Chemical Reactions and Equations — RBSE Class 10 (Science)

Leave a glass of milk out on a Jaipur summer afternoon and by evening it has curdled. Leave an iron tawa by the kitchen window through the monsoon and it grows a brown coat. Nothing was added — yet the substance is no longer the same. That "no longer the same" is the signature of a chemical reaction, and this chapter teaches you to read it, write it, and balance it.

1. How do we know a reaction has happened?

In the RBSE/NCERT book this chapter opens with five everyday situations — souring of milk, rusting of an iron tawa, fermentation of grapes, cooking of food, digestion. In each, the identity of the starting substance changes. That is the test: a physical change (ice melting, sugar dissolving) keeps the substance the same; a chemical change makes a new substance.

You spot a chemical reaction by one or more of these clues:

• Change in state — wax (solid) burning to gases.
• Change in colour — green ferrous sulphate turning brown.
• Evolution of a gas — zinc + dilute acid giving bubbles of hydrogen.
• Change in temperature — quicklime + water becoming hot.
• Formation of a precipitate — a solid that settles out of solution.

The substances you start with are reactants; the new substances are products.

2. Chemical equations — the grammar of reactions

A word equation for the burning of magnesium:

Magnesium + Oxygen → Magnesium oxide

A chemical equation replaces names with formulae:

$\text{Mg}+{\text{O}}_2\to \text{MgO}$

But this is unbalanced — there are 2 oxygen atoms on the left and 1 on the right. The law of conservation of mass (mass is neither created nor destroyed in a reaction) demands equal atoms of each element on both sides.

Balancing — the hit-and-trial method

1. Write the unbalanced (skeletal) equation.
2. Count atoms of each element on both sides.
3. Balance the atom that appears in the fewest formulae first; adjust coefficients only — never change a formula's subscripts.
4. Balance H and O last (they appear in many compounds).
5. Re-count to verify, then add physical states.

$\text{Mg}+{\text{O}}_2\to \text{MgO}\Rightarrow 2\text{Mg}+{\text{O}}_2\to 2\text{MgO}$

Now: 2 Mg ↔ 2 Mg, 2 O ↔ 2 O. Balanced.

Making an equation more informative

Add physical states — (s) solid, (l) liquid, (g) gas, (aq) aqueous — and conditions over the arrow:

$3\text{Fe}(s)+4{\text{H}}_2\text{O}(g)\stackrel{\text{heat}}{\to }{\text{Fe}}_3{\text{O}}_4(s)+4{\text{H}}_2(g)$

3. The five types of chemical reactions

(a) Combination reaction

Two or more reactants form a single product.

$\text{CaO}(s)+{\text{H}}_2\text{O}(l)\to {\text{Ca(OH)}}_2(aq)+\text{heat}$

Quicklime (calcium oxide) reacting with water to give slaked lime — the reaction used for whitewashing walls. Slaked lime then slowly reacts with CO₂ in air to form a shiny CaCO₃ coat.

(b) Decomposition reaction

A single reactant breaks into two or more products. Energy is supplied as heat (thermal), light (photo) or electricity (electrolytic).

$2{\text{FeSO}}_4(s)\stackrel{\text{heat}}{\to }{\text{Fe}}_2{\text{O}}_3(s)+{\text{SO}}_2(g)+{\text{SO}}_3(g)$

Green ferrous sulphate crystals turn brown — the colour change you see in Activity 1.5. Other examples: heating limestone (CaCO₃ → CaO + CO₂), electrolysis of water, and the decomposition of silver chloride/bromide in light (the basis of photography, and why these salts are kept in dark bottles).

Decomposition is the reverse of combination.

(c) Displacement reaction

A more reactive element displaces a less reactive one from its compound.

$\text{Fe}(s)+{\text{CuSO}}_4(aq)\to {\text{FeSO}}_4(aq)+\text{Cu}(s)$

An iron nail dropped into blue copper sulphate solution turns the solution pale green and gets coated with reddish-brown copper — iron is more reactive than copper.

(d) Double displacement reaction

Two compounds exchange ions; often a precipitate forms.

${\text{Na}}_2{\text{SO}}_4(aq)+{\text{BaCl}}_2(aq)\to {\text{BaSO}}_4(s)\downarrow +2\text{NaCl}(aq)$

The white insoluble barium sulphate is the precipitate. Reactions in which an insoluble solid forms are precipitation reactions.

(e) Oxidation–reduction (redox) reaction

• Oxidation = gain of oxygen / loss of hydrogen.
• Reduction = loss of oxygen / gain of hydrogen.

$\text{CuO}+{\text{H}}_2\stackrel{\text{heat}}{\to }\text{Cu}+{\text{H}}_2\text{O}$

Here CuO loses oxygen → reduced to Cu; H₂ gains oxygen → oxidised to H₂O. Both happen together, so it is a redox reaction. A memory aid: OIL RIG — Oxidation Is Loss, Reduction Is Gain (of oxygen, in the Class 10 sense).

4. Exothermic and endothermic reactions

• Exothermic — heat is released. Burning of fuels, respiration (glucose + O₂ → CO₂ + H₂O + energy), decomposition of vegetable matter into compost, and the quicklime + water reaction.
• Endothermic — heat is absorbed. Thermal decomposition reactions generally need a continuous supply of heat.

Respiration is the standout example: it is a controlled, exothermic combustion of glucose that powers your body.

5. Corrosion and rancidity — redox in everyday life

Corrosion — a metal is slowly eaten away by attack of moist air. Iron rusts (reddish-brown Fe₂O₃·xH₂O), copper forms a green coat, silver tarnishes black. Rusting needs both air (oxygen) and water — which is why an iron gate in humid Kota rusts faster than one in dry Bikaner. Prevention: painting, oiling, greasing, galvanisation (zinc coating), and alloying (e.g. stainless steel).

Rancidity — fats and oils in food get oxidised on exposure to air, turning the taste and smell unpleasant (the "off" smell of old namkeen or ghee). Prevention: adding antioxidants, packaging in nitrogen (chip packets), refrigeration, and airtight containers.

Both corrosion and rancidity are oxidation processes — the same chemistry you met in §3(e), now in your kitchen and on your gate.

6. Closing thought

This chapter handed you a new literacy. You can now:

• Read a change in the world and classify it as physical or chemical;
• Write it as a balanced chemical equation that respects conservation of mass;
• Name the reaction — combination, decomposition, displacement, double displacement, redox;
• Explain why your tawa rusts and your snacks go stale.

Every later chemistry chapter — acids and bases, metals, carbon compounds — is just these five reaction types in new costumes. Get the balancing reflex automatic now, and the rest of Class 10 chemistry becomes bookkeeping you already know how to do.