Class 10 Science - Chapter 1: Chemical Reactions & Equations

Q1) Why should a Magnesium ribbon be cleaned before burning in air ?

Ans :- A Magnesium ribbon should be cleaned before burning in air is because due to the presence of a thin layer of magnesium oxide (MgO) on its surface.

Q2) Write the balanced equation for the following chemical reactions.

1. Hydrogen + Chlorine → Hydrogen chloride
2. Barium chloride + Aluminium sulphate → Barium sulphate + Aluminum chloride
3. Sodium + Water → Sodium hydroxide + Hydrogen

Ans :

(i) H₂ (g) + Cl₂ (g) → 2HCl(g)

(ii) 3BaCl₂ (aq) + Al₂(SO₄)₃ (aq) → 3BaSO₄ (s) + 2AlCl₃ (aq)

(iii) 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂ (g)

Q3) Write a balanced chemical equation with state symbols for the following reactions.

1. Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride .
2. Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Ans :

(i) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Q1) A solution of a substance ‘X’ is used for whitewashing.

1. Name the substance ‘X’ and write its formula.
2. Write the reaction of the substance ‘X’ named in (i) above with water.

Ans :

(i) The substance ‘X’ is Calcium oxide and its chemical formula is CaO.

(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide.

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

Q2) Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

Ans : In activity 1.7 water is electrolysed to give H₂ at one electrode and O₂ at the other electrode.

2H₂O(l) → 2H₂(g) + O₂(g)

Thus two molecules of water on electrolysis give two molecules of hydrogen and one molecule of oxygen.

Q1) Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?

Ans :- When an iron nail is placed in a copper sulphate solution, iron displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.

Fe + CuSO₄ → FeSO₄(aq) + Cu(s)

Q2) Give an example of a double displacement reaction other than the one given in Activity 1.10.

Ans :

Na₂CO₃(aq) + CaCl₂(aq) → CaCO₃(s) + 2NaCl(aq)

Q3) Identify the substances that are oxidised and the substances that are reduced in the following reactions.

1. 4Na+O₂ → 2Na₂O
2. CuO+H₂ → Cu(s)+H₂O

Ans :

(i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.

(ii) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen(H₂) gets oxidised to water H₂O.

Q1) Which of the statements about the reaction below are incorrect?

2PbO(s) + C(s) → 2Pb(s) + CO₂(g)

1. Lead is getting reduced.
2. Carbon dioxide is getting oxidised.
3. Carbon is getting oxidised.
4. Lead oxide is getting reduced

(i) (a) and (b)

(ii) (a) and (c)

(iii) (a), (b) and (c)

(iv) all

Ans : (a) and (b) are incorrect, and (i) is correct.

Q2) Fe₂O₃ + 2Al → Al₂O₃ + 2Fe

The above reaction is an example of a

1. combination reaction.
2. double displacement reaction.
3. decomposition reaction.
4. displacement reaction.

Ans : Correct answer is (d).

Q3) What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

1. Hydrogen gas and iron chloride are produced.
2. Chlorine gas and iron hydroxide are produced.
3. No reaction takes place.
4. Iron salt and water are produced.

Ans : (a) Hydrogen gas and iron chloride are produced.

Q4) What is a balanced chemical equation? Why should chemical equations be balanced?

Ans : A balanced chemical equation represents a reaction where the quantity of each type of atom is identical on both the reactant and product sides. This balancing is essential and rooted in the principle of mass conservation. This principle dictates that during a chemical transformation, matter is neither gained nor lost; the total mass of the system remains constant. Therefore, the mass of the reactants must equal the mass of the products. Balancing chemical equations ensures that this conservation of mass is upheld in our representation of the chemical change.

Q5) Translate the following statements into chemical equations and then balance them.

1. Hydrogen gas combines with nitrogen to form ammonia.
2. Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
3. Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
4. Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Ans :

(a) 3H₂ + N₂ → 2NH₃

(b) 2H₂S + 3O₂ → 2H₂O + 2SO₂

(c) 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄

(d) 2K + 2H₂O → 2KOH + H₂

Q6) Balance the following chemical equations.

1. HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
2. NaOH + H₂SO₄ → Na₂SO₄ + H₂O
3. NaCl + AgNO₃ → AgCl + NaNO₃
4. BaCl₂ + H₂SO₄ → BaSO₄ + HCl

Ans :

(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O

(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

(c) NaCl + AgNO₃ → AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

Q7) Write the balanced chemical equations for the following reactions.

1. Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
2. Zinc + Silver nitrate → Zinc nitrate + Silver
3. Aluminium + Copper chloride → Aluminium chloride + Copper
4. Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Ans :

(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag

(c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

(d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

Q8) Write the balanced chemical equation for the following and identify the type of reaction in each case.

1. Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
2. Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
3. Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
4. Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Ans :

(a) 2KBr (aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s)

Type : Double displacement reaction

(b) ZnCO₃ (s) → ZnO (s) + CO₂ (g)

Type : Decomposition reaction

(c) H₂ (g) + Cl₂ (g) → 2HCl(g)

Type : Combination reaction

(d) Mg (s) + 2HCl (aq) → MgCl₂(aq) + H₂ (g)

Type : Displacement reaction

Q9) What does one mean by exothermic and endothermic reactions? Give examples?

Ans :

Exothermic reactions : Those reactions in which heat is evolved are known as exothermic reactions.

Eg :

(a) C (s) + O₂ (g) → CO₂ (g) + Heat

(b) N₂ (g) + 3H₂ (g) → 2NH₃ (g) + Heat

Endothermic reactions : Those reactions in which heat is absorbed are known as endothermic reactions.

Eg :

(a) C (s) + 2S (s) → CS₂ (l) – Heat

(b) N₂ (g) + O₂ (g) → 2NO(g) – Heat

Q10) Why is respiration considered an exothermic reaction? Explain?

Ans : Respiration is exothermic because it releases energy. Glucose is broken down, and while this requires some initial energy, the formation of new bonds in the products (carbon dioxide and water) releases much more energy. This net release of energy makes respiration an exothermic process.

Since energy is released in the whole process, it is an exothermic process.

Equation :- C₆H₁₂O₆ (aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(l) + Energy

Q11) Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions?

Ans : Decomposition reactions are considered the inverse of combination reactions because they represent the reverse process. In a combination reaction, two or more smaller molecules or elements join to form a larger molecule. Conversely, a decomposition reaction involves a larger molecule breaking down into two or more smaller molecules or elements. Essentially, combination reactions build up larger molecules, while decomposition reactions break them down. Few examples of these reactions are:

(i) ZnCO₃ → ZnO + CO₂ [Decomposition reaction]

(ii) C + O₂ → CO₂ [Combination reaction]

Q12) Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Ans :

(a) Decomposition reaction where heat is supplied for energy: Also known as Thermal decomposition reaction

CaCO₃ (Heat) → CaO + CO₂

(b) Decomposition reaction where electricity is supplied for energy: Also known as Electrolytic decomposition reaction :-

2H₂O(l) → 2H₂(g) + O₂(g)

(c) Decomposition reaction where light is supplied for energy: Also known as Photo decomposition

2H₂O₂ → 2H₂O + O₂

Q13) What is the difference between displacement and double displacement reactions? Write equations for these reactions?

Ans :

Displacement Reaction: A displacement reaction is a chemical reaction in which a more reactive element or ion replaces a less reactive element or ion in a compound. It involves a single "substitution" or "replacement."

Double Displacement Reaction: A double displacement reaction is a chemical reaction in which two compounds exchange ions or elements. It involves two "substitutions" or "replacements," essentially a swapping of partners between two reacting molecules.

Eg :

(i) Displacement reaction: Mg + 2HCl → MgCl₂ + H₂

(ii) Double displacement : 2KBr + BaI₂ → 2KI + BaBr₂

Q14) In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Ans :

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

Q15) What do you mean by a precipitation reaction? Explain by giving examples.

Ans : A precipitation reaction is a type of double displacement reaction where two solutions containing soluble salts are mixed. The ions in these salts then exchange partners. If this exchange results in the formation of a new compound that is insoluble in the solution, this insoluble compound forms a solid called a precipitate. The precipitate then separates from the liquid part of the mixture.

Eg :

(i) CdSO₄(aq) + K₂S(aq) → CdS(s) + K₂SO₄(aq)

(ii) 2NaOH(aq) + MgCl₂(aq) → 2NaCl(aq) + Mg(OH)₂(s)

Q16) Explain the following in terms of gain or loss of oxygen with two examples each.

1. Oxidation
2. Reduction

Ans :

(a) Oxidation is the gain of oxygen. For eg :

(i) CO₂ + H₂ → CO + H₂O

(ii) 2Cu + O₂ → 2CuO

(b) Reduction : addition of hydrogen or removal of oxygen in a chemical reaction.

Eg:

ZnO + C → Zn + CO [ZnO is reduced to Zn].

CuO + H₂ → Cu + H₂O [CuO is reduced to Cu].

Q17) A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Ans : The brown coloured element ‘X ‘ is copper. On heating in air it forms copper oxide, which is black in colour.

2Cu(s) + O₂ → 2CuO

(Brown)     (air)     (Black)

Q18) Why do we apply paint on iron articles?

Ans : Painting iron prevents rusting by creating a barrier that blocks oxygen and moisture, the two key ingredients needed for rust to form.

Q19) Oil and fat containing food items are flushed with nitrogen. Why?

Ans : Nitrogen gas is used to flush food packaging because it's inert, meaning it doesn't readily react with the food. Oxygen, on the other hand, can react with the oils and fats in food, causing them to become rancid (spoiled). By replacing the oxygen with nitrogen, food manufacturers prevent this rancidity and keep the food fresher for longer.

Q20) Explain the following terms with one example each.

1. Corrosion
2. Rancidity

Ans:

(a) Corrosion: Corrosion is the gradual deterioration of a material, typically a metal, due to a chemical reaction with its environment. This environment can include air, moisture, chemicals, or other substances.

For eg:

4Fe + 3O₂ + nH₂O → 2Fe₂O₃.nH₂O (Rusting of Iron)

(b) Rancidity: Rancidity is the spoilage of fats and oils in foods due to oxidation, resulting in unpleasant odors and flavors. This deterioration makes the food unpalatable and often unsafe to eat.

For eg: when a chips packet is exposed to atmospheric air, it results in a change in taste and odor.