Chemical Reactions and Equations: Class 10 short notes
The first chapter of Class 10 Science is mostly about recognising a reaction's type from its equation. These notes list each type with the example boards use most, the colour changes worth remembering, and a quick way to tell oxidation from reduction.
Short notes
- Signs that a reaction has happened: a change in state, a change in colour, a gas given off, or a change in temperature.
- A chemical equation is balanced so that each element has the same number of atoms on both sides: mass is neither created nor destroyed (law of conservation of mass).
- Combination: two or more substances form one. CaO + H₂O → Ca(OH)₂ + heat (quicklime to slaked lime). Burning magnesium: 2Mg + O₂ → 2MgO, with a dazzling white flame.
- Decomposition: one substance breaks into two or more. It needs energy: heat (thermal), electricity (electrolysis) or light (photolysis).
- Thermal: CaCO₃ → CaO + CO₂. Heating green ferrous sulphate crystals: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃, with a smell of burning sulphur. Heating lead nitrate gives brown fumes of NO₂.
- Electrolysis of water: 2H₂O → 2H₂ + O₂. The gas at the cathode (hydrogen) is double the volume of the gas at the anode (oxygen).
- Light: 2AgCl → 2Ag + Cl₂ in sunlight; white silver chloride turns grey. This is used in black-and-white photography.
- Displacement: a more reactive element pushes a less reactive one out of its compound. Fe + CuSO₄ → FeSO₄ + Cu: the blue solution turns light green and the iron nail turns brownish.
- Double displacement: two compounds swap ions. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl. The white insoluble solid (BaSO₄) is a precipitate.
- Oxidation is the gain of oxygen or loss of hydrogen; reduction is the loss of oxygen or gain of hydrogen. In CuO + H₂ → Cu + H₂O, CuO is reduced and H₂ is oxidised (a redox reaction).
- Exothermic reactions give out heat (respiration, burning natural gas). Endothermic reactions take in heat (most decomposition reactions).
- Corrosion: a metal is eaten away by air and moisture, like iron rusting. Rancidity: fats and oils oxidise and smell and taste bad; it's slowed by antioxidants, airtight containers and flushing chips packets with nitrogen.
At a glance
| Type | Pattern | Example |
|---|---|---|
| Combination | A + B → AB | CaO + H₂O → Ca(OH)₂ |
| Decomposition | AB → A + B | CaCO₃ → CaO + CO₂ |
| Displacement | A + BC → AC + B | Fe + CuSO₄ → FeSO₄ + Cu |
| Double displacement | AB + CD → AD + CB | Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl |
| Redox | One gains O, one loses O | CuO + H₂ → Cu + H₂O |
Memory tricks
OIL RIG
Oxidation Is Loss (of electrons or hydrogen), Reduction Is Gain. For Class 10, also remember: oxidation gains oxygen.
Decomposition needs energy: HEL
Heat, Electricity, Light: thermal, electrolytic and photolytic decomposition.
Double the H
In electrolysis of water, H₂ is double O₂, just as in the formula H₂O.
Practice questions
When an iron nail is dipped in copper sulphate solution, the solution's colour changes from:
Show answer
Blue to light green. Iron displaces copper, forming ferrous sulphate, which is light green.
2AgCl → 2Ag + Cl₂ in sunlight is an example of:
Show answer
Photolytic decomposition. Light supplies the energy to break silver chloride down.
In CuO + H₂ → Cu + H₂O, the substance oxidised is:
Show answer
H₂. Hydrogen gains oxygen to form water, so it's oxidised.
Chips packets are filled with nitrogen to prevent:
Show answer
Rancidity. Nitrogen keeps oxygen away, so the oils in the chips don't oxidise.
Questions
Which types of reaction are most important for the board exam?
All five types, each with one standard example, plus oxidation and reduction in a given equation. Balancing an equation and naming the reaction type come up most often.
Why is respiration called an exothermic reaction?
Glucose combines with oxygen in our cells and releases energy, so heat is given out.
Can I get notes like these for my own chapter?
Yes. Add your NCERT chapter PDF or photos of the pages to TellBrief, choose CBSE once, and you get short notes, memory tricks and board-style questions, each point linked to its page.