Chemical Reactions and Equations

Change in shape is a physical change, not a chemical change. For example, cutting a paper into pieces changes its shape but no new substance is formed. Chemical reactions always involve formation of new substances, which is shown by colour change, gas evolution, temperature change, precipitate formation, or change in smell.

When magnesium (Mg) burns in oxygen (O₂) from air, it undergoes a combination reaction to form magnesium oxide (MgO). The reaction is: 2Mg + O₂ → 2MgO. A bright white flame is seen during this reaction, which is a sign of a chemical reaction taking place.

In 2H₂ + O₂ → 2H₂O: Left side has 4 H atoms (2×2) and 2 O atoms. Right side has 4 H atoms (2×2) and 2 O atoms (2×1). Both sides are equal, so the equation is balanced. It correctly follows the Law of Conservation of Mass.

• Right side has 3 Fe atoms in Fe₃O₄, so put 3 before Fe on left: 3Fe
• Right side has 4 O atoms in Fe₃O₄, so we need 4 H₂O on left: 4H₂O
• 4H₂O gives 8 H atoms on left, so we need 4H₂ on right
• Check: Fe: 3=3 ✓, O: 4=4 ✓, H: 8=8 ✓

CaO + H₂O → Ca(OH)₂. Here, quicklime (CaO) and water (H₂O) combine to form a single product — calcium hydroxide Ca(OH)₂ (also called slaked lime). Since two substances are joining to form one, it is a combination reaction. This reaction is also highly exothermic — a lot of heat is released.

2AgCl(s) → 2Ag(s) + Cl₂(g). This reaction occurs in presence of sunlight (light energy). Since light causes the decomposition, it is called photolytic decomposition. The white silver chloride turns grey because silver metal is formed. This principle is used in black and white photography.

FeSO₄·7H₂O (ferrous sulphate, green crystals) when heated: FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g). The green crystals lose water first and turn white, then decompose to form reddish-brown iron oxide (Fe₂O₃), and release SO₂ and SO₃ gases with a suffocating smell. This is a thermal decomposition reaction.

Fe + CuSO₄ → FeSO₄ + Cu. Iron (Fe) displaces copper from copper sulphate solution. The blue colour of copper sulphate (CuSO₄) fades because it is consumed, and green iron sulphate (FeSO₄) is formed. Reddish-brown copper deposits on the iron nail. Iron is more reactive than copper, so it displaces copper.

In this reaction, Na₂SO₄ and BaCl₂ exchange their ions — sodium (Na⁺) swaps with barium (Ba²⁺). Two new compounds are formed: BaSO₄ (barium sulphate — a white precipitate, shown by ↓) and NaCl (sodium chloride). Since both compounds exchange ions, it is a double displacement reaction. The ↓ symbol shows BaSO₄ is an insoluble precipitate.

• Carbon (C) gains oxygen (from ZnO) to form CO₂ → Carbon is oxidised (gains oxygen)
• ZnO loses oxygen to form Zn → ZnO is reduced (loses oxygen)

1. (1)Some HCl molecules lose hydrogen (oxidised) — their H joins O to form H₂O
2. (2)Some HCl molecules lose Cl⁻ which forms Cl₂ gas — the Cl is oxidised

Galvanisation is the process of coating iron or steel with a thin layer of zinc to protect it from rusting. Zinc is more reactive than iron, so it reacts with oxygen and moisture instead of iron, protecting the iron beneath. Zinc forms a protective layer of zinc oxide/carbonate on its surface. Galvanised iron pipes and buckets are common examples.

When copper is exposed to air (which contains CO₂ and moisture), it slowly forms a green layer of basic copper carbonate: Cu + CO₂ + H₂O → CuCO₃·Cu(OH)₂ (basic copper carbonate). This is why old copper statues, vessels, and the Statue of Liberty appear green. This is a form of corrosion specific to copper.

The chemical reaction for rusting is: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (hydrated iron oxide — rust). Both oxygen and water are needed for this reaction. When we paint or oil the surface, we create a barrier that stops oxygen and moisture from reaching the iron. Without these two reactants, the rusting reaction cannot occur.

Nitrogen is an inert (unreactive) gas. When nitrogen is flushed into the chips packet, it replaces oxygen. Since fats in chips need oxygen to get oxidised (become rancid), removing oxygen prevents rancidity. This keeps the chips fresh for longer. Nitrogen does not react with the food, so it does not change the taste or quality of chips.

BHA (Butylated Hydroxyanisole) and BHT (Butylated Hydroxytoluene) are synthetic antioxidants added to food products to prevent rancidity. Antioxidants work by getting oxidised themselves, thereby protecting the fats and oils in food from oxidation. Natural antioxidants include Vitamin C (ascorbic acid) and Vitamin E (tocopherol). NaCl is a preservative but not an antioxidant.

Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Glucose is broken down in the presence of oxygen to release energy. This energy is used by our body to perform activities. Since energy (heat) is released, it is an exothermic reaction. Our body temperature remains at 37°C because of this continuous exothermic process.

CaCO₃ + heat → CaO + CO₂. Limestone (CaCO₃) absorbs a large amount of heat when heated to decompose into quicklime (CaO) and carbon dioxide. Since energy is absorbed (not released), it is endothermic. Options A, C, and D all release heat — they are exothermic reactions.

• Opposite of combination = decomposition
• Displacement involves reactivity of metals
• Double displacement involves ion exchange and often forms precipitate
• Redox involves transfer of oxygen, hydrogen, or electrons

Proposed by Antoine Lavoisier. This is why we balance chemical equations — to ensure the same number of atoms are present before and after the reaction. Example: In 2H₂ + O₂ → 2H₂O, we have 4H and 2O on both sides. If the equation is unbalanced, it violates this law.

N₂ + 3H₂ → 2NH₃ is the Haber process for making ammonia. Here, two substances (N₂ and H₂) combine to form one product (NH₃) — that is a combination reaction.

Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Glucose is oxidised (combined with oxygen) to release energy. Since energy is released, it is exothermic. The reason correctly explains why respiration is exothermic — the oxidation of glucose releases heat energy. So both the assertion and reason are correct, and the reason is the correct explanation.

1. (1)It decomposes into lead oxide (PbO), nitrogen dioxide (NO₂) — a brown/reddish-brown gas, and oxygen gas (O₂)
2. (2)Since one substance breaks into multiple products using heat, it is thermal decomposition
3. (3)Observations: Yellow/orange fumes of NO₂ are seen, the white solid turns yellow (PbO)
4. (4)A glowing splint brought near the mouth of test tube relights — confirming O₂ is released

Iron (Fe) is more reactive than copper (Cu) in the reactivity series. So iron displaces copper from copper sulphate solution. As the blue CuSO₄ is consumed and green FeSO₄ is formed, the solution changes colour from blue to green. Copper metal deposits on the iron nail as a reddish-brown coating. This is a displacement reaction.

Zinc is more reactive than hydrogen. So zinc displaces hydrogen from hydrochloric acid. The H₂ gas produced causes bubbling (effervescence). We can test the gas by bringing a burning matchstick near the mouth of the test tube — H₂ burns with a pop sound. The zinc chloride (ZnCl₂) formed dissolves in water (remains in solution).

In Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl, the ions Na⁺ and Ba²⁺ exchange their partners — Na pairs with Cl and Ba pairs with SO₄. Two compounds exchange ions to form two new compounds. A white precipitate of BaSO₄ is also formed. This is a double displacement and also a precipitation reaction.

These 10 equations are frequently asked in exams. Learning them with the type of reaction will help answer most questions from this chapter. Always balance the equation and write physical states for full marks.