ₂₄Cr, ₂₅Mn, ₂₆Fe, ₂₇Co, ₂₈Ni, ₂₉Cu, ₃₀Zn. So the order is Ti (22), V (23), Mn (25), Ni (28), Cu (29). Tip: the book's economic-importance section follows the same order (1-Scandium, 2-Titanium, ... 10-Zinc), so the uses you learn in this unit also help you remember the order.
Choose the correct answer: Zinc (₃₀Zn), the last element of the first transition series, belongs to group ...
- C. IB (11)
- B. IIA (2)
- D. VIII (8, 9, 10)
- A. IIB (12)
الإجابة: IIB (12)
The d-block groups, from left to right: IIIB, IVB, VB, VIB, VIIB, VIII (three columns: 8, 9, 10), IB (11), IIB (12). Zinc (4s²\,3d¹⁰) is in the last column of the d-block, group IIB (12), with cadmium and mercury. Copper is in IB. Zinc is not in group IIA: although it has 4s², the electrons of its series filled the 3d sublevel, so it is a d-block element.
اتعلّمها وراجعها
- Main transition elements (d-block)درس
- Elements of the first transition seriesدرس
- Light and wavesدرس
- Electromagnetic radiation (UV, X-rays, gamma)درس
- Economic importance of scandium and titaniumدرس
- Valency and writing chemical formulasدرس
- The transition elements and their seriesفيديو
- Light and waves: from zeroفيديو
- Electromagnetic radiation (UV, X-rays, gamma): from zeroفيديو
- Acids, bases and salts: from zeroفيديو
- Acidic and basic oxides: from zeroفيديو
- Rusting and corrosion: from zeroفيديو
- أسئلة عليها بإجابات مشروحةتدريب
- أسئلة عليها بإجابات مشروحةتدريب
- أسئلة عليها بإجابات مشروحةتدريب
- أسئلة عليها بإجابات مشروحةتدريب
- Main transition elements (d-block)ريل
- Elements of the first transition seriesريل
- Light and wavesريل
- Electromagnetic radiation (UV, X-rays, gamma)ريل
- Economic importance of scandium and titaniumريل
- Valency and writing chemical formulasريل