Elements, compounds and mixtures

Chemically combined, or just mixed?

1. Elements, compounds and mixtures: from zero

2. Mixed or combined?

Mix grey iron filings with yellow sulphur powder. A magnet pulls the iron out. Now heat a fresh mixture strongly. A black solid forms.

Pick the right answer

Predict: can the magnet still pull the iron out of the black solid?

A. Yes: the iron is still there, so the magnet still works
B. No: iron and sulphur are now chemically joined in a new substance
C. Only after the black solid is dissolved in water

Explanation

The magnet can no longer pull the iron out: heating joined iron and sulphur chemically into a new substance, iron(II) sulphide, FeS, with its own properties.

3. A compound is a new substance

An element has one kind of atom, like Fe or S.

Compound

A substance formed of two or more elements chemically combined in a fixed ratio.

A compound has new properties. FeS is black: not grey like iron, not yellow like sulphur. It always has 1 Fe for 1 S.

4. A mixture is just mixed

Mixture

Two or more substances mixed physically. Each keeps its own properties, and they can be separated by physical means.

	Compound	Mixture
Ratio of the parts	fixed	can change
Properties	new ones	each part keeps its own
Separation	not by physical means	by physical means (a magnet, a filter...)

5. Element, compound or mixture?

Sort into groups

Sort each substance.

Water, $H2O H X_{2} O$ Salt waterCopper, CuAirCarbon dioxide, $CO2 CO X_{2}$ Iron filings and sand

Pick the right answer

A rock contains the compound

Fe2O3 Fe X_{2} O X_{3}

with sand spread through it. How is the whole rock best described?

A. A compound, because it contains iron and oxygen
B. A mixture: the $Fe2O3 Fe X_{2} O X_{3}$ and the sand each keep their properties
C. An element, because it is mostly iron

Explanation

The whole rock is a mixture: the compound

Fe2O3 Fe X_{2} O X_{3}

and the sand are not bonded to each other, each keeps its properties, and the amount of sand can vary.

6. Summary

Element: one kind of atom (Fe, S, O).
Compound: elements chemically combined in a fixed ratio, with new properties ( $FeS FeS$ , $Fe2O3 Fe X_{2} O X_{3}$ ).
Mixture: substances mixed physically. Each keeps its properties, and physical means separate them.

Can you explain why a magnet separates iron from an iron-sulphur mixture, but not from FeS?

Reveal

In the mixture, iron is only mixed physically and keeps its properties, so a magnet pulls it out. In

FeS FeS

, iron is chemically combined with sulphur in a new substance with new properties, so the magnet cannot pull the iron out.

Next: iron is found in ores: iron compounds, mostly oxides, mixed with impurities (unwanted substances).

Concepts in this lesson