Class 8 SELINA Solutions Chemistry Chapter 6: Chemical Reactions
Chemical Reactions Exercise Ex. I
Solution A 1(i)
Correct option: (d) Evolution of gas
Solution:
In many chemical reactions, one of the products is a gas.
Solution A 1(ii)
Correct option: (c) Oxygen
Solution:
Solution A 1(iii)
Correct option: (b) White
Solution:
When a solution of silver nitrate is added to a solution of sodium chloride, a white insoluble substance (precipitate), silver chloride, is formed.
Solution A 1(iv)
Correct option: (c) Iron
Solution:
Finely divided iron is a positive catalyst in manufacturing ammonia from hydrogen and nitrogen.
Solution A 1(v)
Correct option: (c) Light
Solution:
The process of photosynthesis takes place in the presence of light.
Solution B 1
(a) Chemical reaction: Any chemical change in matter which involves transformation into one or more substances with entirely different properties is called a chemical reaction.
(b) Catalyst: A catalyst is a substance that either increases or decreases the rate of a chemical reaction without itself undergoing any chemical change during the reaction.
(c) Promoter: Substance that improves the efficiency of a catalyst is called a promoter.
Solution B 2
A chemical bond is the attractive force that holds the atoms or ions of a molecule together.
Solution B 3
A chemical reaction involves breaking chemical bonds between the atoms or groups of atoms of reacting substances and the rearrangement of atoms, making new bonds to form new substances with absorption or release of energy normally in the form of heat and light.
Solution B 4
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Reactants |
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Products |
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1 |
The substances that take part in a reaction i.e., which undergo change, are called reactants. |
1 |
The new substances formed in the reaction are called products. |
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2 |
Reactants are written on the left-hand side of the chemical equation. |
2 |
Reactants are written on the right-hand side of the chemical equation. |
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3 |
Example: Sodium hydroxide reacts with dilute hydrochloric acid to form sodium chloride (a salt) and water.
Reactants are sodium hydroxide and hydrogen chloride. |
3 |
Example: Sodium hydroxide reacts with dilute hydrochloric acid to form sodium chloride (a salt) and water.
Products are sodium chloride and water. |
Solution B 5
Amylase, pepsin, lipase maltase, zymase and trypsin are some enzymes which help in the digestion of food.
Solution B 6
Characteristics of chemical reactions:
(i) Evolution of gas: In many chemical reactions, one of the products is a gas.
(ii) Change of colour: Certain chemical reactions are characterised by a change in the colour of the reactants.
(iii)Formation of precipitates: Certain chemical reactions are characterised by the formation of insoluble solid substances called precipitates, which settle down at the bottom of the reaction tube.
(iv) Change of state: In many chemical reactions, a change of state is observed.
(v) Change in energy: During a chemical reaction, change in energy takes place.
(Write any two characteristics)
Solution B 7
Solution C 1
(a) Evolution of gas: In many chemical reactions, one of the products is a gas.
Example:
When zinc reacts with dilute sulphuric acid, hydrogen gas evolves with an effervescence.
Zn + H2SO4 ⟶ ZnSO4 + H2
Effervescence is the formation of gas bubbles in a liquid during a chemical reaction. In the above reaction, one of the reactants is a liquid. In such cases, the gas produced forms bubbles in the liquid.
(b) Change of colour: Certain chemical reactions are characterised by a change in the colour of the reactants.
Example:
When a few pieces of iron are dropped into a blue-coloured copper sulphate solution, the solution's blue colour fades and eventually turns light green.
(c) Change of state: In many chemical reactions, a change of state is observed.
For example, the reaction might start with solid or liquid reactants and end with gaseous products, or vice versa.
The physical states in the reaction can be represented by (s) for the solid state, (l) for the liquid state, (g) for the gaseous state, and (aq.) for the aqueous solution.
Example:
The reaction between hydrogen sulphide and chlorine (both gases) produces Sulphur (solid) and hydrogen chloride (gas).
Solution C 2
Positive Catalyst:
When a catalyst increases the rate of a chemical reaction, it is known as a positive catalyst.
Example:
On being heated to 700 °C, potassium chlorate decomposes to evolve oxygen gas. But when manganese dioxide is mixed with it (in the ratio 1:4), the decomposition takes place at a much lower temperature, i.e. at about 300 °C. In this reaction, manganese dioxide acts as a positive catalyst and remains unaffected.
Negative catalyst:
When a substance decreases the rate of a chemical reaction. It is known as a negative catalyst or inhibitor.
Example:
Phosphoric acid acts as a negative catalyst to decrease the rate of decomposition of hydrogen peroxide. Alcohol too acts as a negative catalyst in certain chemical reactions.
Solution C 3(a)
When zinc reacts with dilute sulphuric acid, hydrogen gas is evolved with an effervescence.
Solution C 3(b)
When a few pieces of iron are dropped into a blue coloured copper sulphate solution, the blue colour of the solution fades and eventually turns into light green.
Solution C 3(c)
When a solution of silver nitrate is added to a solution of sodium chloride, a white insoluble substance (precipitate), silver chloride, is formed.
Solution C 3(d)
When ferrous sulphate solution is added to sodium hydroxide solution, a dirty green precipitate of ferrous hydroxide is formed.
Solution C 3(e)
Lead nitrate is a white, crystalline solid. When heated strongly, it decomposes to produce light yellow solid lead monoxide, reddish brown nitrogen dioxide, and colourless oxygen gas.
Solution C 3(f)
When few drops of dilute sulphuric acid are added to an aqueous solution of barium chloride, a white precipitate of barium sulphate is formed.
Solution C 4(a)
Water decomposes into hydrogen and oxygen (gases) when an electric current is passed through it.
Solution C 4(b)
If iron powder and sulphur powder are mixed, they do not react.
However, when heated, they react to form iron sulphide, a new substance,
Solution C 4(c)
When zinc reacts with dilute hydrochloric acid, hydrogen gas evolves with an effervescence,
Solution C 4(d)
When heated to 700 °C, potassium chlorate decomposes to evolve oxygen gas. However, when mixed with manganese dioxide (in the ratio 1/4), the decomposition occurs at a much lower temperature, about 300 °C. In this reaction, manganese dioxide acts as a positive catalyst and remains unaffected.
Chemical Reactions Exercise Ex. II
Solution A 1(i)
Correct option: (a) Combination reaction
Solution:
Metal such as magnesium ribbon when burnt in air reacts with atmospheric oxygen to form magnesium oxide which is a combination reaction.
2Mg + O2 ⟶ 2MgO
Solution A 1(ii)
Correct option: (d) Sodium hydroxide reacts with dilute H2SO4
Solution:
Reaction between a base such as sodium hydroxide and and acid such as sulphuric acid (H2SO4) to form salt and water is termed as neutralisation reaction.
Solution A 1(iii)
Correct option: (c) NO
Solution:
Acidic oxides such as nitric oxide (NO) forms acids in water. Sodium hydroxide is an alkali which reacts with acid to form salt and water.
Solution A 1(iv)
Correct option: (b) NO2
Solution:
Lead nitrate on heating produces nitrogen dioxide gas that is acidic in nature.
Solution A 1(v)
Correct option: (b) hydrogen
Solution:
Fe + dil. H2SO4 ⟶ FeSO4 + H2á
Solution A 1(vi)
Correct option: (b) copper is more reactive than silver
Solution:
The more reactive metals can displace less reactive metals from their salt solution.
2AgNO3 + Cu ⟶ 2Ag + Cu(NO3)2
Solution A 1(vii)
Correct option: (a) 
Solution:
Heat is absorbed during the endothermic reaction.
Solution A 2
(a) A reaction in which two or more substances combine to form a single substance is called a combination reaction.
(b) A catalyst is a substance, which changes the rate of a chemical reaction without undergoing a chemical change itself.
(c) The formation of gas bubbles in a liquid during a reaction is called effervescence.
(d) The reaction between an acid and a base is called neutralisation.
(e) Soluble bases are called alkali.
(f) The chemical change involving iron and hydrochloric acid illustrates a displacement reaction.
(g) In the type of reaction called double displacement two compounds exchange their positive and negative radicals respectively.
(h) A catalyst either increases or decreases the rate of a chemical change but itself remains unreacted at the end of the reaction.
(i) The chemical reaction between hydrogen and chlorine is a combination reaction.
(j) When a piece of copper is added to silver nitrate solution, it turns blue in colour.
Solution A 3
(a) True: When iron nails are put into a solution of copper sulphate, the solution becomes colourless after some time.
(b) True: Burning fuel in air produces a large amount of energy in the form of heat and light.
(c) False: Calcium oxide reacts with water to form a single product, calcium hydroxide; hence, it is an example of a combination reaction.
(d) True: A neutral oxide does not change the colour of an indicator.
(e) False: A yellow solid of lead monoxide is produced when lead nitrate is heated.
Solution A 4
| (a) | It changes the rate of a chemical reaction | (iii) | Catalyst |
| (b) | Attractive force between the atoms to form molecules | (iv) | Chemical bond |
| (c) | Oxides of non- metals | (v) | Acidic oxides |
| (d) | Lead monoxide | (ii) | Amphoteric oxide |
| (e) | Light energy | (i) | Photosynthesis |
Solution A 5
(a) Products
(b) Precipitate
(c) Oxides
(d) Exothermic reaction
(e) Decomposition reaction
Solution A 6
(a)ZnO
(b) CO
(c) H2O
(d) CaO
(e) Na2O
Solution A 7
(a) Decomposition
(b) Displacement
(c) Double displacement
(d) Combination
(e) Precipitation
(f) Displacement
(g) Combination
(h) Neutralisation
(i) Neutralisation
Solution A 8
(a) NaOH + HCl ⟶ NaCl + H2O
(b) 
(c) 
Solution B 1
(a) An endothermic reaction: It causes a fall in temperature.
(b) An exothermic reaction: It causes a rise in temperature.
Solution B 2
(a) When an acid is added to a basic solution, it forms a salt and water. This reaction is called neutralisation reaction.
Acid + Base/Alkali ⟶ Salt + Water
(b) When ammonium chloride is dissolved in water, the beaker becomes colder than it was earlier. This is because dissolution of ammonium chloride is an endothermic reaction in which heat energy is absorbed.
Solution B 3
(a) Precipitation: A chemical reaction in which two compounds in their aqueous solution state react to form an insoluble solid (a precipitate) as one of the products is known as a precipitation reaction.
(b) Neutralisation: A chemical reaction in which a base or an alkali reacts with an acid to produce only salt and water is known as a neutralisation reaction.
(c) Catalyst: A catalyst is a substance that either increases or decreases the rate of a chemical reaction without itself undergoing any chemical change during the reaction.
(d) Alkali: The basic oxides that dissolve in water to produce soluble bases known as alkalis.
Solution B 4
(a) When an iron nail is placed in blue-coloured copper sulphate solution, a reddish brown coating is seen on the iron nail, and the colour of the solution changes gradually from blue to light green.
This is because iron, which is more reactive than copper, displaces copper from the solution, and copper is deposited on an iron nail.
Fe + CuSO4 ⟶ FeSO4 + Cu↓
(b) When phenolphthalein is added to sodium hydroxide solution, the colour of the solution becomes pink since phenolphthalein is pink in colour in alkaline solution.
(c) Blue litmus turns red in an acidic solution such as dilute hydrochloric acid solution.
(d) When lead nitrate is heated, it decomposes to form pale yellow coloured lead monoxide solid, the reddish brown nitrogen dioxide gas, and the colourless oxygen gas.
(e) Magnesium ribbon burns with a dazzling white light and produces a white magnesium oxide powder.
(f) The reaction between ammonia and hydrogen chloride gases produces ammonium chloride, which is a white solid.
NH3 (g) + HCl (g) ⟶ NH4Cl (s)
Solution B 5
(a) The human stomach secretes hydrochloric acid, which helps in food digestion and kills the germs that enter the stomach alongside the food. But if the acid is secreted in more amounts, it can cause acidity and indigestion, which happens when we overeat or do not eat for a long time. Acidity and indigestion can be overcome by taking antacids like magnesia milk [Mg(OH)2] because the extra acid is neutralised by an alkaline solution such as milk of magnesia.
(b) Most plants grow well in neutral soil. If the soil is acidic, it can be treated with bases like quick lime, slaked lime, or chalk to make it neutral. Similarly, basic soils are neutralised by adding sulphate salt or organic matter.
(c) Wasp stings are alkaline. They can be neutralised by vinegar, which is a weak acid. Lemon juice can also be used.
Solution C 1(a)
Combination reaction: A reaction in which two or more substances combine to form a single substance is called a combination reaction. This type of reaction is also called a synthesis reaction.
A + B ⟶ AB
In the above reaction, the combination of substances A and B takes place to give a molecule of a new substance AB.
In combination reactions,
(i) two elements combine to form a compound.
Example:
When iron and sulphur (both elements) are heated together, they combine to form a compound, iron sulphide.
(ii) an element and a compound can combine to give one product.
Example:
Carbon monoxide, a compound, burns in the presence of oxygen, an element, to form a single product, carbon dioxide.
(iii) two or more compounds can combine to form a single product.
Example:
Ammonia and hydrogen chloride, both compounds, combine to form a compound, ammonium chloride.
NH3 (g) + HCl (g) ⟶ NH4Cl (s)
Solution C 1(b)
Decomposition reaction:
A reaction in which a compound breaks up on heating into two or more simpler substances is called decomposition reaction.
Here, the decomposition of the molecule of a compound AB takes place to give two new substances, A and B.
Since, most of the decomposition reactions are carried out by heating, they are called ‘thermal decomposition reactions’.
Due to decomposition:
(i) a compound can break up to form two or more elements.
Example:
The compound mercuric oxide, when heated, decomposes to from two elements, mercury and oxygen.
(ii) a compound can break up to form both elements and compounds.
Example:
The compound potassium nitrate decomposes on heating to produce a compound, potassium nitrite and an element, oxygen.
(iii) a compound can break up to form two or more new simpler compounds.
Example:
The compound calcium carbonate (limestone) decomposes on strong heating to form two compounds, calcium oxide (quicklime) and carbon dioxide.
Solution C 1(c)
Displacement reaction:
A reaction in which a more reactive element displaces a less active element from a compound is called displacement reaction.
In such reactions, one constituent of the reactant molecule is replaced by another. For example,
AB + C ⟶ CB + A
Here, C displaces A from the molecule AB, since C is chemically more reactive as compared to A.
In displacement reaction:
(i) A more reactive metal displaces a less reactive metal from its salt solution.
Example:
Zinc being more reactive than copper, displaces copper from copper sulphate solution. The blue copper sulphate solution turns colourless due to the formation of zinc sulphate and a reddish brown copper is deposited on zinc.
(ii) A metal more reactive than hydrogen displaces hydrogen gas from an acid.
Example: Zinc, being an active metal, displaces hydrogen from dilute hydrochloric (or dilute sulphuric) acid.
Zn + 2HCl (dil.) ZnCl2 + H2
Solution C 1(d)
Double displacement reaction:
A chemical reaction in which two compounds in their aqueous state exchange their ions or radicals to form new compounds is called a double decomposition reaction or a double displacement reaction.
Example:
AB + CD ⟶ CB + AD
Here, AB and CD are the two reactants, molecules. They exchange their ions (or radicals) to form two new molecules, CB and AD. Double displacement reactions are of two types:
i. Precipitation reaction
ii. Neutralisation reaction
i. Precipitation reaction:
A chemical reaction in which two compounds in their aqueous solution state react to form an insoluble solid (a precipitate) as one of the products is known as a precipitation reaction.
Example:
When barium chloride solution reacts with sodium sulphate solution, a white precipitate of barium sulphate and a soluble salt, sodium chloride, are obtained.
BaCl2 (aq) + Na2SO4 (aq) ⟶ BaSO4 ↓ + 2NaCl (aq)
White ppt.
ii. Neutralisation reaction:
A chemical reaction in which a base or an alkali reacts with an acid to produce a salt and water only is known as a neutralisation reaction.
Acid + Base/Alkali ⟶ Salt + Water
Example:
When an alkali, sodium hydroxide, reacts with hydrochloric acid, it forms a salt, sodium chloride, and water.
NaOH (aq) + HCl (aq) ⟶ NaCl (aq) + H2O
Solution C 2
(a) Basic oxide:
Most of the metallic oxides are basic in nature hence called as basic oxide.
Some of these basic oxides dissolve in water to produce soluble bases known as alkalis.
Na₂O + H₂O → 2NaOH
Sodium hydroxide is a strong alkali. They change the colour of indicators. In an alkaline solution, red litmus turns blue.
All alkalis are bases but all bases are not alkalis because all bases are not soluble in water.
(b) Acidic oxide
These non-metallic oxides, being acidic in nature, turn moist litmas paper red. They dissolve in water to produce acidic solutions. Examples Carbon dioxide, sulphur dioxide, sulphur trioxide. phosphorus pentoxide, nitrogen dioxide, etc.
CO2 + H2O ⟶ H2CO3
SO2 + H2O ⟶ H2SO3
(c) Amphoteric oxide
These metallic oxides show both acidic and basic properties. They react with acids to produce salt and water showing basic properties, while they react with bases to produce salt and water showing acidic properties. Examples: zinc oxide, lead monoxide, aluminium oxide, etc.
ZnO + 2HCl ⟶ ZnCl2 + H₂O
ZnO + 2NaOH ⟶ Na2ZnO2 + H2O
(d) Neutral oxide
These non-metallic oxides are neutral in nature. They do not change the colour of indicators. They do not reасt with acids and bases to produce salt and water.
Example: Carbon monoxide (CO), nitric oxide (NO), nitrous oxide NO2, H2O, etc.
Solution C 3
Exothermic Reaction:
A chemical reaction in which heat (a form of energy) is given out is called an exothermic reaction. It causes a rise in temperature,
Examples:
(a) When carbon burns in oxygen to form carbon dioxide, a lot of heat is produced.
C + O2 ⟶ CO2
(b) When water is added to quicklime, a lot of heat energy is produced [along with alkaline calcium hydroxide (slaked lime)], which boils the water.
CaO + H₂O → Ca(OH)2 + Heat
Respiration, rusting, burning of fuels like coal, petrol, kerosene, etc. are some common exothermic reactions. All neutralization reactions are exothermic in nature.
Endothermic Reaction:
A chemical reaction in which heat is absorbed is called an endothermic reaction. It causes a fall in temperature.
Example:
When nitrogen and oxygen together are heated to a temperature of about 3000°C, nitric oxide gas is formed. This is an endothermic reaction.
Solution C 4
(a) Take a magnesium ribbon and hold it with a pair of tongs and heat it in the air. Now, plunge it into a gas jar containing oxygen gas. You will observe that magnesium ribbon burns with a dazzling white light and produces a white powder which is magnesium oxide.
The reaction can be represented as
2Mg + O2 ⟶ 2MgO (white powder)
(b) When a solution of silver nitrate is added to a solution of sodium chloride, a white insoluble substance (precipitate), silver chloride, is formed.
AgNO3 (aq) + NaCl (aq) ⟶ AgCl↓ + NaNO3 (aq)
(c) Lead nitrate on heating produces lead monoxide.
(d) When zinc reacts with dilute hydrochloric acid, hydrogen gas is evolved with an effervescence.
Zn + HCl ⟶ ZnCl2 + H2á
(e) The reaction between nitrogen and hydrogen at high pressure (200 to 900 atm) and high temperature (450 °C) produces ammonia gas in the presence of the iron catalyst and molybdenum as the promoter.
Solution C 5
(a) A list in which the metals are arranged in decreasing order of their chemical reactivity is called the metal reactivity series.
Among the commonly known metals, the most active metal (potassium) is at the top of the list, and the least active metal (platinum) is at the bottom.
The metals have been arranged in order of their chemical reactivity based on the rate of their reaction with oxygen (air), water, and dilute acids.
(b)
(i) Metal P is more reactive than silver.
(ii) Displacement reaction
(iii) Iron displaces copper from blue copper sulphate solution to form a green iron sulphate solution since iron is more reactive than copper.
Fe + CuSO4 ⟶ FeSO4 + Cu↓
Solution C 6
(a) Na (Sodium) can displace all the metals from their salt solution since it is the most reactive metal in the list.
(b) Ag (Silver) cannot react with dilute acid to produce hydrogen gas as it is a noble metal and does not react with acids.
(c) Zn (Zinc) reacts with oxygen upon heating to produce an amphoteric oxide, zinc oxide (ZnO).
(d) The decreasing order of reactivity is:
Na > Zn > Fe > Pb > Cu > Hg > Ag
Solution C 7
(a) Compound X is white ring of ammonium chloride formed due to the combination of ammonia and hydrogen chloride gases.
(b) NH3 (g) + HCl (g) ⟶ NH4Cl (s)
(c) Combination reaction.