NCERT Solutions for Class 10 Science chapter 1 – Chemical Reactions and Equations

Magnesium is a reactive metal. It will react with oxygen and form a white layer of magnesium oxide (MgO) on its surface. Magnesium oxide is a stable compound and will prevent the further reaction of the metal. So, a magnesium ribbon is cleaned before burning in air to remove the layer of magnesium oxide from its surface.

i) Hydrogen + Chlorine → Hydrogen chloride

A balanced equation consists of the same number of moles on the reactants and the products side. The balanced equation for the given reaction is:

H2(g)+Cl2(g)→2HCl(g)

ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride

A balanced equation consists of the same number of moles on the reactants and the products side. The balanced equation for the given reaction is:

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

iii) Sodium + Water → Sodium hydroxide + Hydrogen 

A balanced equation consists of the same number of moles on the reactants and the products side. The balanced equations for the given reaction is:

2Na(s)+2H2O(l)→2NaOH(aq)+H2(g)

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

A balanced chemical equation has the same number of moles of reactants and products. The balanced chemical equation is:

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

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

A balanced chemical equation has the same number of moles of reactants and products. The balanced chemical equation is:

NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l) 

i) Name the substance 'X' and write its formula.

Substance ‘X’ is calcium oxide also known as quicklime that is used in white washing. Its chemical formula is CaO.

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

Calcium oxide reacts with water in a combination reaction to from calcium hydroxide also called slaked lime, the reaction is:

CaO(s)+H2O(l)→Ca(OH)2(aq) 

Water consists of hydrogen and oxygen atoms. It is a combination of two hydrogen and one oxygen atom. In electrolysis, hydrogen moves towards the cathode and oxygen towards the anode. So, the ratio of hydrogen and oxygen is 2:1. And hydrogen is double the amount of oxygen. 

2H2O→2H2+O2

A double displacement reaction consists of two ions replacing each other’s position from the reactants to form new compounds in the products. A double displacement reaction occurs when Potassium iodide reacts with lead nitrate to form lead iodide (yellow precipitate) and potassium nitrate. 

2KI+Pb(NO3)2→PbI2↓+2KNO3

i) 4Na(s)+O2(g)→2Na2O(s) 

Any species is said to be oxidised when it loses electrons and increases the oxidation state, while any species is said to be reduced when it gains electrons and decreases the oxidation state. Sodium (Na) is oxidised and oxygen gets reduced.

4Na+O2→2Na2O

ii) CuO(s)+H2(g)→Cu(s)+H2O(l) 

Any species is said to be oxidized when it loses electrons and increases the oxidation state, while any species is said to be reduced when it gains electrons and decreases the oxidation state. Copper oxide (CuO) is reduced to copper (Cu) as it loses oxygen and hydrogen(H2) is oxidized to water(H2O).

CuO+H2→ΔCu+H2O

When an iron nail is dipped in a solution of copper sulphate, then the more reactive metal displaces the less reactive metal. As a result the blue solution of copper sulphate gets faded and forms a green compound known as ferrous sulphate.

Incorrect option is (i) (a) and (b)

Explanation:

(a) Oxygen is being removed

(b) The Oxygen removed from lead is transferred to the elemental Carbon.

(d) The given reaction is an example of a single displacement reaction.

Explanation: The oxygen from ferrous oxide is transferred to aluminum metal, forming aluminum oxide. In this reaction, aluminum, being more reactive than iron, displaces iron from its oxide. This process is known as a displacement reaction, where a more reactive element replaces a less reactive one.

(a) Hydrogen gas and iron chloride are produced. The reaction is as follows:

Fe(s)+2HCl(aq)→FeCl2(aq)+H2↑

A balanced chemical equation consists of the equal number of moles of the elements in the reactants and that in the products.

Chemical equations should be balanced, as according to the law of conservation of mass, mass can neither be created nor can be destroyed. Therefore, in a chemical reaction, the total mass of reactant should be equal to the total mass of products. Hence, a balanced chemical equation has total mass of reactants equal to total mass of products.

a) Hydrogen gas combines with nitrogen to form ammonia.

3H2(g)+N2(g)→2NH3(g)

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

2H2S(g)+3O2(g)→2H2O(l)+2SO2(g)

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

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

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

2K(s)+2H2O(l)→2KOH(aq)+H2(g)

a) HNO3+Ca(OH)2→Ca(NO3)2+H2O 

    2HNO3+Ca(OH)2→Ca(NO3)2+2H2O

b) NaOH+H2SO4→Na2SO4+H2O 

    2NaOH+H2SO4→Na2SO4+2H2O

c) NaCl+AgNO3→AgCl+NaNO3 

    NaCl+AgNO3→AgCl+NaNO3

d) BaCl2+H2SO4→BaSO4+HCl 

    BaCl2+H2SO4→BaSO4+2HCl

a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

    CaOH(2)+CO2 → CaCo(3) + H2O

b) Zinc + Silver nitrate →Zinc nitrate +Silver

    Zn+2AgNO3→Zn(NO3)2+2Ag

c) Aluminium + Copper chloride →Aluminium chloride + Copper

    2Al+3CuCl2→2AlCl3+3Cu

d) Barium chloride + Potassium sulphate →Barium sulphate + Potassium chloride

    BaCl2+K2SO4→BaSO4+2KCl 

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

    2KBr(aq)+BaI2(aq)→2KI(aq)+BaBr2(aq) ; Double displacement reaction

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

    ZnCO3(s)→ZnO(s)+CO2(g) ; Decomposition reaction

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

    H2(g)+Cl2(g)→2HCl(g) ; Combination reaction

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

    Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) ; Single displacement reaction

Exothermic Reaction: The reactions in which heat is evolved along with the formation of new products are called exothermic reactions. In these reactions the energy of the reactants is more than that of the products, so energy is released to complete the reaction.

Energy of reactants > Energy of products

Example: Complete combustion of methane gas produces carbon dioxide and water along with heat and light.

CH4( g)+2O2( g)→CO2+2H2O+heat+light

Another example of exothermic reactions is respiration and decomposition of vegetable matter.

Endothermic Reaction: The reactions in which energy is absorbed by the reactants to carry on the reaction are called endothermic reactions. In this type of reaction, the energy of the reactants is less than that of the products, so energy is needed and absorbed.

Energy of reactants <Energy of products

Example: Process of Photosynthesis, where plants absorb sunlight in the presence of carbon dioxide and water and make their food in the form of glucose and release oxygen.

6CO2(g)+6H2O(l)→sunlightC6H12O4(aq) +6O2(g)

Exothermic reactions are the reactions in which heat is going to be released. These reactions result in the formation of energy as the energy of reactants is greater than that of products. Respiration is the process in which the glucose from our body combines with oxygen in the cells to provide us with energy. The glucose is broken down through the process of digestion that along with oxygen provides us with energy and hence respiration is an exothermic reaction. The reaction that happens is as follows:

C6H12O6(aq)+6O2(g)→6CO2(g)+6H2O(l)+energy

A decomposition reaction consists of a single reactant that breaks down into two or more simpler products.

Example: Decomposition of calcium carbonate to give calcium oxide and carbon dioxide as, 

CaCO3+ Energy →CaO+CO2

A combination reaction consists of two reactants that combine or mix together to form a single product.

Example: Combination of Calcium oxide and carbon dioxide to form one single product calcium carbonate

CaO+CO2→CaCO3+ energy 

Hence, decomposition reactions are the opposite of combination reactions.

Decomposition reactions are the reactions where a reactant breaks down into two or more products.

a) Decomposition by heat:

Ferrous sulphate decomposes to give ferrous oxide, sulphur dioxide and sulphur trioxide as,

2FeSO4(s)→ΔFe2O3(s)+SO2(g)+SO3(g)

b) Decomposition by light:

Silver chloride decomposes in light to form silver and chlorine as,

2AgCl(s)→Light2Ag(s)+Cl2(g)

c) Decomposition by electricity:

Water decomposes in presence of electricity to form hydrogen and oxygen gases as,

2H2O→Electricity2H2+O2

A displacement reaction is a reaction in which a more reactive element displaces a less reactive element from its compound. The elements at the top in the activity series can replace the elements at the bottom. 

Example: Zinc is more reactive than copper and replaces copper from copper sulphate as:

CuSO4(aq)+Zn(s)→ZnSO4(aq)+Cu(s)

Double displacement reaction is the type of reaction where two compounds react in the way that there is exchange of positive and negative ions and new compounds are formed as products.

Example: On mixing sodium sulphate solution with barium chloride solution, a white precipitate of barium sulphate is formed as:

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

The equation of recovery of silver from silver nitrate solution in silver refining is as follows:

2AgNO3(aq)+Cu(s)→Cu(NO3)2(aq)+2Ag(s)

Here displacement of silver by copper happens as copper is more reactive than silver.

The type of reaction where an insoluble substance called a precipitate is formed when there is exchange of ions between the reactants is called a precipitation reaction.

Example 1: On mixing sodium carbonate solution with calcium chloride solution, a white precipitate of calcium carbonate is formed.

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

Example 2: On mixing sodium sulphate solution with barium chloride solution, a white precipitate of barium sulphate is formed.

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

a) Oxidation

Ans: Oxidation:  A type of reaction in which oxygen or an electronegative species is added to a substance or there is removal of hydrogen or a positive species from a substance is called oxidation.

For Example:

2Cu+O2→2CuO (Addition of oxygen to Cu)

b) Reduction 

Ans: Reduction: A type of reaction in which hydrogen or an electropositive species is added to a substance or oxygen or an electronegative species is removed, is called reduction.

For Example:

CuO+H2→ΔCu+H2O (Oxygen is removed from CuO)

Copper (Cu) is ‘X’ and the black colour compound is copper (II) oxide (CuO). Copper reacts with oxygen to form a black colour compound called copper (II) oxide as: 

2Cu+O2→2CuO

Iron is a reactive metal and can react with moisture and air. Iron articles, if exposed for a long time in moisture or air, can get corroded and form rust. So, paint is applied on iron articles to prevent rusting and form a protective layer for exposure to air and moisture.

Oils and fat containing items are perishable and can be spoiled when exposed to oxygen. This is because oil and fats are reactive and can react with oxygen easily and get oxidised. To prevent oxidation, these items are flushed with nitrogen gas. Nitrogen is an inert gas and cannot react with oils or fats easily. So, food items with oils and fats are kept in packets having nitrogen gas that increases the shelf life and makes these items last for a long time.

a) Corrosion

Ans: Corrosion: Corrosion is defined as a process where a substance, generally metal deteriorates and forms a layer of oxide on its surface. Metals get converted into their hydrated oxides or sulphides. Ex; Iron, Cu, Ag

b) Rancidity

Ans: Rancidity: It is a process in which food items like fats and oils are oxidised. This results in the change in taste and smell of the food item and is called rancidity. For example, when any fried food is exposed to air for a long time then it has a change in smell and taste and it becomes rancid.

Rancidity can be avoided by:

• Adding antioxidants like BHA (Butylated tri hydroxy anisole)

• Refrigerate the food items

• keep food items in airtight containers

• Adding nitrogen to food packets to prevent oxidation.