# Types of chemical reactions

> Break down, swap one, swap two


## Objectives

- Recognize thermal decomposition, simple displacement and double displacement from their equations.
- Explain what the ↓ sign after a product means (a precipitate).

## 1. Types of chemical reactions: from zero




## 2. Green to black

Predict: what happened to the green solid?

- It just melted
- It broke down into two simpler substances ✓
- It took in a gas from the air

The green copper(II) carbonate broke down into two simpler substances: heat split it into a black solid and a gas. Melting would keep the same substance, and here a gas comes out of the solid instead of going in.

[ar-EG] الـ copper(II) carbonate الأخضر اتكسّر لمادتين أبسط: الحرارة فصلته لـ solid أسود و gas. الانصهار كان هيسيب المادة زي ما هي، وهنا الـ gas خارج من الـ solid مش داخل فيه.
## 3. Thermal decomposition

Thermal decomposition Breaking down a compound by heat into simpler substances. green solid → black solid + gas Pattern: one compound in, two or more substances out. The $\Delta$ over the arrow means heat.


## 4. Simple displacement

Simple displacement A more active element takes the place of a less active one in its compound. Zinc is above copper in the activity series. The blue colour fades, and reddish copper coats the zinc. Metals above hydrogen also displace it from dilute acids: $\ce{Mg + 2HCl -> MgCl2 + H2}$.


## 5. Double displacement

Double displacement Two compounds exchange their partners: AB + CD → AD + CB. A white solid appears in the solution. AgCl does not dissolve, so it separates as a solid. Such a solid is a **precipitate**, shown by ↓. Another double displacement you know acid + base → salt + water


## 6. Spot the type

In $\ce{CuSO4 + 2NaOH -> Cu(OH)2 v + Na2SO4}$, what does ↓ after $\ce{Cu(OH)2}$ tell you?

- An insoluble solid forms in the solution ✓
- A gas leaves the reaction
- The reaction needs heat

The down arrow ↓ after $\ce{Cu(OH)2}$ means it is a precipitate: an insoluble solid that forms in the solution. A gas leaving is shown by ↑, and heat by $\Delta$ over the arrow.

[ar-EG] السهم اللي لتحت ↓ بعد الـ $\ce{Cu(OH)2}$ معناه إنه precipitate: solid مش بيدوب وبيتكوّن في الـ solution. أما خروج gas فبيتكتب ↑، والحرارة بتتكتب $\Delta$ فوق السهم.
## 7. Summary

**Thermal decomposition**: heat breaks one compound into simpler substances. **Simple displacement**: a more active element replaces a less active one. **Double displacement**: two compounds swap partners. A precipitate (↓) or salt + water may form. Can you explain how to tell simple from double displacement at a glance? Next: roasting heats iron ores, and some of its changes are thermal decompositions.



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-chem.reaction-types