Chapter 1
Chemical Reactions and Equations
Q1
When a magnesium ribbon burns in air, the white powder formed is:
✅ Explanation
Magnesium burns vigorously in air (oxygen) to form magnesium oxide (MgO) — a white powder. Reaction: 2Mg + O₂ → 2MgO
Q2
Which of the following is NOT an indicator that a chemical reaction has taken place?
✅ Explanation
Chemical reactions are indicated by: change in state, colour, evolution of gas, or temperature change. Change in shape is a physical change, not a chemical one.
Q3
The correctly balanced equation for burning of magnesium in air is:
✅ Explanation
To balance, 2 atoms of Mg react with 1 molecule of O₂ (2 oxygen atoms) to form 2 molecules of MgO. Law of conservation of mass must be satisfied.
Q4
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat. This reaction is an example of:
✅ Explanation
Calcium oxide (quick lime) combines with water to form a single product — calcium hydroxide. Since heat is also released, it is both a combination and an exothermic reaction.
Q5
Reactions in which heat is released along with products are called:
✅ Explanation
Exothermic reactions release heat (exo = outside). Examples: burning of coal, respiration, formation of slaked lime. Endothermic reactions absorb energy.
Q6
2FeSO₄(s) →(Heat) Fe₂O₃(s) + SO₂(g) + SO₃(g). What type of reaction is this?
✅ Explanation
A single reactant (FeSO₄) breaks down into multiple products when heated. This is decomposition. Since it is caused by heat, it is specifically called thermal decomposition.
Q7
When silver chloride is kept in sunlight, it turns grey. This is because it decomposes into:
✅ Explanation
2AgCl(s) →(Sunlight) 2Ag(s) + Cl₂(g). The grey colour is due to formation of silver metal. This photochemical decomposition is used in black and white photography.
Q8
In electrolysis of water, the ratio of hydrogen gas to oxygen gas collected is:
✅ Explanation
2H₂O → 2H₂ + O₂. For every 2 molecules of water, 2 molecules of H₂ and 1 molecule of O₂ are produced. So H₂ : O₂ = 2 : 1 (by volume).
Q9
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). This is an example of:
✅ Explanation
Iron (more reactive) displaces copper from copper sulphate solution. One element replaces another in its compound — this is a displacement reaction. The iron nail becomes copper-coated and the blue solution fades.
Q10
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq). The white insoluble precipitate formed is:
✅ Explanation
BaSO₄ (barium sulphate) is an insoluble white precipitate formed by the exchange of ions between sodium sulphate and barium chloride. This is a double displacement (precipitation) reaction.Advertisement
Q11
In the reaction CuO + H₂ →(Heat) Cu + H₂O, which substance is oxidised?
✅ Explanation
Hydrogen gains oxygen (to form H₂O) → it is oxidised. CuO loses oxygen (to form Cu) → it is reduced. This is a redox reaction where both oxidation and reduction occur simultaneously.
Q12
Rancidity in food is caused by:
✅ Explanation
When fats and oils get oxidised, they become rancid — their smell and taste change. Antioxidants and airtight containers help slow down oxidation and prevent rancidity.
Q13
Chips manufacturers flush bags with nitrogen gas. Why?
✅ Explanation
Nitrogen is an inert gas. It displaces oxygen inside the bag, thereby preventing oxidation of the fat/oil in chips. This prevents rancidity and keeps chips fresh for longer.
Q14
The process by which iron objects get coated with reddish-brown powder when exposed to moist air is called:
✅ Explanation
Corrosion is the process where metals are attacked by substances like moisture and acids. The reddish-brown powder on iron is iron oxide (rust). The black coating on silver and green on copper are also examples of corrosion.
Q15
Which of the following represents a double displacement reaction?
✅ Explanation
In double displacement reactions, two compounds exchange their ions to form two new compounds. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl is a classic example where SO₄²⁻ and Cl⁻ ions are exchanged.
Q16
In the reaction 2Pb(NO₃)₂ →(Heat) 2PbO + 4NO₂ + O₂, the brown fumes observed are of:
✅ Explanation
When lead nitrate is heated, it undergoes thermal decomposition. The brown fumes produced are of nitrogen dioxide (NO₂). Lead oxide (yellow solid) and oxygen are also produced.
Q17
Decomposition of calcium carbonate on heating is called thermal decomposition. The products formed are:
✅ Explanation
CaCO₃(s) →(Heat) CaO(s) + CO₂(g). Calcium oxide (quick lime) is used in the manufacture of cement. This is an important industrial reaction.
Q18
Which of the following is an endothermic reaction?
✅ Explanation
Endothermic reactions absorb energy. Decomposition of CaCO₃ requires continuous heating (energy input) to break down. Burning of coal, respiration, and formation of slaked lime all release heat (exothermic).
Q19
In the reaction 2Cu + O₂ →(Heat) 2CuO, copper is said to be:
✅ Explanation
Oxidation is the gain of oxygen (or loss of hydrogen). Here copper gains oxygen to form copper oxide (black). This is why the surface of copper turns black on heating in air.
Q20
Fe₂O₃ + 2Al → Al₂O₃ + 2Fe. This reaction is used to join railway tracks. It is called:
✅ Explanation
Aluminium (more reactive) displaces iron from iron(III) oxide. This is a displacement reaction also known as the Thermite or Aluminothermic reaction. It produces molten iron and releases enormous heat, used to weld railway tracks.Advertisement