CBSE Class 10 Science Acids Bases and Salts VBQs Set 05

Understanding the Chemical Properties of Acids and Bases

Question. Vinay observed that the stain of curry on a white shirt becomes reddish-brown when soap is scrubbed on it, but it turns yellow again when the shirt is washed with plenty of water. What might be the reason for his observation?
(i) Soap is acidic in nature.
(ii) Soap is basic in nature.
(iii) Turmeric is a natural indicator which gives reddish tinge in bases.
(iv) Turmeric is a natural indicator which gives reddish tinge in acids.
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) (ii) and (iv)
Answer: (b) (ii) and (iii)

What Do All Acids and All Bases have in Common?

Question. How will you protect yourself from the heat generated while diluting a concentrated acid?
(a) By adding acid to water with constant stirring.
(b) By adding water to acid with constant stirring.
(c) By adding water to acid followed by base.
(d) By adding base to acid with constant stirring.
Answer: (a) By adding acid to water with constant stirring.

How Strong are Acid or Base Solutions?

Question. Anita added a drop each of diluted acetic acid and diluted hydrochloric acid on pH paper and compared the colours. Which of the following is the correct conclusion?
(a) pH of acetic acid is more than that of hydrochloric acid.
(b) pH of acetic acid is less than that of hydrochloric acid.
(c) Acetic acid dissociates completely in aqueous solution.
(d) Acetic acid is a strong acid.
Answer: (a) pH of acetic acid is more than that of hydrochloric acid.

More About Salts

Question. With the reference to four gases \( CO_2, CO, Cl_2 \) and \( O_2 \), which one of the options in the table is correct?
(a) Acidic oxide: \( CO \), Used in treatment of water: \( Cl_2 \), Product of respiration: \( O_2 \), Product of incomplete combustion: \( CO \)
(b) Acidic oxide: \( CO_2 \), Used in treatment of water: \( Cl_2 \), Product of respiration: \( CO_2 \), Product of incomplete combustion: \( CO \)
(c) Acidic oxide: \( CO_2 \), Used in treatment of water: \( O_2 \), Product of respiration: \( O_2 \), Product of incomplete combustion: \( CO_2 \)
(d) Acidic oxide: \( CO \), Used in treatment of water: \( O_2 \), Product of respiration: \( CO_2 \), Product of incomplete combustion: \( CO_2 \)
Answer: (b) Acidic oxide: \( CO_2 \), Used in treatment of water: \( Cl_2 \), Product of respiration: \( CO_2 \), Product of incomplete combustion: \( CO \)

Question. Which of the given options correctly represents the parent acid and base of calcium carbonate?
(a) Parent Acid: \( HCl \), Parent Base: \( NaOH \)
(b) Parent Acid: \( H_2CO_3 \), Parent Base: \( Ca(OH)_2 \)
(c) Parent Acid: \( H_3PO_3 \), Parent Base: \( CaSO_4 \)
(d) Parent Acid: \( H_2SO_4 \), Parent Base: \( CaSO_4 \)
Answer: (b) Parent Acid: \( H_2CO_3 \), Parent Base: \( Ca(OH)_2 \)

Question. Assertion (A) : Fresh milk in which baking soda is added, takes a longer time to set as curd.
Reason (R): Baking soda decreases the pH value of fresh milk to below 6.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true and R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (c) A is true but R is false.

Question. Write the chemical name and chemical formula of the salt used to remove permanent hardness of water.
Answer: The salt used to remove permanent hardness of water is washing soda. Its chemical name is sodium carbonate decahydrate and its chemical formula is \( Na_2CO_3 \cdot 10H_2O \)

Question. Match the following pH values 1, 7, 10, 13 to the solutions given below:
— Milk of magnesia
— Gastric juices
— Brine
— Aqueous sodium hydroxide.
Amit and Rita decided to bake a cake and added baking soda to the cake batter. Explain with a balanced reaction, the role of the baking soda. Mention any other use of baking soda.
Answer: Matching pH values:
— Gastric juices: pH = 1
— Brine: pH = 7
— Milk of magnesia: pH = 10
— Aqueous sodium hydroxide: pH = 13
Role of baking soda in baking: Baking soda (sodium hydrogen carbonate) undergoes thermal decomposition when the cake is baked, releasing carbon dioxide gas which makes the cake fluffy and soft.
Balanced equation: \( 2NaHCO_3 \xrightarrow{\text{heat}} Na_2CO_3 + H_2O + CO_2 \)
Other use of baking soda: It is used as an antacid to neutralize excess acid in the stomach.

Question. (i) Four samples A, B, C and D change the colour of pH paper or solution to green, reddish pink, blue and orange. Their pH was recorded as 7, 2, 10.5 and 6 respectively. Which of the samples has the highest amount of hydrogen ion concentration? Arrange the four samples in the decreasing order of their pH.
(ii) Rahul found that the plaster of Paris, which he stored in a container, has become very hard and lost its binding nature. What is the reason for this? Also, write a chemical equation to represent the reaction taking place.
(iii) Give any one use of plaster of Paris other than for plastering or smoothening of walls.
Answer: (i) Sample B with pH = 2 is the most acidic and has the highest amount of \( H^+ \) ion concentration. The decreasing order of their pH is: C (10.5) > A (7) > D (6) > B (2).
(ii) Plaster of Paris absorbs moisture from the atmosphere and forms a hard mass of gypsum, thereby losing its binding nature.
Equation: \( CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O \)
(iii) Plaster of Paris is used to make toys, dolls, and statues.

Case Based Questions

Read the passage given below and answer the following questions :
A compound, X of sodium forms a white powder. It is a constituent of baking powder and is used in some antacids. When heated it gives a compound, Y which is anhydrous and absorbs water to become a hydrated salt. When this salt is kept in open air, it loses water molecules in a process called efflorescence. When dissolved in water it forms a strong base and a weak acid, Z.

Question. Identify the compound X and Z.
Answer: The compound of sodium that is a constituent of baking powder and is used in antacids is sodium hydrogen carbonate (\( NaHCO_3 \)). Thus, X is \( NaHCO_3 \). Z is carbonic acid (\( H_2CO_3 \)), a weak acid formed when \( Na_2CO_3 \) is dissolved in water.

Question. Name the compound Y. What is the nature of the solution formed by dissolving Y in water?
Answer: The compound Y is anhydrous sodium carbonate (\( Na_2CO_3 \)). Sodium carbonate dissolves in water to form sodium hydroxide (a strong base) and carbonic acid (a weak acid). Since the base is strong and the acid is weak, the resulting solution is basic or alkaline in nature.

Question. Predict whether sodium carbonate is basic or acidic in nature. Give one use of it.
Answer: Sodium carbonate is basic in nature because it is a salt of a strong base (NaOH) and a weak acid (\( H_2CO_3 \)). One use of sodium carbonate is in the manufacturing of glass.

Read the passage given below and answer the following questions :
pH is quite useful to us in a number of ways in daily life. Some of its applications are :
Control of pH of the soil : Plants need a specific pH range for proper growth. The soil may be acidic, basic or neutral depending upon the relative concentration of \( H^+ \) and \( OH^- \). The pH of any soil can be determined by using pH paper. If the soil is too acidic, it can be corrected by adding lime to it. If the soil is too basic, it can be corrected by adding organic manure which contains acidic materials.
Regaining shine of a tarnished copper vessel by use of acids : A copper vessel gets tarnished due to formation of an oxide layer on its surface. On rubbing lemon on the vessel, the surface is cleaned and the vessel begins to shine again. This is due to the fact that copper oxide is basic in nature, which reacts with the acid (citric acid) present in lemon to form a salt (copper citrate) which is washed away with water. As a result, the layer of copper oxide is removed from the surface of the vessel and the shining surface is exposed.
Self-defence by animals through chemical warfare : Stings of bees and ants contain methanoic acid. When stung, it causes lot of pain and irritation. This can be cured by rubbing the affected area with mild base like baking soda.

Question. When black copper oxide placed in a beaker is treated with dilute HCl, its colour changes to
(a) white
(b) dark red
(c) bluish green
(d) no change.
Answer: (c) bluish green

Question. P is an aqueous solution of acid and Q is an aqueous solution of base. When these two are diluted separately, then
(a) pH of P increases while that of Q decreases till neutralisation.
(b) pH of P decreases while that of Q increases till neutralisation.
(c) pH of both P and Q decrease.
(d) pH of both P and Q increase.
Answer: (a) pH of P increases while that of Q decreases till neutralisation.

Question. Which of the following acids is present in bee sting?
(a) Formic acid
(b) Acetic acid
(c) Citric acid
(d) Hydrochloric acid
Answer: (a) Formic acid

Question. Sting of ant can be cured by rubbing the affected area with soap because
(a) it contains oxalic acid which neutralises the effect of formic acid
(b) it contains aluminium hydroxide which neutralises the effect of formic acid
(c) it contains sodium hydroxide which neutralises the effect of formic acid
(d) None of the options.
Answer: (c) it contains sodium hydroxide which neutralises the effect of formic acid

Question. The pH of soil X is 7.5 while that of soil Y is 4.5. Which of the two soils, should be treated with powdered chalk to adjust its pH?
(a) X only
(b) Y only
(c) Both X and Y
(d) None of the options.
Answer: (b) Y only

A & R Questions

Question. Assertion : pH of ammonium nitrate solution is acidic.
Reason : Solution of a salt of weak base and strong acid is acidic.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.

Question. Assertion : \( NH_4Cl \) is a basic salt.
Reason : \( NH_4Cl \) is a salt of strong acid and a weak base.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (d) Assertion is wrong statement but reason is correct statement.

Question. Assertion : HCl is a stronger acid than acetic acid.
Reason : On dissociation, HCl yields lesser hydrogen ions for the same concentration as compared to acetic acid.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (c) Assertion is correct statement but reason is wrong statement.

Question. Assertion : Solutions of glucose and alcohol do not conduct electricity.
Reason : Substances like alcohol and glucose though contain hydrogen but when dissolved in water do not produce \( H^+ \) ions.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.

Multiple Choice Questions

Question. Phenolphthalein is _______ colour in basic medium whereas methyl orange is _______ colour in acidic medium.
(a) colourless, red
(b) pink, red
(c) pink, yellow
(d) colourless, yellow
Answer: (b) pink, red

Question. Which of the following two substances react to give salt and water only?
(a) Copper(II) oxide and ethanoic acid
(b) Magnesium and sulphuric acid
(c) Sodium oxide and water
(d) Zinc carbonate and hydrochloric acid
Answer: (a) Copper(II) oxide and ethanoic acid

Question. A student was given four unknown colourless samples labelled A, B, C and D and asked to test their pH using pH paper. He observed that the colour of pH paper turned to light green, dark red, light orange and dark blue with samples A, B, C, and D respectively. The correct sequence of increasing order of the pH value of samples is
(a) A < B < C < D
(b) A < D < C < B
(c) C < B < A < D
(d) B < C < A < D
Answer: (d) B < C < A < D

Question. Washing soda has the formula
(a) \( Na_2CO_3 \cdot 7H_2O \)
(b) \( Na_2CO_3 \cdot 10H_2O \)
(c) \( Na_2CO_3 \cdot H_2O \)
(d) \( Na_2CO_3 \)
Answer: (b) \( Na_2CO_3 \cdot 10H_2O \)

VSA Type Questions

Question. Name two substances from daily life which contain acid and two substances which contain base.
Answer: Substances containing acid: Lemon juice and vinegar. Substances containing base: Soap solution and baking powder.

Question. What is dead burnt plaster?
Answer: Dead burnt plaster is anhydrous calcium sulphate (\( CaSO_4 \)). It is formed when plaster of Paris is heated above 373 K.

Question. What do you call the property of losing water of crystallisation?
Answer: The property of losing water of crystallisation when exposed to air is called efflorescence.

SA I Type Questions

Question. Give general equations for the reactions of acids with (i) metal carbonates (ii) metal oxides.
Answer: (i) Metal carbonate + Acid \(\rightarrow\) Salt + carbon dioxide + water
(ii) Metal oxide + Acid \(\rightarrow\) Salt + water

Question. Write the chemical formula of washing soda. What happens when crystals of washing soda are exposed to air?
Answer: The chemical formula of washing soda is \( Na_2CO_3 \cdot 10H_2O \). When crystals of washing soda are exposed to air, they lose nine molecules of water of crystallisation and form a monohydrate. This process is known as efflorescence.
\( Na_2CO_3 \cdot 10H_2O \xrightarrow{\text{Air}} Na_2CO_3 \cdot H_2O + 9H_2O \)

Question. Write the balanced equation in molecular form illustrating the complete neutralisation of \( Al(OH)_3 \) with \( H_2SO_4 \).
Answer: Complete neutralisation requires one \( H^+ \) for each \( OH^- \). Since \( Al(OH)_3 \) has three available \( OH^- \) ions and \( H_2SO_4 \) can provide only two \( H^+ \) ions, the reaction requires two moles of \( Al(OH)_3 \) for three moles of \( H_2SO_4 \).
\( 2Al(OH)_3 + 3H_2SO_4 \rightarrow Al_2(SO_4)_3 + 6H_2O \)

Question. How can you test that metal oxides are basic while non-metal oxides are acidic? Explain with one example in each case.
Answer: Metal oxides react with acids to form salt and water, showing they are basic in nature. For example: \( CuO + 2HCl \rightarrow CuCl_2 + H_2O \).
Non-metal oxides react with bases to form salt and water, showing they are acidic in nature. For example: \( CO_2 + 2NaOH \rightarrow Na_2CO_3 + H_2O \).

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 rain water flows into the river, the pH of the river water decreases. The pH range suitable for aquatic survival is usually 7.0 to 7.8. Due to the decrease in pH caused by acid rain, the aquatic environment becomes too acidic, making it difficult for aquatic life to survive.

Question. A housewife found that the cake prepared by her is hard and small in size. Which ingredient has she forgotten to add that would have made the cake fluffy? Give reason.
Answer: She has forgotten to add baking powder. When baking powder (a mixture of sodium bicarbonate and tartaric acid) is added during cake preparation and heated, sodium bicarbonate reacts with tartaric acid to form carbon dioxide gas. The evolved \( CO_2 \) makes the cake rise and become light and fluffy.

SA II Type Questions

Question. (a) Why is plaster of Paris written as \( CaSO_4 \cdot 1/2H_2O \)? How is it possible to have half a water molecule attached to \( CaSO_4 \)?
(b) Why is sodium hydrogencarbonate an essential ingredient in antacids?
(c) When electricity is passed through an aqueous solution of sodium chloride, three products are obtained. What are these products and why is the process called chlor-alkali?
Answer: (a) The formula is written as \( CaSO_4 \cdot 1/2H_2O \) because one molecule of water is shared by two formula units of \( CaSO_4 \), i.e., the formula is \( (CaSO_4)_2 \cdot H_2O \). Thus, it represents half a molecule of \( H_2O \) per formula unit of \( CaSO_4 \).
(b) Sodium hydrogencarbonate, being basic, neutralises the excess acid produced in the stomach. Hence, it is used as an antacid.
(c) The three products are sodium hydroxide (NaOH), chlorine (\( Cl_2 \)), and hydrogen (\( H_2 \)). The process is called chlor-alkali because of the products formed: "chlor" for chlorine and "alkali" for sodium hydroxide.

Question. What is the role of acid in our stomach? Why pain occurs in the stomach during indigestion? What is done to get rid of this pain?
Answer: Our stomach produces hydrochloric acid which helps in the digestion of food without harming the stomach. During indigestion, the stomach produces too much acid, causing pain and irritation. To get rid of this pain, people use mild bases called antacids, like magnesium hydroxide (milk of magnesia), to neutralise the excess acid.

Question. How is bleaching powder prepared? Why does bleaching powder (a) smell strongly of chlorine? (b) not dissolve completely in water?
Answer: Bleaching powder is prepared by the action of chlorine (\( Cl_2 \)) on dry slaked lime [\( Ca(OH)_2 \)].
\( Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O \)
(a) It smells strongly of chlorine because \( CO_2 \) present in the air reacts with it and liberates free chlorine: \( CaOCl_2 + CO_2 \rightarrow CaCO_3 + Cl_2 \).
(b) It does not dissolve completely in water because it contains some unreacted lime which is insoluble in water.

Question. Name three chemicals which can be obtained from common salt.
Answer: Sodium hydroxide (NaOH), chlorine (\( Cl_2 \)), and hydrogen (\( H_2 \)) are obtained by the electrolysis of an aqueous solution of common salt.

Question. Write the chemical formula for washing soda. How can it be obtained from baking soda? Describe a household application of washing soda.
Answer: Chemical formula: \( Na_2CO_3 \cdot 10H_2O \). It is obtained by heating baking soda followed by recrystallisation.
\( 2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2 \)
\( Na_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O \)
Application: It is used in the laundry for washing clothes and also for removing permanent hardness of water.

LA Type Questions

Question. (a) Write the chemical formula of hydrated copper sulphate and anhydrous copper sulphate. How will you illustrate that these two are interconvertible?
(b) What is the chemical name and formula of plaster of Paris and gypsum?
Answer: (a) Hydrated copper sulphate: \( CuSO_4 \cdot 5H_2O \). Anhydrous copper sulphate: \( CuSO_4 \).
Interconvertibility: Heat blue hydrated copper sulphate crystals in a boiling tube. They turn white (anhydrous form) as they lose water of crystallisation. If you add 2-3 drops of water to the white sample, the blue colour is restored.
\( CuSO_4 \cdot 5H_2O \xrightarrow{\text{Heat}} CuSO_4 + 5H_2O \)
\( CuSO_4 + 5H_2O \rightarrow CuSO_4 \cdot 5H_2O \)
(b) Plaster of Paris: Calcium sulphate hemihydrate (\( CaSO_4 \cdot 1/2H_2O \)). Gypsum: Calcium sulphate dihydrate (\( CaSO_4 \cdot 2H_2O \)).

Question. What is observed when
(i) dilute sulphuric acid is added to solid sodium carbonate?
(ii) hot concentrated sulphuric acid is added to sulphur?
(iii) sulphur dioxide is passed through lime water?
(iv) sodium hydroxide reacts with Al.
(v) bleaching powder is added to limited quantity of dilute \( H_2SO_4 \).
Write chemical equations to represent the chemical reactions, taking place in each case.
Answer: (i) Effervescence occurs due to the formation of \( CO_2 \) gas.
\( Na_2CO_3 + H_2SO_{4(dil.)} \rightarrow Na_2SO_4 + CO_2 \uparrow + H_2O \)
(ii) \( SO_2 \) gas is formed.
\( S + 2H_2SO_{4(conc.)} \rightarrow 3SO_2 \uparrow + 2H_2O \)
(iii) Lime water turns milky.
\( SO_2 + Ca(OH)_2 \rightarrow CaSO_3 + H_2O \)
(iv) Hydrogen gas is evolved.
\( 2Al + 2NaOH + 2H_2O \rightarrow 2NaAlO_2 + 3H_2 \uparrow \)
(v) \( 2CaOCl_2 + H_2SO_{4(dil.)} \rightarrow CaSO_4 + CaCl_2 + 2HOCl \)

Question. Complete the following equations:
(i) \( NaHCO_3 \xrightarrow{\text{Heat}} \)
(ii) \( CaSO_4 \cdot 2H_2O \xrightarrow{373\text{ K}} \)
(iii) \( Na_2CO_3 \cdot 10H_2O \xrightarrow{\text{Air}} \)
(iv) \( CuSO_4 \cdot 5H_2O \xrightarrow{\text{Heat}} \)
(v) \( Ca(OH)_2 + Cl_2 \rightarrow \)
Answer: (i) \( 2NaHCO_3 \xrightarrow{\text{heat}} Na_2CO_3 + H_2O + CO_2 \)
(ii) \( CaSO_4 \cdot 2H_2O \xrightarrow{373\text{ K}} CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \)
(iii) \( Na_2CO_3 \cdot 10H_2O \xrightarrow{\text{air}} Na_2CO_3 \cdot H_2O + 9H_2O \)
(iv) \( CuSO_4 \cdot 5H_2O \xrightarrow{230^\circ\text{C}} CuSO_4 + 5H_2O \)
(v) \( Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O \)

Question. Identify two salts among the following whose solutions have a pH greater than 7.
(i) \( K_2S \)
(ii) \( NH_4Cl \)
(iii) \( KNO_3 \)
(iv) \( CaCO_3 \)
(v) \( NH_4NO_3 \)
Answer: The salts whose solutions have a pH greater than 7 are basic salts. These are salts formed from a strong base and a weak acid. From the list:
- \( K_2S \) is formed from KOH (strong base) and \( H_2S \) (weak acid).
- \( CaCO_3 \) is formed from \( Ca(OH)_2 \) (strong base) and \( H_2CO_3 \) (weak acid).
Therefore, \( K_2S \) and \( CaCO_3 \) have a pH greater than 7.

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?
(c) what is the role of tartaric acid added to baking soda?
Answer: (a) If baking soda is used instead of baking powder, the cake will taste bitter due to the formation of sodium carbonate (\( Na_2CO_3 \)) when the soda is heated.
(b) Baking soda can be converted into baking powder by adding a small amount of an edible acid like tartaric acid to it.
(c) Tartaric acid neutralises the sodium carbonate formed and prevents the cake from tasting bitter.