2. Acid , Bases and Salts

CBSE Class 10 Science Chapter 2 : Acid , Bases and Salts

Chapter 2 : Acid , Bases and Salts

Class 10 Science Chapter 2 . Acid , Bases and Salts CBSE Previous Years Questions and Answers :

CBSE Previous Years Questions and Answers for 2008

Question : How does the flow of acid rain water into a river make the survival of aquatic life in the river difficult ?

Answer: When acid rainwater, with a pH below 5.6, enters rivers, it reduces the river water's pH. As aquatic organisms thrive within a specific pH range (7.0 to 7.8), this acidity disrupts their environment, making survival challenging for aquatic life in affected rivers .

Question : Write the chemical formula for washing soda. How may it be obtained from baking soda ? Name an industrial use of washing soda other than washing clothes . [2M]

Answer : The chemical formula for washing soda is .

When baking soda(sodium bicarbonate, ) is heated, then produces sodium carbonate, carbon dioxide, and water .

By heating sodium carbonate kept at atmospheric temperature, recrystallisation of sodium carbonate occurs, resulting in the formation of washing soda.

An industrial use of washing soda, besides laundry, is in the treatment of water. It can be used to soften hard water, as it reacts with calcium and magnesium ions, reducing the hardness of water and preventing the precipitation of mineral deposits in pipes and appliances.

CBSE Previous Years Questions and Answers for 2009

Question : Fresh milk has a pH of 6 . When it changes into curd (yogurt), will its pH value increase or decrease ? why ?

Answer : When fresh milk changes into curd (yogurt), its pH value decreases. This decrease occurs due to the fermentation process conducted by lactic acid bacteria, which convert lactose (milk sugar) into lactic acid, leading to an increase in acidity and a decrease in pH.

Question : A compound which is prepared from gypsum the property of hardening when mixed with a proper quantity of water . Identify the compound . Write the chemical equation for its preparation . For what purpose is it used in hospitals ? [2M]

Answer : The compound prepared from gypsum that hardens when mixed with water is plaster of Paris (calcium sulfate hemihydrate, ).

Chemical equation for its preparation:

Plaster of Paris is used in hospitals as a plaster for supporting fractured bones in the right position.

It hardens quickly, forming a sturdy cast to immobilize and support the injured limb during the healing process.

CBSE Previous Years Questions and Answers for 2012

Question : Which of the following observations is true about dilute solution of acetic acid ?

(a) It smells like vinegar and turns red litmus blue (b) It smells like onion and turns blue litmus blue

Answer : (d) It smells like vinegar and turns blue litmus red .

[Acetic acid, which is the main component of vinegar, smells like vinegar and is acidic in nature. Blue litmus paper turns red in the presence of an acidic solution, which confirms the acidic nature of acetic acid. ]

Question : A student adds 4 ml of acetic to a test tube containing 4 ml of distilled water . He then shakes the test tube and leaves it to settle . After about 10 minutes he observes :

(a) A layer of water over the layer of acetic acid

(b) A layer of acetic acid over the layer of water

(d) A clear colourless solution

Answer : (d) A clear colorless solution .

[ When acetic acid () is added to water, it completely mixes with the water to form a clear, colorless solution. Acetic acid is highly soluble in water, so there won't be separate layers or a precipitate forming. ]

CBSE Previous Years Questions and Answers for 2013

Question : A student prepared 20% sodium hydroxide solution in a beaker containing water . The observations noted by him are given below .

(i) Sodium hydroxide is in the form of pellets .

(ii) It dissolves in water readity .

(iii) The beaker appears cold when touches from outside .

(iv) Red litmus paper turns blue when dipped into the solution .

The correct observations are :

(a) (i) , (ii) and (iii) (b) (ii) , (iii) and (iv) (c) (iii) , (iv) and (i) (d) (i) , (ii) and (iv)

Answer: (b) (ii), (iii), and (iv)

[ (ii) Sodium hydroxide dissolves in water readily. This is true because sodium hydroxide is highly soluble in water.

(iii) The beaker appears cold when touched from the outside. The dissolution of sodium hydroxide in water is an exothermic process, which means it releases heat. The sensation of coldness is due to the heat being absorbed from the surroundings, including the beaker.

(iv) Red litmus paper turns blue when dipped into the solution. Sodium hydroxide is a strong base, and it turns red litmus paper blue due to its alkaline nature. ]

Question : In an experiment to study the properties of acetic acid, a student takes about 2 ml of acetic acid in a dry test tube . He adds about 2 ml of water to it and shakes the test tube well . He is likely to observe that :

(a) The acetic acid dissolves readily in water .

(b) The solution becomes light orange .

(d) Acetic acid floats over the surface of water .

Answer : (a) The acetic acid dissolves readily in water.

[ Acetic acid is highly soluble in water, and when the student adds water to acetic acid and shakes the test tube, the acetic acid will readily dissolve in the water. ]

CBSE Previous Years Questions and Answers for 2018

Question : 2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube . When the contents are warmed , a gas evolves which is bubbled through a soap solution before testing . Write the equation of the chemical reaction involved and the test to detect the gas . Name the gas which will be evolved when the same metal reacts with dilute solution of a strong acid . [3M]

Answer : The chemical reaction involved when sodium hydroxide solution reacts with granulated zinc metal is:

This reaction produces hydrogen gas () and sodium zincate () .

The test to detect the evolved gas (hydrogen gas) is the "pop" test. In this test, a lighted wooden splint is brought near the mouth of the test tube containing the evolved gas. If hydrogen gas is present, it will ignite with a "pop" sound.

When the same metal (Zinc) reacts with a dilute solution of a strong acid, such as hydrochloric acid (HCl) or sulphuric acid ( ),then produce salt and hydrogen gas .

In this reaction, zinc chloride () is also produced along with hydrogen gas.

Question : The pH of a salt used to make tasty and crispy pakoras is 14 . Identify the salt and write a chemical equation for its formation . List its two uses . [3M]

Answer : The salt with a pH of 14 is sodium bicarbonate, commonly known as baking soda( ) .

Formation of sodium bicarbonate () :

Uses of sodium bicarbonate ():

(i) Sodium bicarbonate is used in cooking as a leavening agent, helping dough to rise and producing carbon dioxide gas bubbles that make baked goods light and fluffy. It's commonly used in recipes for pakoras, cakes, bread, and other baked goods.

(ii) Sodium bicarbonate is also used as an antacid to relieve heartburn, acid indigestion, and upset stomach. It works by neutralizing excess stomach acid.

CBSE Previous Years Questions and Answers for 2019

Question : Blue litmus solution is added to two test tubes A and B containing dilute HCl and NaOH solution respectively . In which test tube a colour change will be observed ? State the colour change and give its reason . [2M]

Answer: When blue litmus solution is added to two test tubes A and B containing dilute HCl and NaOH solution respectively, a color change will be observed in test tube A containing dilute HCl.

In test tube A (containing dilute HCl), the blue litmus solution will turn red. This color change occurs because hydrochloric acid (HCl) is a strong acid .

In test tube B (containing NaOH solution), there will be no color change observed. Sodium hydroxide (NaOH) is a strong base .

Question : What is observed when 2 mL of dilute hydrochloric acid is added to 1 g of sodium carbonate taken in a clean and dry test tube ? Write chemical equation for the reaction involved . [2M]

Answer : When 2 mL of dilute hydrochloric acid is added to 1 g of sodium carbonate in a clean and dry test tube, effervescence is observed due to the release of carbon dioxide gas. Sodium chloride and water are also formed as products.

The chemical equation for the reaction is:

Question : Identity the acid and the base from which sodium chloride is obtained . Which type of salt is it ? When is it called rock salt ? How is rock salt formed ? [3M]

Answer : In the reaction where sodium chloride (NaCl) is obtained, hydrochloric acid (HCl) acts as the acid, and sodium hydroxide (NaOH) acts as the base.

Acid: Hydrochloric acid (HCl)

Base: Sodium hydroxide (NaOH)

Sodium chloride is a type of salt known as a "neutral salt" because it is formed from the reaction between a strong acid (HCl) and a strong base (NaOH), resulting in a neutralization reaction.

Rock salt is typically a neutral salt, which is the mineral form of sodium chloride. It is called rock salt because it occurs naturally in large deposits underground in the form of solid, crystalline rocks.

Rock salt is formed over long periods of time through the evaporation of ancient bodies of water, such as lakes or seas. As the water evaporates, it leaves behind deposits of salt minerals, including sodium chloride. Over time, these deposits accumulate and form thick layers of rock salt

CBSE Previous Years Questions and Answers for 2020

Question : The chemical formula for plaster of Paris is :

(a) (b) (c) (d)

Answer : (a)

Question : Baking soda is a mixture of :

(a) Sodium carbonate and acetic acid

(b) Sodium carbonate and tartaric acid

(d) Sodium hydrogen carbonate and acetic acid

Answer : (c) Sodium hydrogen carbonate and tartaric acid .

Question : A clock strip dipped in onion juice is used for testing a liquid ‘X’ . The liquid ‘X’ changes its odour .Which type of an indicator is onion juice ? The liquid ‘X’ turns blue litmus red . List the observations the liquid ‘X’ will show on reacting with the following :

(a) Zinc granules

(b) Solid sodium carbonate

Write the chemical equations for the reactions involved . [5]

Answer : Onion juice acts as a pH indicator, functioning as an acid-base indicator.

It changes its odor in the presence of acidic or basic substances.

When exposed to acidic substances, onion juice tends to change its odor, indicating the presence of an acid.

Since liquid 'X' turns blue litmus red, it indicates that 'X' is acidic (HCl) in nature.

When reacting with the following substances:

(a) Reaction with Zinc granules: Chemical Equation:

(b) Reaction with Solid sodium carbonate: Chemical Equation:

Question : Define water of crystallization. Give the chemical formula for two compounds as examples . How can it be proved that the water of crystallization makes a difference in the state and colour of the compounds ? [5]

Answer : Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt.

The chemical formula for two compounds are :

(i) Copper (II) sulphate pentahydrate :

(ii) Calcium sulphate hemihydrate :

The presence of water of crystallization significantly impacts the state and color of compounds. Dry copper sulphate crystals contain water of crystallization. Heating the crystals removes water, turning the salt white. Moistening the crystals with water causes the blue color to reappear. This reversible process demonstrates the crucial role of water of crystallization in determining the state and color of compounds.

Question : List the important products of the Chlor-alkali process . Write one important use each . [3M]

Answer : The Chlor-alkali process is an industrial method for the electrolysis of sodium chloride (common salt) to produce chlorine gas, sodium hydroxide (caustic soda), and hydrogen gas.

The important uses of the products are :

Chlorine Gas (Cl₂):

Chlorine is used in the production of a wide range of chemicals, including PVC (polyvinyl chloride) for plastics, chlorinated solvents, and various organic chemicals. It is also employed in water treatment for disinfection.

Sodium Hydroxide (NaOH) - Caustic Soda:

Caustic soda is a versatile industrial chemical used in the manufacturing of soaps and detergents, textiles, paper, aluminum, and various chemicals. It is also a key component in water treatment and serves as a strong base in chemical processes.

Hydrogen Gas (H₂): Hydrogen has numerous applications, including being used in the production of ammonia for fertilizers, as a reducing agent in various industrial processes, and in fuel cells for generating electricity. It is also utilized in the food industry for hydrogenation processes.

Question : How is washing soda prepared from sodium carbonate ? Give its chemical equation . State the type of this salt . Name the type of hardness of water which can be removed by it . [3M]

Answer : When sodium hydrogen carbonate is heated, it produces sodium carbonate, water, and carbon dioxide.

When sodium carbonate () is hydrated with water and the resulting solution is crystallised, the product is .

Sodium carbonate is a basic salt .

Washing soda is effective in removing temporary hardness of water.

CBSE Previous Years Questions and Answers for 2021

Question : Which of the following oxides(s) is/are soluble in water to form alkalies ?

(i) (ii) (iii) (iv)

(a) (i) and (iii) (b) (i) only (c) (ii) and (iv) (d) (iii) only

Answer : (a) (i) and (iii)

[ Sodium oxide () and potassium oxide () are both soluble in water and form alkaline solutions.]

Question : Study the diagram given below and identity the gas formed in the reaction .

(a) Carbon dioxide which extinguishes the burning candles

(b) Oxygen due to which the candle burns more brightly

(d) Hydrogen which while burning produces a popping sound

Answer : (d) Hydrogen which while burning produces a popping sound .

Question : Which of the options in the given table are correct ?

Option	Natural source	Acid present
(i)	Orange	Oxalic
(ii)	Sour milk	Lactic acid
(iii)	Ant sting	Methanoic acid
(iv)	Tamarind	Acetic acid

(a) (i) and (ii) (b) (i) and (iv) (c) (ii) and (iii) (d) (iii) and (iv)

Answer : (c) (ii) and (iii)

[ Sour milk contains lactic acid, and ant sting contains methanoic acid (also known as formic acid).]

Question : Select from the following the statement which is true for bases

(a) Bases are bitter and turn blue litmus red .

(b) Bases have a pH less than 7 .

(d) Bases turn pink when a drop of phenolphthalein is added to them .

Answer : (d) Bases turn pink when a drop of phenolphthalein is added to them.

[ Phenolphthalein is an indicator that turns pink in basic solutions.]

Question : Study the following table and choose the correct option :

	Salt	Parent Acid	Parent Base	Nature
(a)	Sodium chloride	HCl	NaOH	Basic
(b)	Sodium Carbonate		NaOH	Neutral
(c)	Sodium Sulphate		NaOH	Acidic
(d)	Sodium Acetate		NaOH	Basic

Answer : (d) Sodium Acetate - - NaOH - Basic

Sodium acetate is the salt of a weak acid (acetic acid,) and a strong base (sodium hydroxide, NaOH). Therefore, it exhibits basic nature.

Question : Cosider the pH value of the following acidic samples :

S. No.	Sample	pH value
1.	Lemon juice	2.2
2.	Gastric juice	1.2
3.	Vinegar	3.76
4.	Dil. Acetic acid	3.0

The decrease order of their ion concentration is : [2021 1M]

(a) 3>4>1>2 (b) 2>1>3>4 (c) 2>1>4>3 (d) 3>4>2>1

Answer : (c) 2>1>4>3

Question : Study the experimental set up shown in given figure and choose the correct option from the following :

	P	Q	Change observed in calcium hydroxide solution
(a)		gas	No change
(b)		gas	No change
(c)		gas	Turns milky
(d)		gas	Turns milky

Answer : (d) – gas – Turns milky

Question : Which of the following salts do not have the water of crystallization ? [2021 1M]

(i) Beaching powder (ii) Plaster of Paris (iii) Washing soda (iv) Baking soda

(a) (ii) and (iv) (b) (i) and (iii) (c) (ii) and (iii) (d) (i) and (iv)

Answer : (d) (i) and (iv)

Question : Assertion (A) : Sodium hydrogen carbonate is used as an ingredient in antacids .

Reason (R) : is a mild non-corrosive basic salt .

(a) Both (A) and (R) are true and (R) is the correct explanation of (A) .

(b) Both (A) and (R) are true but (R) is not the correct explanation of (A).

(d) (A) is false , but (R) is true .

Answer: (a) Both (A) and (R) are true and (R) is the correct explanation of (A) .

Question : 50.0 mL of tap water was taken in a beaker . Hydrochloric acid was added drop by drop to water . The temperature and pH of the solution was noted . The following graph was obtained . Choose the correct statement related to this activity .

(i) The process of dissolved an acid in water is highly endothermic .

(ii) The pH of the solution increases rapidly on addition of acid .

(iii) The pH of the solution decreases rapidly on addition of acid .

(iv) The pH of tap water was around 7.0 .

(a) (i) and (ii) (b) (i) and (iii) (c) (iii) and (iv) (d) (ii) and (iv)

Answer : (d) (ii) and (iv)