Complete Chapter Notes • Simple Language • Exam Focused
In this chapter, we learn how substances change during chemical reactions, how to write and balance chemical equations, different types of chemical reactions, oxidation and reduction, corrosion, and rancidity.
Think about milk turning sour, an iron nail rusting, or food being digested in your body.
In each of these examples, a substance changes into something with a different identity — not just a different shape or state, but a genuinely new substance.
A chemical reaction is a process in which one or more substances change into new substances with different properties.
Look for any of these four signs:
| Sign | What you may observe |
|---|---|
| 1. Change in state | Solid → Gas, Liquid → Gas, etc. |
| 2. Change in colour | A new colour appears |
| 3. Gas is evolved | Bubbles or gas are produced |
| 4. Change in temperature | Heat is released or absorbed |
When a small piece of magnesium ribbon is burned:
It produces a dazzling white flame and leaves behind a white powder of magnesium oxide.
Describing every chemical reaction in a full sentence can be tedious. Chemists therefore represent reactions using chemical equations.
A simple way is to write the names of the substances.
Written on the left.
Written on the right.
Shows the direction of reaction.
Instead of writing complete names, we can use chemical formulae.
Mass can neither be created nor destroyed during a chemical reaction.
Therefore, atoms cannot simply disappear or appear during a reaction.
| Element | Reactant Side | Product Side |
|---|---|---|
| Zinc (Zn) | 1 | 1 |
| Hydrogen (H) | 2 | 2 |
| Sulphur (S) | 1 | 1 |
| Oxygen (O) | 4 | 4 |
Consider:
For example: 3Fe is correct, but Fe3 would change the substance.
Check the number of atoms of every element on both sides.
In Fe3O4, there are 4 oxygen atoms. There is only 1 oxygen atom in H2O.
There are now 4 × 2 = 8 hydrogen atoms on the left.
There are 3 iron atoms in Fe3O4. Therefore, put 3 before Fe.
| Element | Left | Right |
|---|---|---|
| Fe | 3 | 3 |
| O | 4 | 4 |
| H | 8 | 8 |
This trial-and-improvement approach is called the hit-and-trial method.
Chemical equations become more informative when we mention the physical state of each substance.
| Symbol | Physical State |
|---|---|
| (s) | Solid |
| (l) | Liquid |
| (g) | Gas |
| (aq) | Aqueous — dissolved in water |
Carbon dioxide + Water → Glucose + Oxygen
Sunlight + Chlorophyll are required for the reaction.
There are several important types of chemical reactions:
A combination reaction is a reaction in which two or more substances combine to form a single new product.
Calcium oxide (quick lime) reacts with water to form calcium hydroxide (slaked lime).
A large amount of heat is released during this reaction.
Reactions that release heat are called exothermic reactions.
Examples: Burning of natural gas, respiration, and calcium oxide reacting with water.
During respiration, glucose combines with oxygen and releases energy.
Therefore, respiration is an exothermic reaction.
Slaked lime solution is used for whitewashing walls. Over a period of a few days, it slowly reacts with carbon dioxide from the air.
This produces a thin layer of calcium carbonate on the wall, giving the freshly whitewashed wall its shiny finish.
A decomposition reaction is a reaction in which one reactant breaks down into two or more simpler products.
This is essentially the opposite of a combination reaction.
Heating ferrous sulphate crystals, which are initially green, causes them to lose water and then decompose.
When calcium carbonate (limestone) is heated, it decomposes into calcium oxide and carbon dioxide.
Silver chloride is normally white. When exposed to sunlight, it decomposes and turns grey.
Silver bromide behaves similarly.
Electricity can also cause decomposition. Water can be decomposed into hydrogen and oxygen.
Exothermic → Heat is released
Endothermic → Energy is absorbed/required
A displacement reaction occurs when a more reactive element displaces a less reactive element from its compound.
Here, A is more reactive than B.
Iron is more reactive than copper. Therefore, iron displaces copper from copper sulphate.
In a double displacement reaction, two compounds exchange their ions to form two new compounds.
The ions exchange partners.
Barium sulphate (BaSO4) is insoluble in water. Therefore, it forms a white insoluble solid that settles out of the solution.
Oxidation and reduction often occur together.
A substance is oxidised when it:
A substance is reduced when it:
When copper powder is heated in air, it develops a black coating of copper oxide.
Copper gains oxygen. Therefore, copper is oxidised.
Hydrogen gas is passed over heated copper oxide. The black copper oxide changes back to brown copper metal.
CuO loses oxygen → Reduction
H2 gains oxygen → Oxidation
Oxidation reactions are involved in several everyday processes. Two important examples are corrosion and rancidity.
Rusting is the corrosion of iron. It produces a characteristic reddish-brown coating called rust.
| Metal | Common Observation |
|---|---|
| Iron | Reddish-brown rust |
| Silver | Black tarnish |
| Copper | Green coating |
Iron articles are often painted.
Antioxidants are added to fatty foods to slow down oxidation.
Reducing contact with oxygen slows the oxidation of fats and oils.
Packaged snacks such as chips are often flushed with nitrogen gas before sealing.
Nitrogen is relatively unreactive with the food.
Important concepts to revise before the exam
A process in which one or more substances change into new substances.
Change in state + Change in colour + Gas evolved + Temperature change
The number of atoms of every element is equal on both sides.
| Type | Basic Idea |
|---|---|
| Combination | Two or more substances → One product |
| Decomposition | One substance → Two or more products |
| Displacement | More reactive element displaces less reactive element |
| Double Displacement | Two compounds exchange ions |
| Redox | Oxidation and reduction occur together |
| Type | Energy |
|---|---|
| Exothermic | Heat is released |
| Endothermic | Energy is absorbed / required |
| Oxidation | Reduction |
|---|---|
| Gains oxygen | Loses oxygen |
| Loses hydrogen | Gains hydrogen |
Metals deteriorate.
Fats and oils develop unpleasant smell and taste.
High-priority equations for revision
Focus especially on balancing equations, identifying reaction types, oxidation and reduction, observations in reactions, corrosion, rancidity, and the key chemical equations.