Chemical Reactions and Equations
CBSE Class 10 · Science · Notes, formulas and practice questions
Revise the CBSE Class 10 Science chapter Chemical Reactions and Equations: balancing equations, types of chemical reactions, oxidation and reduction, and everyday effects like corrosion and rancidity.
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A chemical reaction is an event in which substances change into new substances. This chapter opens with how to describe such events by word equations and then by symbol equations. Balancing an equation means adjusting whole-number coefficients so that both sides contain the same number of atoms of every element; subscripts are never changed. Equations also carry symbols such as (s), (l), (g) and (aq) to show physical states.
Reactions are grouped by the pattern of change. In a combination reaction two or more reactants give one product, while a decomposition reaction does the reverse and needs heat, light or electricity. A displacement reaction occurs when a more reactive element takes the place of a less reactive one in a compound. In a double-displacement reaction two ionic compounds exchange their ions, and one product may separate as a precipitate.
Oxidation and reduction always take place together, and this pair is called redox. Oxidation was originally defined as addition of oxygen and loss of hydrogen, and reduction as the loss of oxygen and gain of hydrogen; the electron-loss and electron-gain definitions extend the same ideas to all reactions. In any redox equation one reactant is oxidised while another is reduced, and knowing which is which is a standard exam skill.
The same science explains two real problems. Corrosion is the slow reaction of a metal with air and moisture; rusting of iron, the green coating on copper and the blackening of silver are examples. Rancidity is the oxidation of fats and oils in food, which changes the taste and smell. Using airtight containers, antioxidants and refrigeration all slow rancidity by limiting oxygen or slowing the reaction.
Key terms
- Chemical equation
- A chemical reaction written with symbols and formulas, with reactants on the left of an arrow and products on the right. For example, 2Mg + O₂ → 2MgO shows magnesium reacting with oxygen to form magnesium oxide.
- Balanced chemical equation
- An equation in which the number of atoms of each element is the same on both sides. It is achieved by adding coefficients before formulas; subscripts in formulas are never changed.
- Combination reaction
- A reaction in which two or more reactants combine to form one single product. Example: CaO + H₂O → Ca(OH)₂, in which quicklime reacts with water to form slaked lime.
- Decomposition reaction
- A reaction in which a single compound breaks down into two or more simpler substances. It usually needs heat, light or electricity; for example, 2H₂O → 2H₂ + O₂ during electrolysis.
- Displacement reaction
- A reaction in which a more reactive element removes a less reactive element from its compound. Example: Fe + CuSO₄ → FeSO₄ + Cu, where iron displaces copper from copper sulphate solution.
- Double-displacement reaction
- A reaction between two ionic compounds in solution in which their ions exchange partners. A precipitate is often formed; example: BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl.
- Redox reaction
- A reaction in which oxidation and reduction occur simultaneously. Oxidation is gain of oxygen, loss of hydrogen or loss of electrons; reduction is loss of oxygen, gain of hydrogen or gain of electrons. In 2CuO + C → 2Cu + CO₂, CuO is reduced and carbon is oxidised.
- Corrosion
- The slow destruction of a metal by reaction with air, moisture or chemicals around it. Rusting of iron, the green coating on copper and the blackening of silver are common examples of corrosion.
- Rancidity
- The oxidation of fats and oils in food when exposed to air, producing an unpleasant smell and taste. It is slowed by adding antioxidants, using airtight containers, or keeping food cool.
Formula sheet
| What | Formula | Notes |
|---|---|---|
| Balanced chemical equation | 2Mg + O₂ → 2MgO | Magnesium burning in oxygen to form magnesium oxide. The same number of atoms of each element appears on both sides, which is what balancing means. |
| Combination reaction | CaO + H₂O → Ca(OH)₂ | Two or more reactants form a single product. Here quicklime reacts with water to form slaked lime, releasing heat. |
| Decomposition reaction | 2H₂O → 2H₂ + O₂ | A single reactant breaks into two or more products, usually needing heat, light or electricity. This one happens during electrolysis of water. |
| Displacement reaction | Fe + CuSO₄ → FeSO₄ + Cu | A more reactive element displaces a less reactive one from its compound. Iron displaces copper from copper sulphate solution. |
| Double-displacement reaction | BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl | Two compounds exchange ions, often forming a precipitate. Barium sulphate is the insoluble white precipitate here. |
| Redox reaction | 2CuO + C → 2Cu + CO₂ | One substance is reduced while another is oxidised in the same reaction. Copper oxide is reduced to copper and carbon is oxidised to carbon dioxide. |
Practice questions with answers
1. What is a balanced chemical equation? Why must an equation be balanced?
A balanced chemical equation has the same number of atoms of each element on both sides of the arrow. It must be balanced because matter is conserved during a chemical reaction; atoms are neither created nor destroyed.
2. What is a combination reaction? Give the balanced equation for the reaction of quicklime with water.
A combination reaction is one in which two or more substances combine to form a single product. CaO + H₂O → Ca(OH)₂, and this reaction releases heat, so it is an exothermic combination reaction.
3. Complete and balance the reaction between barium chloride and sodium sulphate. Name the precipitate and state the reaction type.
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq). Insoluble white barium sulphate precipitates. This is a double-displacement reaction because the two compounds exchange their ions.
4. Iron filings are added to blue copper sulphate solution. Write the balanced equation, identify the reaction type, and state the colour change.
Fe + CuSO₄ → FeSO₄ + Cu. This is a displacement reaction because iron is more reactive than copper and displaces it from its salt. The blue copper sulphate solution becomes pale green, and a red-brown coating of copper forms on the iron.
5. What two conditions are necessary for rusting of iron? What type of change is rusting?
Rusting requires both oxygen from air and moisture or water in contact with iron. It is a slow oxidation reaction, and the reddish-brown product formed on iron is called rust.
6. Define oxidation in terms of oxygen and hydrogen, and also in terms of electrons.
Oxidation is gain of oxygen by a substance, loss of hydrogen from a substance, or loss of electrons. Reduction is the opposite process: loss of oxygen, gain of hydrogen, or gain of electrons.
7. In the reaction ZnO + C → Zn + CO, which substance is oxidised and which is reduced? Give reasons.
Zinc oxide loses oxygen to give zinc metal, so ZnO is reduced. Carbon gains oxygen to form carbon monoxide, so carbon is oxidised. This is a redox reaction.
8. Potato chip packets are often flushed with nitrogen gas before sealing. Why?
Nitrogen is an unreactive gas, so it does not react with the food. Flushing the packet with nitrogen removes oxygen, preventing the oxidation of fats and oils in the chips. This slows rancidity and keeps the chips fresh for a longer time.
9. Silver chloride turns grey when kept in sunlight. Write an equation for this change and identify the type of reaction.
2AgCl → 2Ag + Cl₂. This is a decomposition reaction in which light energy is used to break silver chloride into silver and chlorine, so it is called photolytic decomposition.
10. For the reaction 2CuO + C → 2Cu + CO₂, identify the oxidising agent and the reducing agent.
Copper oxide gives oxygen to carbon, so CuO is the oxidising agent; it is reduced to copper. Carbon accepts oxygen and forms carbon dioxide, so carbon is the reducing agent; it is oxidised.
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