Selina Concise Solutions for Chapter 6 Chemical Reactions Class 8 Chemistry ICSE
Exercise I
1. (a) Define a chemical reaction.
(b) What happens during a chemical reaction?
(c) What do you understand by a chemical bond?
Answer
(a) Any chemical change in matter which involves transformation into one or more substances with entirely different properties is called a chemical reaction.
(b) A chemical reaction involves breaking of chemical bonds between the atoms or groups of atoms of reacting substances and rearrangement of atoms making new bonds to form new substances.
(c) A chemical bond is the attractive force that holds the atoms of a molecule together, in a compound.
2. Give one example each of which illustrates the following characteristics of a chemical reaction:
(a) evolution of a gas
(b) change of colour
(c) change in state
Answer
(a) When Zinc reacts with dil. sulphuric acid. Hydrogen gas is evolved, with an effervescence
(c) What do you understand by a chemical bond?
Answer
(a) Any chemical change in matter which involves transformation into one or more substances with entirely different properties is called a chemical reaction.
(b) A chemical reaction involves breaking of chemical bonds between the atoms or groups of atoms of reacting substances and rearrangement of atoms making new bonds to form new substances.
2. Give one example each of which illustrates the following characteristics of a chemical reaction:
(a) evolution of a gas
(b) change of colour
(c) change in state
Answer
(a) When Zinc reacts with dil. sulphuric acid. Hydrogen gas is evolved, with an effervescence
(b) When blue coloured copper sulphate reacts with hydrogen sulphide gas, a black coloured substance copper sulphide is formed.
3. How do the following help in bringing about a chemical change?
(a) pressure
(a) pressure
(b) light
(c) catalyst
(c) catalyst
(d) heat.
Answer
(a) Some chemical reactions take place when reactants are subjected to high pressure.
e.g: Nitrogen and hydrogen when subjected to high pressure produce ammonia gas
(b) Some chemical reactions can take place in the presence of light. Ex. Photosynthesis.
(c) A catalyst can either increases or decreases the rate of chemical reaction and some chemical reactions need a catalyst to change the rate of the reaction, in case it is too slow or too fast.
e.g. When lead nitrate is heated, it breaks into lead monoxide, nitrogen dioxide and oxygen.
4. (a) Define catalyst.
(b) What are (i) positive catalysts and (ii) negative catalysts? Support your answer with one example for each of them.
(c) Name three biochemical catalysts found in the human body.
Answer
(a) 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.
(b) (i) Positive catalyst: When a catalyst increases the rate of chemical reaction, it is called positive catalyst.
e.g. when potassium chlorate heated to 700°C decomposes to evolve oxygen gas, when MnO2 is added the decomposition takes place at 300°C
(ii) Negative catalyst: When a catalyst decreases the rate of chemical reaction it is called negative catalyst.
Example. Phosphoric acid acts as a negative catalyst to decrease the rate of the decomposition of hydrogen peroxide. Alcohol too acts as a negative catalyst in certain chemical reactions.
(c) Biochemical catalysts found in human body:
5. What do you observe when
(a) dilute sulphuric acid is added to granulated zinc?
(b) a few pieces of iron are dropped in a blue solution of copper sulphate?
(c) silver nitrate is added to a solution of sodium chloride?
(d) ferrous sulphate solution is added to an aqueous solution of sodium hydroxide.
(e) solid lead nitrate is heated?
(f) when dilute sulphuric acid is added to barium chloride solution?
Answer
(a) When Zinc reacts with dilute sulphuric acid, hydrogen gas is evolved with an effervesence.
Zn (dil.) + H2SO4 → Zn SO4 + H2.
Answer
(a) Some chemical reactions take place when reactants are subjected to high pressure.
e.g: Nitrogen and hydrogen when subjected to high pressure produce ammonia gas
(b) Some chemical reactions can take place in the presence of light. Ex. Photosynthesis.
(c) A catalyst can either increases or decreases the rate of chemical reaction and some chemical reactions need a catalyst to change the rate of the reaction, in case it is too slow or too fast.
- Positive catalyst: When a catalyst increase the rate of reaction finely divided iron is used as a positive catalyst in the manufacturing of ammonia from hydrogen and nitrogen.
- Negative Catalyst: When a catalyst decreases the rate of reaction.
Ex.ample: Phosphoric acid act as a negative catalyst to decrease the rate of the decomposition of hydrogen peroxide.
e.g. When lead nitrate is heated, it breaks into lead monoxide, nitrogen dioxide and oxygen.
4. (a) Define catalyst.
(b) What are (i) positive catalysts and (ii) negative catalysts? Support your answer with one example for each of them.
(c) Name three biochemical catalysts found in the human body.
Answer
(a) 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.
(b) (i) Positive catalyst: When a catalyst increases the rate of chemical reaction, it is called positive catalyst.
e.g. when potassium chlorate heated to 700°C decomposes to evolve oxygen gas, when MnO2 is added the decomposition takes place at 300°C
Example. Phosphoric acid acts as a negative catalyst to decrease the rate of the decomposition of hydrogen peroxide. Alcohol too acts as a negative catalyst in certain chemical reactions.
(c) Biochemical catalysts found in human body:
- Pepsin
- Trypsin
- lipase.
5. What do you observe when
(a) dilute sulphuric acid is added to granulated zinc?
(b) a few pieces of iron are dropped in a blue solution of copper sulphate?
(c) silver nitrate is added to a solution of sodium chloride?
(d) ferrous sulphate solution is added to an aqueous solution of sodium hydroxide.
(e) solid lead nitrate is heated?
(f) when dilute sulphuric acid is added to barium chloride solution?
Answer
(a) When Zinc reacts with dilute sulphuric acid, hydrogen gas is evolved with an effervesence.
Zn (dil.) + H2SO4 → Zn SO4 + H2.
(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 green.
(c) When a solution of silver nitrate is added to a solution of sodium chloride, white insoluble ppt. of silver chloride is formed.
AgNO3 (aq) + NaCl (aq) → AgCl (ppt) + NaNO3 (aq)
AgNO3 (aq) + NaCl (aq) → AgCl (ppt) + NaNO3 (aq)
(d) When ferrous sulphate solution is added to sodium hydroxide solution, a dirty green ppt. of ferrous hydroxide is formed.
FeSO4 (aq) + 2NaOH (aq) → Fe(OH)2 ↓ + Na2SO4(aq)
FeSO4 (aq) + 2NaOH (aq) → Fe(OH)2 ↓ + Na2SO4(aq)
(e) When solid lead nitrate is heated, it decomposes to produce light yellow solid lead monoxide, reddish brown nitrogen dioxide gas and colourless oxygen gas.
(f) When few drops of dilute sulphuric acid is added to barium chloride solution, a white precipitate of barium sulphate is formed.
(f) When few drops of dilute sulphuric acid is added to barium chloride solution, a white precipitate of barium sulphate is formed.
6.Complete and balance the following chemical equations:
Answer
1. Fill in the blanks.
(a) A reaction in which two or more substances combine to form a single substance is called a reaction.
(b) A is a substance which changes the rate of a chemical reaction without undergoing a chemical change.
(c) The formation of gas bubbles in a liquid during a reaction is called
(d) The reaction between an acid and a base is called .
(e) Soluble bases are called .
(f) The chemical change involving iron and hydrochloric acid illustrates a reaction.
(g) In the type of reaction called , two compounds exchange their positive and negative radicals ions respectively.
(h) A catalyst either or decreases the rate of a chemical change but itself remains unchanged at the end of the reaction.
(i) The chemical reaction between hydrogen and chlorine is a reaction.
(j) When a piece of copper is added to silver nitrate solution, it turns in colour.
(a) N2 + O2 ⟶
(b) H2S + Cl2 ⟶
(c) Na + H2O ⟶
(d) NaCl + AgNO3 ⟶
(e) Zn + H2SO4 (dil) ⟶
(f) FeSO4 (aq.) + NaOH (aq.) ⟶
(g) Pb(NO3)2 (heat) ⟶
(h) BaCl2 (aq.) + H2SO4 (aq.) ⟶
Exercise II
1. Fill in the blanks.
(a) A reaction in which two or more substances combine to form a single substance is called a reaction.
(b) A is a substance which changes the rate of a chemical reaction without undergoing a chemical change.
(c) The formation of gas bubbles in a liquid during a reaction is called
(d) The reaction between an acid and a base is called .
(e) Soluble bases are called .
(f) The chemical change involving iron and hydrochloric acid illustrates a reaction.
(g) In the type of reaction called , two compounds exchange their positive and negative radicals ions respectively.
(h) A catalyst either or decreases the rate of a chemical change but itself remains unchanged at the end of the reaction.
(i) The chemical reaction between hydrogen and chlorine is a reaction.
(j) When a piece of copper is added to silver nitrate solution, it turns in colour.
Answer
(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.
(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 neutralization reaction.
(e) Soluble bases are called alkalis.
(f) The chemical change involving iron and hydrochloric acid illustrates a displacement reaction.
(g) In the type of reaction called double decomposition reaction, ions two compounds exchange their positive and negative radicals ions respectively.
(h) A catalyst either increases or decreases the rate of a chemical change but itself remains unchanged 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.
2. Classify the following reactions as combination, decomposition, displacement, precipitation and neutralization. Also balance the equations.
Answer
3. Define:
(a) precipitation (b) neutralization (c) catalyst
Answer
(a) Precipitation: A chemical reaction in which two compounds in their aqueous state react to form an insoluble salt as one of the product.
Acid + Base → Salt + Water
Example.
(c) Catalyst: A catalyst is a substance that either increases or decreases the rate of a chemical reaction without itself undergoing any chemical change.
here iron act as a catalyst and increases the rate of chemical reaction.
4. Explain the following types of chemical reactions giving two examples for each of them.
(a) combination reaction
(b) decomposition reaction
(c) displacement reaction
(d) double decomposition reaction
Answer
(a) Combination reaction: A reaction in which two or more substances combine to form a single substance is called combination reaction.
A + B → AB
Example:
(b) A catalyst is a substance which changes the rate of a chemical reaction without undergoing a chemical change.
(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 neutralization reaction.
(e) Soluble bases are called alkalis.
(f) The chemical change involving iron and hydrochloric acid illustrates a displacement reaction.
(g) In the type of reaction called double decomposition reaction, ions two compounds exchange their positive and negative radicals ions respectively.
(h) A catalyst either increases or decreases the rate of a chemical change but itself remains unchanged 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.
2. Classify the following reactions as combination, decomposition, displacement, precipitation and neutralization. Also balance the equations.
(a) precipitation (b) neutralization (c) catalyst
Answer
(a) Precipitation: A chemical reaction in which two compounds in their aqueous state react to form an insoluble salt as one of the product.
Acid + Base → Salt + Water
Example.
(b) Neutralization: A chemical reaction in which a base or an alkali reacts, with an acid to produce a salt and water only.
(c) Catalyst: A catalyst is a substance that either increases or decreases the rate of a chemical reaction without itself undergoing any chemical change.
4. Explain the following types of chemical reactions giving two examples for each of them.
(a) combination reaction
(b) decomposition reaction
(c) displacement reaction
(d) double decomposition reaction
Answer
(a) Combination reaction: A reaction in which two or more substances combine to form a single substance is called combination reaction.
A + B → AB
Example:
- When iron and sulphur are heated together, they combine to form iron sulphide.
- When carbon bums in oxygen to form a gaseous compound called carbon dioxide.
(b) Decomposition reaction: A reaction in which a compound breaks up due to the application of heat into two or more simple substances is called decomposition reaction.
- Mercuric oxide when heated, decomposes to form two elements mercury and oxygen.
- CaCO3 when heated decomposes to calcium oxide and carbon dioxide.
(c) Displacement reaction: A reaction in which a more active element displaces a less active element from a compound is called displacement reaction.
AB + C → CB + A
Example:
- Zinc, displaces copper from copper sulphate solution.
Zn + CuSO4 (aq) → ZnSO4 (aq) + Cu - Iron piece when added to copper sulphate solution, copper is displaced.
Fe + CuSO4 → FeSO4 + Cu.
AB + CD → CB + AD
Example:
(i) AgNO3 + HCl → AgCl + HNO3 (aq)
(ii) NaOH (aq) + HCl (aq) → NaCl (aq) + H2O.
5. Write the missing reactants and products and balance the equations.
(a) NaOH + .... → NaCl + ……
(b) KClO3 → ….. + ……
(c) …… + HCl → NaCl + H2O + ……
Answer
6. How will you obtain?
(a) Magnesium oxide from magnesium.
(b) Silver chloride from silver nitrate.
(c) Nitrogen dioxide from lead nitrate.
(d) Zinc chloride from zinc.
(e) Ammonia from nitrogen.
Also give balanced equations for the reactions
Answer
(a) Magnesium when burnt in air (oxygen) Magnesium oxide is formed
(b) When silver nitrate solution reacts with sodium chloride, silver chloride is formed
(c) Lead nitrate when heated nitrogen oxide is obtained
(d) Zinc when reacts with hydrochloric acid zinc chloride and hydrogen (g) is formed.
Answer
6. How will you obtain?
(a) Magnesium oxide from magnesium.
(b) Silver chloride from silver nitrate.
(c) Nitrogen dioxide from lead nitrate.
(d) Zinc chloride from zinc.
(e) Ammonia from nitrogen.
Also give balanced equations for the reactions
Answer
(a) Magnesium when burnt in air (oxygen) Magnesium oxide is formed
(b) When silver nitrate solution reacts with sodium chloride, silver chloride is formed
(c) Lead nitrate when heated nitrogen oxide is obtained
(d) Zinc when reacts with hydrochloric acid zinc chloride and hydrogen (g) is formed.
Zn + 2HCl → ZnCl2 + H2
(e) Nitrogen when reacts with hydrogen at 450°C and under 200 atm, ammonia is formed.
7. What do you observe when
(a) Iron nail is kept in copper sulphate solution for sometime.
(b) Phenolphthalein is added to sodium hydroxide solution.
(c) Blue litmus paper is dipped in dilute hydrochloric acid.
(d) Lead nitrate is heated.
(e) Magnesium ribbon is burnt in oxygen.
(f) Ammonia is brought in contact of hydrogen chloride.gas.
Answer
(a) A brown layer of copper gets deposited on iron nail. This is due to chemical reaction.
Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s)
(b) Solution turns pink.
(c) Blue litmus turns red in an acid solution.
(d) The pale yellow solid is lead monoxide, the reddish brown gas is nitrogen dioxide and the colourless gas is oxygen.
(e) Magnesium ribbon bums 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)
(f) Ammonia and hydrogen chloride, both compounds, combine to form a compound, ammonium chloride.
8. Give reason:
(a) A person suffering from acidity is advised to take an antacid.
(b) Acidic soil is treated with quick lime.
(c) Wasp sting is treated with vinegar.
Answer
(a) An antacid neutralizes stomach acidity.
(b) If the soil is acidic it can be treated with base like quick lime, to make it neutral.
(c) Wasp stings are alkaline and they can be neutralized by vinegar which is a weak acid.
9. What is meant by the metal reactivity series? State its importance, (any two points).
Answer
A list in which the metals are arranged in the decreasing order of their chemical reactivity is called the metal reactivity series.
Special features of the activity series:
10. What are oxides ? Give two examples of each of the following oxides.
(a) Basic oxide (b) Acidic oxide
(c) Amphoteric oxide (d) Neutral oxide
Answer
An oxide is a compound which essentially contains oxygen in its molecule, chemically combined with a metal or a non-metal.
11. Define exothermic and endothermic reactions. Give two examples of each.
Answer
Exothermic reactions: The chemical reaction in which heat is given out is called exothermic reactions. It causes rise in temperature. .
e.g. (i) When carbon bums in oxygen to form carbon dioxide, a lot of heat is produced.
C + O2 → CO2 + heat.
When water is added to quicklime a lot of heat is produced which boils the water.
CaO + H2O → Ca (OH)2 + Heat.
Endothermic reaction: A chemical reaction in which heat is absorbed is called endothermic reaction. It causes fall in temperature.
Example:
(e) Nitrogen when reacts with hydrogen at 450°C and under 200 atm, ammonia is formed.
(a) Iron nail is kept in copper sulphate solution for sometime.
(b) Phenolphthalein is added to sodium hydroxide solution.
(c) Blue litmus paper is dipped in dilute hydrochloric acid.
(d) Lead nitrate is heated.
(e) Magnesium ribbon is burnt in oxygen.
(f) Ammonia is brought in contact of hydrogen chloride.gas.
Answer
(a) A brown layer of copper gets deposited on iron nail. This is due to chemical reaction.
Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s)
(b) Solution turns pink.
(c) Blue litmus turns red in an acid solution.
(d) The pale yellow solid is lead monoxide, the reddish brown gas is nitrogen dioxide and the colourless gas is oxygen.
The reaction can be represented as
2Mg + O2 → 2MgO (white powder)
(f) Ammonia and hydrogen chloride, both compounds, combine to form a compound, ammonium chloride.
8. Give reason:
(a) A person suffering from acidity is advised to take an antacid.
(b) Acidic soil is treated with quick lime.
(c) Wasp sting is treated with vinegar.
Answer
(a) An antacid neutralizes stomach acidity.
(b) If the soil is acidic it can be treated with base like quick lime, to make it neutral.
(c) Wasp stings are alkaline and they can be neutralized by vinegar which is a weak acid.
9. What is meant by the metal reactivity series? State its importance, (any two points).
Answer
A list in which the metals are arranged in the decreasing order of their chemical reactivity is called the metal reactivity series.
- The ease with which a metal in solution loses electron(s) and forms a positive ion decreases down the series, i.e. from potassium to gold.
- Hydrogen is included in the activity series because, like metals do, it too loses an electron and becomes positively charged (H+) in most chemical reactions.
- The series facilitates the comparative study of metals in terms of the degree of their reactivity.
- The compounds of the metals (oxides, carbonates, nitrates and hydroxides) too can be easily compared.
10. What are oxides ? Give two examples of each of the following oxides.
(a) Basic oxide (b) Acidic oxide
(c) Amphoteric oxide (d) Neutral oxide
Answer
An oxide is a compound which essentially contains oxygen in its molecule, chemically combined with a metal or a non-metal.
Following are the examples:
(a) Basic oxides - [Eg. Na2O, CaO, MgO, etc.]
(b) Acidic oxides - [Eg. CO2, SO2, P2O5, etc.]
(c) Amphoteric - [Eg. ZnO, Al2O3, PbO, etc.]
(d) Neutral oxides - [Eg, H2O, NO, CO, etc.]
11. Define exothermic and endothermic reactions. Give two examples of each.
Answer
Exothermic reactions: The chemical reaction in which heat is given out is called exothermic reactions. It causes rise in temperature. .
e.g. (i) When carbon bums in oxygen to form carbon dioxide, a lot of heat is produced.
C + O2 → CO2 + heat.
When water is added to quicklime a lot of heat is produced which boils the water.
CaO + H2O → Ca (OH)2 + Heat.
Endothermic reaction: A chemical reaction in which heat is absorbed is called endothermic reaction. It causes fall in temperature.
Example:
- When nitrogen and oxygen together are heated to a temperature of about 3000°C, nitric oxide gas is formed.
N2 + O2 + heat → 2NO (g) - Decomposition of calcium carbonate into carbon dioxide and calcium oxide when heated to a 1000°C.
CaCO3 + Heat → CaO (s) + CO2 (g)
12. State the effect of:
(a) an endothermic reaction
(b) an exothermic reaction on the surroundings.
Answer
(a) Carbon dioxide present in the atmosphere is trapped by infrared radiations, gives rise to temperature which is exothermic reaction.
(b) The melting of glaciers by global warming.
13. What do you observe when
(a) an acid is added to a basic solution.
(b) ammonium chloride is dissolved in water.
Answer
(a) A chemical reaction in which a base or an alkali reacts with an acid to produce a salt and water.
Acid + Base → Salt + Water
(b) Dissolution of ammonium chloride in water is an endothermic reaction in which heat energy is absorbed.
Additional Questions
Multiple Choice Questions
Tick the most appropriate answer.
1. Which one of the following alters the rate of the chemical reaction without itself undergoing any change?
1. temperature and Pressure
2. concentration of reactants
3. presence of catalyst
4. none of these
1. temperature and Pressure
2. concentration of reactants
3. presence of catalyst
4. none of these
Answer
3. presence of catalyst
2. Heat energy is evolved in these reactions
1. Thermal decomposition reactions
2. Exothermic reactions
3. Endothermic reactions
4. None of these
Answer
2. Exothermic reactions
3. Thermal decomposition of a substance is brought about with the help of
1. reactants
2. water
3. wind
4. heat
Answer
4. heat
4. Reduction reaction is the exact opposite of
1. displacement reaction
2. decomposition reaction
3. oxidation reaction
4. none of these
1. displacement reaction
2. decomposition reaction
3. oxidation reaction
4. none of these
Answer
3. oxidation reaction
5. Which of the following is a reducing agent?
1. oxygen
2. chlorine
3. hydrogen
4. none of these
Answer
3. hydrogen
6. Compounds, which in their molten state allow an electric current to pass through them are called
1. electrolytes2. non-electrolytes
3. non-conductors
4. none of these
Answer
1. electrolytes
7. Reduction is a chemical reaction in which there is
1. removal of hydrogen
2. removal of oxygen
3. addition of oxygen
4. none of these
Answer
2. removal of oxygen
8. Miscible liquids having slightly different boiling points can be separated by the
1. evaporation
2. distillation
3. fractional distillation
4. sedimentation
2. removal of oxygen
8. Miscible liquids having slightly different boiling points can be separated by the
1. evaporation
2. distillation
3. fractional distillation
4. sedimentation
Answer
3. fractional distillation
1. A precipitation reaction takes place only in a solution.
True.
2. Copper can displace iron from iron sulphate.
False.
Copper cannot displace iron from iron sulphate.
3. Ammonia is a good oxidizing agent.
False.
Oxygen is a good reducing agent.
4. Weak electrolytes dissociate partially in their aqueous or fused state.
True.
5. Electrolytic refining is a method in which a thin coating of a metal is done over another metal.
False.
Electroplating is a method in which a thin coating of a metal is done over another metals are refined to purity.
6. Evaporation of water is a physical change. True or false ?
True.
7. The evolution of a gas during a chemical reaction is shown by an arrow pointing downwards. True or false ?
False.
8. Positively charged electrode is called the anode. True or false ?
True.
9. Temperature at which a liquid starts boiling is known as the boiling point of that liquid. True or false ?
True.
1. What are chemical reactions?
Answer
Chemical reactions are defined as changes in which substances undergo an irreversible change, resulting in the formation of a new substances.
Fill in the Blanks
1. A change is a temporary change.
2. In a reaction heat energy is absorbed.
3. In a double decomposition precipitation reaction both the reactants must be soluble.
4. In a reaction, an acid reacts with a base to form salt and water as the only products.
5. Distillation is a method of obtaining pure from a solution.
6. On reaction with sulphuric acid solution, barium chloride forms a white precipitate of .
7. electrolytes dissociate completely in aqueous state and allow large amount of electricity to flow through it.
8. Negatively charged ions are called .
9. In a change, new substances are formed.
10. The new substances formed during a chemical reaction are called the .
11. Positively charged ions are called .
Answer
1. A physical change is a temporary change.
2. In a endothermic reaction heat energy is absorbed.
3. In a double decomposition precipitation reaction both the reactants must be water soluble.
4. In a neutralization reaction, an acid reacts with a base to form salt and water as the only products.
5. Distillation is a method of obtaining pure solvent from a solution.
6. On reaction with sulphuric acid solution, barium chloride forms a white precipitate of barium sulphate.
7. Strong electrolytes dissociate completely in aqueous state and allow large amount of electricity to flow through it.
8. Negatively charged ions are called anions.
9. In a chemical change, new substances are formed.
10. The new substances formed during a chemical reaction are called the products.
11. Positively charged ions are called cations.
Match the following
True/False
1. A precipitation reaction takes place only in a solution.
True.
2. Copper can displace iron from iron sulphate.
False.
Copper cannot displace iron from iron sulphate.
3. Ammonia is a good oxidizing agent.
False.
Oxygen is a good reducing agent.
4. Weak electrolytes dissociate partially in their aqueous or fused state.
True.
5. Electrolytic refining is a method in which a thin coating of a metal is done over another metal.
False.
Electroplating is a method in which a thin coating of a metal is done over another metals are refined to purity.
6. Evaporation of water is a physical change. True or false ?
True.
7. The evolution of a gas during a chemical reaction is shown by an arrow pointing downwards. True or false ?
False.
8. Positively charged electrode is called the anode. True or false ?
True.
9. Temperature at which a liquid starts boiling is known as the boiling point of that liquid. True or false ?
True.
Write Short Answers
1. What are chemical reactions?
Answer
Chemical reactions are defined as changes in which substances undergo an irreversible change, resulting in the formation of a new substances.
2. What are biocatalysts ?
Answer
Biocatalysts are enzymes which help our body to perform various metabolic functions.
Example: amylase, lipase etc.
Answer
Biocatalysts are enzymes which help our body to perform various metabolic functions.
Example: amylase, lipase etc.
3. How is precipitate formation in a chemical reaction indicated?
Answer
A precipitate is an insoluble substance formed when two substances or chemicals react in solution form. The precipitate formed is indicated by writing an arrow pointing downwards (↓) beside the precipitate.
8. Is kerosene an electrolyte or a non-electrolyte ?
Answer
Non-electrolyte.
Answer
A precipitate is an insoluble substance formed when two substances or chemicals react in solution form. The precipitate formed is indicated by writing an arrow pointing downwards (↓) beside the precipitate.
NaCl + AgNO3 → AgCl ↓ + NaNO3
4. What are homogenous reactions ?
Answer
Homogeneous reactions are those in which the reactants and products are in the same physical state and are miscible with each other.
5. Name the type of reaction which is governed by the position of a metal in the metal activity series ?
Answer
Displacement reaction.
6. Define electrolysis.
Answer
Electrolysis is the process of decomposition of an electrolyte, in its aqueous state by passing direct electric current through it.
7. Name three reducing agents ?
Answer
(i) Hydrogen
(ii) Ammonia
(iii) Carbon
8. Is kerosene an electrolyte or a non-electrolyte ?
Answer
Non-electrolyte.
Answer in Detail
1. What are the characteristics of a chemical reaction ? A chemical reaction is characterised by an irreversible change in which a new substance is formed.
Answer
Characteristics of a chemical reaction:
2. Explain the type of reactions on the basis of the direction of reaction.
Answer
On the basis of whether or not a reaction goes to completion, chemical reactions are classified into two categories. These are discussed as under:
Zinc carbonate on heating decomposes to form Zinc oxide and carbon dioxide.
Answer
Characteristics of a chemical reaction:
- Evolution of a gas: A gas may evolve during a chemical reaction. This is shown by writing an arrow pointing upwards (↑) besides the gas. Some gases have characteristic smell also.
e.g. 2Na + 2HCl → 2NaCl + H2 ↑ - Change in colour: Change in colour takes place in some chemical reactions e.g.
- Formation of precipitate: Some chemical reactions involve the formation of a precipitate which is characterised by an arrow pointing downwards (↓) beside the precipitate.
- Change of physical state: In some chemical reactions, a change of state takes place from reactants to products, e.g.
2. Explain the type of reactions on the basis of the direction of reaction.
Answer
On the basis of whether or not a reaction goes to completion, chemical reactions are classified into two categories. These are discussed as under:
- 1. Irreversible reactions: A reaction in which the reactants are completely converted into products is called an irreversible reactions, e.g.
C + O2 → CO2 - 2. Reversible reactions: A reaction which proceeds in both forward and reverse directions is called as a reversible reaction.
e.g. N2 + 3H2 ⇌ 2NH3
3. What are thermal decomposition reactions ? Explain with an example.
Answer
Thermal decomposition reactions are those reactions which break up or decomposes on heating to form many products.
This is represented by a general formula format:
Zinc carbonate on heating decomposes to form Zinc oxide and carbon dioxide.
4. What are chemical combination or synthesis reactions?
Answer
In chemical combination or synthesis reactions, two or more elements or substances combine to form a single product.
For example: H2 + Cl2 ⟶ 2HCl
When hydrogen combine with chlorine in the presence of diffused sunlight, it forms hydrogen chloride.
For example: H2 + Cl2 ⟶ 2HCl
When hydrogen combine with chlorine in the presence of diffused sunlight, it forms hydrogen chloride.
5. What do you mean by redox reaction ? Explain with the help of an example.
Answer
A redox reaction is defined as a reaction in which oxidation and reduction takes place simultaneously. In fact both these processes take place simultaneously.
For example in the reaction of Copper oxide and Hydrogen.
In the above process.
6. Write a short note on the information derived from a balanced chemical equation.
Answer
A chemical equation is a statement that represents a chemical change in terms of symbols and formulae.
A lot of information can be obtained from balanced chemical equation.
7. How is a strong electrolyte different from a weak electrolyte?
Answer
Strong electrolytes are compounds which dissociate completely in their aqueous or fused state. It allows electricity to flow through them. e.g. all strong acids and alkalis. Weak electrolytes are chemical compounds which dissociate partially in their aqueous or fused state. They are not good conductors of electricity, e.g. weak acids like carbonic acid, weak alkalis like ammonium hydroxide.
Answer
A redox reaction is defined as a reaction in which oxidation and reduction takes place simultaneously. In fact both these processes take place simultaneously.
For example in the reaction of Copper oxide and Hydrogen.
- Removal of oxygen from copper oxide to form copper.
- Addition of oxygen to hydrogen to form water.
Answer
A chemical equation is a statement that represents a chemical change in terms of symbols and formulae.
A lot of information can be obtained from balanced chemical equation.
- It emphasizes the fact that matter can neither be destroyed nor be created.
- Simple calculation can be done on the basis of chemical equation.
- Problems based on percentage calculation can be solved.
Answer
Strong electrolytes are compounds which dissociate completely in their aqueous or fused state. It allows electricity to flow through them. e.g. all strong acids and alkalis. Weak electrolytes are chemical compounds which dissociate partially in their aqueous or fused state. They are not good conductors of electricity, e.g. weak acids like carbonic acid, weak alkalis like ammonium hydroxide.