2. Acids , Bases and Salts Class 10 Science NCERT – Chapterwise Important Questions & Answers for CBSE Exam Preparation

Class 10 Science Chapter 2 : Acids , Bases and Salts NCERT Important Questions and Answers for CBSE

Chapter 2 : Acids , Bases and Salts

Multiple Choose Questions and Answers :

Question: The chemical formula for plaster of Paris is : [2020]

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

Answer: (a)

Question: Baking soda is a mixture of : [2020]

(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 .

[Baking powder (commonly confused with baking soda) is a mixture of sodium hydrogen carbonate and tartaric acid, which helps release CO₂ during baking.]

Question: A student prepared 20% sodium hydroxide solution in a beaker containing water . The observations noted by him are given below . [2013]

(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 : [2013]

(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. ]

Question: A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10

Answer: (d) 10

[A solution turning red litmus blue is basic. Basic solutions have pH greater than 7, so pH 10 confirms it is strongly basic.]
Question: A solution reacts with crushed egg-shells to give a gas that turns lime-water milky.The solution contains :
(a) NaCl (b) HCl (c) LiCl (d) KCl

Answer: (b) HCl

[ Crushed egg-shells mainly contain calcium carbonate (CaCO₃).

When an acid (HCl) reacts with calcium carbonate, it produces carbon dioxide (CO₂) gas, which turns lime water milky. ]

Question: 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be :
(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL

Answer: (d) 16mL

[ Hint: 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a solution HCl .

So, 20 mL of a solution of NaOH is found to be completely neutralised by = 2 × 8 mL = 16 mL of a solution HCl . ]
Question: Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic

Answer: (c) Antacid .

[ Indigestion is caused by excess acid in the stomach. Antacids neutralise this acid and give relief. ]

3. Which of the following observations is true about dilute solution of acetic acid ? [2012]

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

(b) It smells like onion and turns blue litmus blue

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

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 : [2012]

(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. ]

Question: Which of the options in the given table are correct ? [2021]

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 [2021]

(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: 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)

[Salts without water of crystallisation are those that do not contain fixed water molecules in their structure. Bleaching powder and baking soda do not have it. ]

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

Reason (R) : is a mild non-corrosive basic salt . [2021 1M]

(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) .

[Sodium hydrogen carbonate (NaHCO₃) is used in antacids because it is a mild, non-corrosive base that neutralises excess stomach acid safely. ]

Question: An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change?

(a) Baking powder (b) Lime (c) Ammonium hydroxide solution (d) Hydrochloric acid

Answer: (d) Hydrochloric acid (HCl) .

[ If a solution turns red litmus blue, it is basic. To reverse this, we need an acid.Hydrochloric acid neutralises the base and turns the solution acidic again.]

Question: Which of the following salts does not contain water of crystallisation?

(a) Blue vitriol (b) Baking soda (c) Washing soda (d) Gypsum

Answer: (b) Baking soda.

[ Water of crystallisation means fixed water molecules in a salt. Baking soda (NaHCO₃) does not contain any water of crystallisation. ]

Question: Sodium carbonate is a basic salt because it is a salt of

(a) strong acid and strong base

(b) weak acid and weak base

(d) weak acid and strong base

Answer: (d) weak acid and strong base

[ Sodium carbonate (Na₂CO₃) is a basic salt because it is a salt of a weak acid (carbonic acid, H₂CO₃) and a strong base (sodium hydroxide, NaOH). So, the correct option is: (d) weak acid and strong base ]

Question: Calcium phosphate is present in tooth enamel. Its nature is

(a) basic (b) acidic (c) neutral (d) amphoteric

Answer: (a) basic

[Calcium phosphate is a salt formed from a strong base and weak acid, so it shows basic nature.]

Question: A sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the pH paper yellowish-orange. Which of the following would change the colour of this pH paper to greenish-blue?

(a) Lemon juice (b) Vinegar (c) Common salt (d) An antacid

Answer: (d) An antacid

[ Antacids are substances that are often basic and are used to neutralize excess stomach acid. Therefore, adding an antacid to the solution would likely change the color of the pH paper to greenish-blue.]

Question: Which of the following gives the correct increasing order of acidic strength?

(a) Water < Acetic acid < Hydrochloric acid

(b) Water < Hydrochloric acid < Acetic acid

(d) Hydrochloric acid < Water < Acetic acid

Answer: (a) Water < Acetic acid < Hydrochloric acid

[ In this order, water is neutral, hydrochloric acid is a strong acid, and acetic acid is a weak acid.]

Question: Sodium hydrogencarbonate when added to acetic acid evolves a gas. Which of the following statements are true about the gas evolved?

(i) It turns lime water milky

(ii) It extinguishes a burning splinter

(iii) It dissolves in a solution of sodium hydroxide

(iv) It has a pungent odour

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

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

[ Sodium hydrogencarbonate reacts with acetic acid to produce carbon dioxide (CO₂) gas.

CO₂ turns lime water milky, extinguishes fire, and dissolves in NaOH, but it is odourless (no pungent smell). ]

Short Questions and Answers :

Question: How does the flow of acid rain water into a river make the survival of aquatic life in the river difficult ? [2008 1M]

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: Why does dry HCl gas not change the colour of the dry litmus paper? [1M]

Answer : Dry HCl gas does not change the color of dry litmus paper because litmus paper requires the presence of water (moisture) to exhibit a color change. In the absence of water, the dry litmus paper remains unaffected by dry HCl gas.

Question: Why should curd and sour substances not be kept in brass and copper vessels? [1M]

Answer: Curd and sour substances should not be kept in brass and copper vessels because they contain acids that can react with the metal, leading to the formation of harmful compounds that can be toxic if consumed.

Question: Fresh milk has a pH of 6 . When it changes into curd (yogurt), will its pH value increase or decrease ? why ? [2009 1M]

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: You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube? [2M]

Answer: To identify the contents of each test tube using red litmus paper:

Dip the litmus paper in the first test tube. If it remains red, it is distilled water.

Dip the litmus paper in the second test tube. If it turns blue, it is a basic solution.

Dip the litmus paper in the third test tube. If it remains red, it is an acidic solution.

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 ? [2009 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.

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: What is a neutralisation reaction? Give two examples. [2M]
Answer: The reaction between an acid and a base to give a salt and water is called a neutralisation reaction.

Two examples are : (i) (ii)

Question: Give two important uses of washing soda and baking soda. [2M]

Answer: Uses of washing soda :

(i) Washing soda is used for removing permanent hardness of water.

(ii) Washing soda is used in glass, soap and paper industries.

Uses of baking soda :

(i) Baking soda is used in soda-acid fire extinguishers.

(ii) For making baking powder, which is a mixture of baking soda and a mild edible acid such as tartaric acid.

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 . [2019 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 .

Qusetion: 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 . [ 2008 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.

34. Name the acid present in ant sting and give its chemical formula. Also give the common method to get relief from the discomfort caused by the ant sting. [2M]

Answer The acid present in an ant sting is Formic acid. Its chemical formula is HCOOH.

Relief from discomfort: (i) Apply a mild base like baking soda (sodium hydrogen carbonate) solution on the affected area.

(ii) This helps because the base neutralises the acid, reducing irritation and pain.

Long Questions and Answers (3 Marks):

Question: Salt A commonly used in bakery products on heating gets converted into another salt B which itself is used for removal of hardness of water and a gas C is evolved. The gas C when passed through lime water, turns it milky. Identify A, B and C. [3M]

Answer: Salt A commonly used in bakery products is sodium bicarbonate (). On heating, it gets converted into another salt B, sodium carbonate (), and a gas C, carbon dioxide (), is evolved.

When gas C (carbon dioxide) is passed through lime water (calcium hydroxide, ), it turns milky due to the formation of calcium carbonate ().

Question: For making cake, baking powder is taken. If at home your mother uses baking soda instead of baking powder in cake,

(a) how will it affect the taste of the cake and why?

(b) how can baking soda be converted into baking powder?

Answer: (a) The cake may taste bitter because baking soda (NaHCO₃) on heating forms sodium carbonate, which is alkaline and gives a bitter taste.

(b) Baking soda can be converted into baking powder by mixing it with a mild edible acid like tartaric acid.

(c) Tartaric acid neutralises the sodium carbonate formed and helps in releasing CO₂, preventing bitterness and improving taste.

Question: In one of the industrial processes used for manufacture of sodium hydroxide, a gas X is formed as by product. The gas X reacts with lime water to give a compound Y which is used as a bleaching agent in chemical industry. Identify X and Y giving the chemical equation of the reactions involved. [3M]

Answer: In the industrial process for the manufacture of sodium hydroxide (NaOH), chlorine gas () is often formed as a byproduct.

The chlorine gas (X) reacts with lime water (calcium hydroxide, ) to give a compound Y, which is calcium oxychloride ).

The chemical equation for the reaction is:

Compound Y, Calcium Oxychloride, is used as a bleaching agent in the chemical industry.

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

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 . [2018 3M]

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

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.

Question: A metal carbonate X on reacting with an acid gives a gas which when passed through a solution Y gives the carbonate back. On the other hand, a gas G that is obtained at anode during electrolysis

of brine is passed on dry Y, it gives a compound Z, used for disinfecting drinking water. Identity X, Y, G and Z. Also, given the reaction . [3M]

Answer: X: Calcium carbonate (CaCO₃)

Y: Lime water (calcium hydroxide, Ca(OH)₂)

G: Chlorine gas (Cl₂)

Z: Bleaching powder (CaOCl₂)

The Reactions : (i) CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
(ii) CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O

(iii) Cl₂ + Ca(OH)₂ → CaOCl₂ + H₂O

Long Questions and Answers (5 Marks):

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 . [5M]

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.

(a) Reaction with Zinc granules: Chemical Equation:

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

Q. 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 ? [M5]

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.