Living Science Chemistry Class 7 ICSE Solutions Chapter 5 Acids, Bases and Salts

ICSE Living Science Chemistry for Class 7 Solutions Chapter 5 Acids, Bases and Salts

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Answer these questions.
Question 1.
Which acid is produced in the stomach ?
Answer:
Hydrochloric acid

Question 2.
What is aqua regia ?
Answer:
Aqua regia is a mixture containing 1 part concentrated nitric acid and 3 parts concentrated hydrochloric acid by volume.

Question 3.
What happens when an acid reacts with a base ?
Answer:
When an acid reacts with a base, it forms salt and water. This reaction is called a neutralization reaction.
acid + base → salt + water

Question 4.
Which one is an alkali-sodium hydroxide or copper (II) hydroxide ?
Answer:
Sodium hydroxide

A. Tick the most appropriate answer.

Question 1.
The gas evolved when an active metal reacts with an acid is
a. hydrogen
b. oxygen
c. nitrogen
d. carbon dioxide
Answer:
a. hydrogen

Question 2.
Which of these is a weak acid ?
a. Formic acid
b. Sulphuric acid
c. Nitric acid
d. Hydrochloric acid
Answer:
a. Formic acid

Question 3.
Which of the following hydroxides is not an alkali ?
a. Ammonium hydroxide
b. Calcium hydroxide
c. Copper hydroxide
d. Sodium hydroxide
Answer:
c. Copper hydroxide

Question 4.
Which of the following is used as an antacid ?
a. Mg(OH)₂
b. NH4OH
c. KOH
d. NaOH
Answer:
a. Mg(OH)₂

Question 5.
Which of the following is not an indicator ?
a. Methyl orange
b. Phenolphthalein
c. Blue litmus
d. Iron (II) hydroxide
Answer:
d. Iron (II) hydroxide

Question 6.
Which of the following is a natural indicator ?
a. Blue vitriol
b. Baking soda
c. Washing soda
d. Turmeric
Answer:
d. Turmeric

Question 7.
A basic solution has a pH
a. < 7
b. > 7
c. = 7
d. None of these
Answer:
b. > 7

Question 8.
Which compound is used in the paper industry for bleaching wood pulp ?
a. Bleaching powder
b. Washing soda
c. Sodium hydroxide
d. Sodium chloride
Answer:
a. Bleaching powder

B. Fill in the blanks.

Question 1.
An acid is defined as a compound which, when dissolved in water, gives ion as the only positively charged
ion.
Answer:
An acid is defined as a compound which, when dissolved in water, gives hydronium ion as the only positively charged ion.

Question 2.
When acids react with metal carbonates, gas is evolved.
Answer:
When acids react with metal carbonates, carbon dioxide gas , is evolved.

Question 3.
The reaction between an acid and a base is called
Answer:
The reaction between an acid and a base is called neutralization reaction.

Question 4.
A/An salt is the one in which the hydrogen of an acid has been partially replaced by a metal.
Answer:
An acid salt is the one in which the hydrogen of an acid has been partially replaced by a metal.

Question 5.
An alkali is a compound which, when dissolved in water, yields ion as the only negatively charged ion.
Answer:
An alkali is a compound which, when dissolved in water, yields hydroxyl ion as the only negatively charged ion.

Question 6.
Bases give colour with phenolphthalein.
Answer:
Bases give pink colour with phenolphthalein.

Question 7.
An is a substance that changes its colour in acidic and basic solutions. .
Answer:
An acid-base indicator is a substance that changes its colour in acidic and basic solutions.

C. State if the following statements are true or false. Correct the statement if it is false.

Question 1.
Aqua regia is a mixture of sulphuric acid and nitric acid.
Answer:
False. Aqua regia is a mixture of hydrochloric acid and nitric acid.

Question 2.
Aluminium hydroxide is an alkali.
Answer:
False. Ammonium hydroxide is an alkali.

Question 3.
Copper(II) oxide (CuO) is a base that is soluble in water.
Answer:
False. Copper(II) oxide (CuO) is a base that is not soluble in water.

Question 4.
Sodium hydrogencarbonate is commonly known as baking soda. ,
Answer:
True

Question 5.
Litmus is a natural indicator obtained from a plant.
Answer:
True

Question 6.
Bleaching powder is used for disinfecting drinking water.
Answer:
True

D. Name the following.

Question 1.
The neutral compound formed when a base and an acid react together
Answer:
Salt

Question 2.
The king of chemicals
Answer:
Sulphuric acid (H₂SO₄)

Question 3.
An acid used for making fertilizers
Answer:
Phosphoric acid (H₃PO₄)

Question 4.
A compound which reacts with an acid to form salt and water
Answer:
Base

Question 5.
A base which is soluble in water.
Answer:
Alkali

Question 6.
A base used for neutralizing acidic soil
Answer:
Calcium hydroxide [Ca(OH)₂]

Question 7.
A chemical substance used to identify acids or bases
Answer:
Indicator (e.g. methyl orange)

Question 8.
A salt used for quick sedimentation of suspended particles in drinking water
Answer:
Potash alum, K₂SO₄ Al₂ (SO₄)₃.24H₂O

Question 9.
A salt used for the preparation of plaster of Paris
Answer:
Gypsum (CaSO₄.2H₂O)

E. Match the following :

Column A	Column B
1. Baking soda	a. hydrated sodium carbonate
2. Blue vitriol	b. sodium hydrogencarbonate
3. Washing soda	c. hydrated copper sulphate
4. Carbonic acid	d. vinegar
5. Acetic acid	e. aerated drinksc
6. Manufacture of cement	f. antacid
7. Milk of magnesia	g. loadingc
8. Sulphuric acid	h. gypsum
9. Manufacture of soaps	i. lead-acid storage batteries
	j. sodium hydroxide

Answer:

Column A	Column B
1. Baking soda	b. sodium hydrogencarbonate
2. Blue vitriol	c. hydrated copper sulphate
3. Washing soda	a. hydrated sodium carbonate
4. Carbonic acid	e. aerated drinks
5. Acetic acid	d. vinegar
6. Manufacture of cement	h. gypsum
7. Milk of magnesia	f. antacid
8. Sulphuric acid	i. lead-acid storage batteries
9. Manufacture of soaps	j. sodium hydroxide

F. What do you observe in the fallowing cases 

Question 1.
Dilute sulphuric acid is added to zinc
Answer:
Hydrogen gas is released along with formation of zinc sulphate

Question 2.
Dilute hydrochloric acid is added to baking soda (NaHC03)
Answer:
Carbon dioxide gas is released with brisk effervescence along with the formation of sodium chloride (salt) and water.

Question 3.
Hydrochloric acid is added to calcium carbonate
Answer:
Carbon dioxide gas is formed along with calcium chloride and water.

Question 4.
Methyl orange is added to dilute hydrochloric acid
Answer:
The colour of methyl orange changes to red in dilute hydrochloric acid.

Question 5.
A drop of phenolphthalein is added to sodium hydroxide solution
Answer:
Phenolphthalein turns pink when added to sodium hydroxide solution (a base)

G. Define the following terms.

Question 1.
1. Acid
2. Alkali
3. Neutralization
4. pH scale
Answer:
An acid is a compound which, when dissolved in water, yields hydronium ions (H₃O⁺)
Example :
Hydrochloric acid

Question 2.
An alkali is a base soluble in water.
Example :
sodium hydroxide (NaOH), potassium hydroxide, etc.

Question 3.
A neutralization reaction is a reaction between an acid and a base to form salt (which is neutral) and water i.e.
acid + base → salt + water
Example:

Question 4.
pH scale is a scale (0-14) which gives the strength of an acid and a base as the measure of hydrogen ion concentration in the solution. Neutral solutions have pH equal to 7. Acidic solutions have pH less than 7. Basic solutions have pH more than 7.

H. Answer the following in short.

I. Choose strong and weak acids as well as strong and weak bases from the following :
H₂CO₃, HNO₃, CH₃COOH, CU(OH)₂, NH₄OH, KOH, Ca(OH)₂, HCl
Answer:
Strong acids – HCl, HNO₃
Weak acids – H2CO₃, CH₃COOH Strong bases – KOH, Ca(OH)₂ Weak bases – NH₄OH, Cu(OH)₂

Question 2.
Which gas is liberated when an acid reacts with a metal?
Answer:
Hydrogen gas

Question 3.
Name the gas evolved when sodium carbonate is added to dilute hydrochloric acid.
Answer:
Carbon dioxide gas

Question 4.
What happens when an acid reacts with sodium carbonate ? Write the balanced chemical equation of the reaction involved.
Answer:
When an acid reacts with sodium carbonate, carbon dioxide gas is liberated with brisk effervescence and sodium salt and water are formed.

Question 5.
Name the products formed when an acid reacts with a base.
Answer:
When an acid reacts with a base, it forms salt (which is neutral) and water. This reaction is called neutralization reaction.
acid + base → salt + water

Question 6.
State two uses for each of the following acids :
a. Hydrochloric acid
b. Nitric acid
c. Sulphuric acid
Answer:
(a) Uses of hydrochloric acid – It is used

1. in the preparation of aqua regia, which dissolves noble metals.
2. in the stomach to provide an acidic medium for the digestion of food.

(b) Uses of nitric acid – It is used

1.  in the manufacture of fertilizers such as ammonium nitrate.
2. in the manufacture of explosives such as TNT (trinitrotoluene) and nitroglycerine.

(c) Uses of sulphuric acid – It is used

1. in the refining of petroleum
2. in lead-acid storage batteries and Tire extinguishers.

Question 7.
What is the difference between an acid salt and a normal salt ?
Answer:
A salt formed by the complete replacement of the hydrogen ions of an acid molecule (monobasic) by a metal ion or a positive ion is called a normal salt.
Example :

A salt formed by the partial replacement of the hydrogen ions of an acid molecule (dibasic or tribasic) by a metal ion or a positive ion is called an acid salt.
Example:

Question 8.
Name :
a. Two bases which are not alkalis
Answer:
Copper hydroxide [Cu(OH)₂],
iron hydroxide [Fe(OH)₂]

b. A normal salt and an acid salt of the same acid
Answer:
Normal salt of carbonic acid – sodium carbonate (Na₂CO₃)
Acid salt of carbonic acid – sodium hydrogencarbonate (NaHCO₃)

Question 9.
Two solutions have pH equal to 3 and 5, respectively. Which one is more acidic and why ?
Answer:
The solution having pH equal to 3 is more acidic as acidic strength increases with decreasing pH number.

Question 10.
Name the salt used in :
a. The manufacture of glass and soap.
Answer:
Washing soda (hydrated sodium carbonate) – Na₂CO₃.10H₂O

b. The manufacture of fertilizers
Answer:
Nitre (potassium nitrate) – KNO₃

c. Fire extinguishers
Answer:
Baking soda (sodium hydrogencarbonate) – NaHCO₃

I. Answer the following in detail.

Question 1.
What is the difference between a strong acid and a weak acid ? Give two examples of each of them.
Answer:
Acids that undergo complete dissociation in aqueous solution producing a high concentration of hydrogen ions (or hydronium ions) are called strong acids. Examples : nitric acid (HNO₃), sulphuric acid (H₂SO₄), hydrochloric acid (HCl), etc.
Acids that undergo partial dissociation in aqueous solution producing a low concentration of hydrogen ions (or hydronium ions) are called weak acids. Examples : carbonic acid (H₂CO₃), phosphoric acid (H₃PO₄), formic acid, acetic acid, etc.

Question 2.
Define the basicity of an acid. Also differentiate between^ monobasic, dibasic and tribasic acids.
Answer:
The basicity of an acid is the number of hydrogen ions (H⁺) produced by the dissociation of one molecule of the acid when dissolved in water. Depending upon the number of hydrogen ions produced per molecule, acids are classified as monobasic,- dibasic and tribasic.

Monobasic acid – Acids which dissociate on dissolving in water to produce one hydrogen ion (H⁺) per molecule of the acid are called monobasic acids.

Basicity = 1 as the number of H⁺ = 1
Examples : hydrochloric acid (HCl), nitric acid (HNO₃),) and
acetic acid (CH₃),COOH).
Dibasic acid – Acids which dissociate on dissolving in water to produce two hydrogen ions (H+) per molecule of the acid are called dibasic acids.
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Basicity = 2 as the number of H⁺ = 2
Examples : sulphuric acid (H₂SO₄), carbonic acid (H₂CO₃), oxalic acid, etc.
Tribasic acid – Acids which dissociate in water to produce three hydrogen ions (H⁺) per molecule of the acid are called
tribasic acids.

Basicity = 3 as the number of H⁺ = 3
Examples : Phosphoric acid (H₃PO₄) and phosphorous acid (H₃PO₃)

Question 3.
While diluting a concentrated acid why is it recommended that the acid should be added to water and not water to the acid ?
Answer:
Diluting a concentrated acid :
The process of dissolving an acid in water is a highly exothermic reaction. Care must be taken while mixing concentrated acids (specifically, strong acids) with water. Always add acid to water and not the other way round. The acid must always be added slowly to water with constant stirring.

But, if water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause bums. The glass container may also break due to excessive heating.

Question 4.
What happenswhen an acid reacts with a base ? Explain by giving a suitable example. What is the special name for such a reaction ?
Answer:
When an acid reacts with a base, it forms salt and water. This reaction is called a neutralization reaction.
acid + base → salt + water
The base neutralizes the acid and the salt formed is neutral in nature, and therefore, the reaction is called neutralization reaction.
For example, when hydrochloric acid reacts with sodium hydroxide solution a neutralization reaction takes place to form sodium chloride and water.

Question 5.
What is the difference between a strong base and a weak base ? Explain with examples.
Answer:
Bases that undergo complete dissociation in aqueous solution and produce a high concentration of hydroxyl ions (OH^–) in solution are called strong bases. Examples : Potassium hydroxide (KOH) and sodium hydroxide (NaOH). Bases that undergo partial dissociation in aqueous solution and produce a low concentration of hydroxyl ions (OH^–) in solution are called weak bases. Examples : Magnesium hydroxide [Mg(OH)₂] and ammonium hydroxide [NH₄OH].

Question 6.
What are indicators ? Name the three common acid- base indicators and give their colours in acidic and basic solutions.
Answer:
An indicator is a chemical substance that is used to identify whether a given substance is an acid or a base by a sharp change in its colour. The three most common acid-base indicators are litmus, methyl orange and phenolphthalein

Question 7.
State one important use of the following :
a. Caustic soda
b. Slaked lime
c. Ammonium hydroxide
d. Magnesium hydroxide
Answer:
(a) Uses of caustic soda (sodium hydroxide, NaOH) : Used in the manufacture of soap, paper, medicines, artificial silk (rayon) and in the refining of petroleum.
(b) Uses of slaked lime [calcium hydroxide, Ca(OH)₂]: Used to make cement, mortar, bleaching powder and neutralizing acidic soil.
(c) Uses of ammonium hydroxide (NH₄OH) : Used to remove grease stains from glass panes and in the manufacture of nylon, plastic, fertilizers, dyes, etc.
(d) Uses of magnesium hydroxide [milk of magnesia, Mg(OH₂)] : Used as an antacid to neutralize the acidity caused by the excess secretion of hydrochloric acid in the stomach.

J. Give reasons for the following.

Question 1.
Acids are not stored in metal containers.
Answer:
Acids are not stored in metal containers as acids can react with the metal of the container, forming harmful salts which can corrode the container.

Question 2.
One molecule of acetic acid has four hydrogen atoms, yet it is called a monobasic acid.
Answer:
In acetic acid (CH₃COOH), four hydrogen atoms are present but only one hydrogen is capable of ionizing. Hence, it is called a monobasic acid. Its dissociation in aqueous solution is
CH₃COOH → CH₃COO + H⁺

Question 3.
Ammonium hydroxide is a weak base.
Answer:
Ammonium hydroxide is a weak base because it undergoes partial dissociation in aqueous solution and produce a low concentration of hydroxyl ions (OH^–) in solution.

Question 4.
All alkalis are bases but all bases are not alkalis.
Answer:
Bases are either soluble or insoluble in water. An alkali is a base soluble in water. So all alkalis are bases but all bases are not alkalis.

Living Science Chemistry Class 7 ICSE Solutions