NCERT Solutions for Class 10 Science Chapter 1 Exercise

Answer: Magnesium is a very reactive metal. If kept open in the air, it reacts with oxygen to form magnesium oxide (MgO) which sits on the metal surface. Gradually, the magnesium oxide layer envelops the metal and cuts off its contact with atmospheric air. In the absence of oxygen, the burning of magnesium metal cannot happen. Hence, the magnesium ribbon should be cleaned properly before burning in air.

(i) Hydrogen + Chlorine ⟶ Hydrogen chloride

Answer: H₂ + Cl₂ ⟶ 2HCl

(ii) Barium chloride + Aluminium sulphate ⟶ Barium sulphate + Aluminium chloride

Answer: 3BaCl₂ + Al₂(SO₄)₃ ⟶ 3BaSO₄ + 2AlCl₃

(iii) Sodium + Water ⟶ Sodium hydroxide + Hydrogen

Answer: 2Na + 2H₂O ⟶ 2NaOH + H₂

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

Answer: BaCl_{2 (aq)} + Na₂SO_{4 (aq)} ⟶ BaSO_{4 (s)} + 2NaCl _(aq)

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer: NaOH _(aq) + HCl _(aq) ⟶ NaCl _(aq) + H₂O _(l)

(i) Name the substance ‘X’ and write its formula.

Answer: The substance ‘X’ is Calcium Oxide (quick lime). Its chemical formula is CaO.

(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Answer: CaO + H₂O ⟶ Ca(OH)₂

Answer: Water (H₂O) contains two parts of hydrogen and one part of oxygen. In activity 1.7, the electrolysis process separates hydrogen and oxygen gases and sends them into two different test tubes. The process releases two molecules of hydrogen and one molecule of oxygen. Hence, the amount of hydrogen gas collected in one test tube is double the amount of oxygens gas collected in the other.

Answer: When an iron nail is dipped in copper sulphate solution, iron displaces copper from the copper sulphate because iron is more reactive than copper.

Fe + CuSO₄ ⟶ FeSO₄ + Cu

In this displacement reaction, copper sulphate vanishes and iron sulphate is formed. As a result, the blue colour of copper sulphate solution fades and the green colour of iron sulphate emerges.

Answer: Reaction between sodium carbonate and calcium chloride is an example of a double displacement reaction. Both the reactants exchange ions to form calcium carbonate and sodium chloride.

Na₂CO₃ + CaCl₂ ⟶ CaCO₃ + 2NaCl

(i) 4Na _(s) + O_{2 (g)} ⟶ 2Na₂O _(s)

Answer: Sodium (Na) is oxidised as it gains oxygen to form sodium oxide Na₂O and oxygen (O₂) is reduced.

(ii) CuO _(s) + H_{2 (g)} ⟶ Cu _(s) + H₂O _(l)

Answer: Hydrogen (H₂) is oxidised to water (H₂O) and copper oxide (CuO) is reduced to copper (Cu).

Answer: Statements (a) and (b) are incorrect. So, option (i) (a) and (b) is the right answer.

Answer: (d) displacement reaction.

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

Answer: A chemical equation in which the number of atoms of each element in the reactants is the same as the number of atoms of each element in the products, is called a balanced chemical equation.

The law of conservation of mass states that mass is neither created nor destroyed. This is why the total mass of reactants remains equal to the total mass of products in a chemical reaction. Hence, the chemical equations should be balanced to satisfy the law of conservation of mass.

(a) Hydrogen gas combines with nitrogen to form ammonia.

Answer: 3H_{2 (g)} + N_{2 (g)} ⟶ 2NH_{3 (g)}

(b) Hydrogen sulphide gas burns in air to give water and sulpur dioxide.

Answer: 2H₂S _(g) + 3O_{2 (g)} ⟶ 2H₂O _(l) + 2SO_{2 (g)}

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

Answer: 3BaCl_{2 (aq)} + Al₂(SO₄)_{3 (aq)} ⟶ 2AlCl_{3 (aq)} + 3BaSO_{4 (s)}

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer: 2K _(s) + 2H₂O _(l) ⟶ 2KOH _(aq) + H_{2 (g)}

(a) HNO₃ + Ca(OH)₂ ⟶ Ca(NO₃)₂ + H₂O

Answer: 2HNO₃ + Ca(OH)₂ ⟶ Ca(NO₃)₂ + 2H₂O

(b) NaOH + H₂SO₄ ⟶ Na₂SO₄ + H₂O

Answer: 2NaOH + H₂SO₄ ⟶ Na₂SO₄ + 2H₂O

(c) NaCl + AgNO₃ ⟶ AgCl + NaNO₃

Answer: NaCl + AgNO₃ ⟶ AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ ⟶ BaSO₄ + HCl

Answer: BaCl₂ + H₂SO₄ ⟶ BaSO₄ + 2HCl

(a) Calcium hydroxide + Carbon dioxide ⟶ Calcium carbonate + Water

Answer: Ca(OH)₂ + CO₂ ⟶ CaCO₃ + H₂O

(b) Zinc + Silver nitrate ⟶ Zinc nitrate + Silver

Answer: Zn + 2AgNO₃ ⟶ Zn(NO₃)₂ + 2Ag

(c) Aluminium + Copper chloride ⟶ Aluminium chloride + Copper

Answer: 2Al + 3CuCl₂ ⟶ 2AlCl₃ + 3Cu

(d) Barium chloride + Potassium sulphate ⟶ Barium sulphate + Potassium chloride

Answer: BaCl₂ + K₂SO₄ ⟶ BaSO₄ + 2KCl

(a) Potassium bromide (aq) + Barium iodide (aq) ⟶ Potassium iodide (aq) + Barium bromide (s)

Answer: 2KBr _(aq) + BaI_{2 (aq)} ⟶ 2KI _(aq) + BaBr_{2 (s)}
This is a double displacement reaction.

(b) Zinc carbonate (s) ⟶ Zinc oxide (s) + Carbon dioxide (g)

Answer: ZnCO_{3 (s)} ⟶ ZnO _(s) + CO_{2 (g)}
This is a decomposition reaction.

(c) Hydrogen (g) + Chlorine (g) ⟶ Hydrogen chloride (g)

Answer: H_{2 (g)} + Cl_{2 (g)} ⟶ 2HCl _(g)
This is a combination reaction.

(d) Magnesium (s) + Hydrochloric acid (aq) ⟶ Magnesium chloride (aq) + Hydrogen (g)

Answer: Mg _(s) + 2HCl _(aq) ⟶ MgCl_{2 (aq)} + H_{2 (g)}
This is a displacement reaction.

Answer: Exothermic Reaction – An exothermic chemical reaction releases energy in the form of heat, light or sound while forming new products. For example, in sufficient amounts of oxygen, the burning of methane gives a great amount of heat and light while forming carbon dioxide (CO₂) and water (H₂O). Chemical equation –

CH₄ + 2O₂ ⟶⟶ CO₂ + 2H₂O + Heat + Light

Endothermic Reaction – An endothermic chemical reaction absorbs energy from the surroundings to break/form chemical bonds and give off new products. For example, in photosynthesis, plants use sunlight in the presence of carbon dioxide and water to prepare their food in the form of glucose. Oxygen is also released. Chemical equation –

Answer: Respiration – We eat food because we need energy to live and work. The digestion process breaks down complex food substances into simpler absorbable substances such as glucose. Glucose reacts with oxygen in the cells to release energy in the human body. Carbon dioxide and water are also formed in the reaction –

C₆H₁₂O₆ + 6O₂ ⟶⟶ 6CO₂ + 6H₂O + Energy

Since energy is released in the respiration process, it is considered an exothermic reaction.

Answer: A decomposition reaction requires energy to break down a single compound into two or more substances. Whereas a combination reaction combines two or more substances to produce a single new compound with the release of energy. Because of their opposite characteristics, decomposition reactions and combination reactions are called opposite to each other.

Answer: A decomposition reaction requires energy in the form of heat, light or electricity to break down a single compound into two or more substances. See examples below —

(i) Decomposition by heat –
CaCO₃ ⟶ CaO + CO₂

(ii) Decomposition by light –
2AgCl ⟶ 2Ag + Cl₂

(iii) Decomposition by electricity –
2H₂O ⟶ 2H₂ + O₂

Answer: Displacement Reactions – In a displacement reaction, a more reactive element displaces a less reactive element from its compound. Only one displacement takes place in such reactions –

A + BC ⟶ AC + B

Example –
Fe + CuSO₄ ⟶ FeSO₄ + Cu

Double Displacement Reactions – In a double displacement reaction, the two reactants exchange positive and negative ions to form new compounds. Two displacements take place in such reactions –

AB + CD ⟶ AD + CB

Example –
AgNO₃ + NaCl ⟶ AgCl + NaNO₃

Answer: Cu _(s) + 2AgNO_{3 (aq)} ⟶ Cu(NO₃)_{2 (aq)} + 2Ag _(s)

Answer: In a precipitation reaction, two soluble salts in aqueous solution combine to form an insoluble salt which settles down at the bottom of the container. This insoluble solid is called precipitate.

For example, when sodium carbonate solution and calcium chloride solution are mixed together, a white precipitate of calcium carbonate is formed along with sodium chloride –

Na₂CO₃ + CaCl₂ ⟶ CaCO₃ + 2NaCl

(a) Oxidation

Answer: Oxidation is the gain of oxygen. It happens when an element gains oxygen to form its respective oxide. Two examples are –

(i) C + O₂ ⟶ CO₂

(ii) 2Cu + O₂ ⟶ 2CuO

(b) Reduction

Answer: Reduction is the loss of oxygen. It happens when oxygen is removed from a substance in a chemical reaction. Two examples are –

(ii) ZnO + C ⟶ Zn + CO

Answer: The element X is copper (Cu) which reacts with oxygen to form black colored copper oxide (CuO).

Answer: We apply paint on iron articles to prevent them from rusting. Rusting is a type of corrosion process which occurs when iron comes in contact with oxygen (or air) in the presence of water (or moisture). It results in the gradual degradation of iron metal. A layer of paint on the iron surface cuts off the contact of iron from air and moisture. This protects our iron articles from damage.

Answer: Food exposed to air for a long time starts smelling bad and becomes undesirable to eat due to rancidity. Rancidity is the oxidation of fats and oils in the food that causes the food to smell and taste bad. We can avoid rancidity by packing food in oxygen-free bags. So, oil and fat containing food items are flushed with nitrogen which does not react with oil and fat easily.

(a) Corrosion

Answer: Corrosion – In the corrosion process, metals deteriorate when they react with air, moisture or a chemical on their surface and a layer of oxide is formed on the surface.

Example – Rusting of iron is a type of corrosion. When iron is exposed to moist air for a long time, a coating of iron oxide or rust is formed on its surface.

(b) Rancidity

Answer: Rancidity – In the rancidity process, fats and oils in food are oxidised when food is exposed to air for a long time. This causes the food to smell and taste bad and makes them unfit for eating.

Example – Butter goes rancid when it is left exposed to air or kept at room temperature for too long.