CBSE - Acids Bases and Salts

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?

Answer:

If the colour of red litmus paper gets changed to blue, then it is a base and if there is no colour change, then it is either acidic or neutral. Thus, basic solution can be easily identified. 

Let us mark the three test tubes as A, B, and C. A drop of the solution in A is put on the red litmus paper. Same is repeated with solution B and C. If either of them changes colour to blue, then it is basic. Therefore, out of three, one is eliminated.
Out of the remaining two, any one can be acidic or neutral. Now a drop of basic solution is mixed with a drop of each of the remaining two solutions separately and then the nature of the drops of the mixtures is checked. If the colour of red litmus turns blue, then the second solution is neutral and if there is no change in colour, then the second solution is acidic. 
This is because acidic and basic solutions neutralize each other. Hence, we can distinguish between the three types of solutions. 

Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful products, thereby spoiling the food.

Hydrogen gas is usually liberated when an acid reacts with a metal.



Take few pieces of zinc granules and add 5 ml of dilute H₂SO₄. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.
H₂SO₄ + Zn → ZnSO₄ + H₂ ↑
We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.

Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.

Answer

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + CO₂ (g) + H₂O (l)
Calcium Carbonate + Hydrochloric acid → Calcium Chloride + Carbon dioxide + Water

When HCl or HNO₃ are mixed with water then they dissolve in water to form H⁺ or H₃O⁺ ions which shows their acidic character. For example just see the following reactions

HCl (aq) → H⁺ + Cl^–

H⁺ + H₂O → H₃O⁺

When alcohols and glucose are mixed with water then they do not dissolve to form ions. Hence they do not show acidic character.

The presence of hydrogen (H⁺) or hydronium (H₃O⁺) ions in the aqueous solution of an acid are responsible for conducting electricity.

Dry HCl gas not change the colour of the dry litmus paper because it has no Hydrogen ions (H⁺) in it.

Since the process of dissolving an acid in water is exothermic, it is always recommended that acid should be added to water. If it is done the other way, then it is possible that because of the large amount of heat generated, the mixture splashes out and causes burns.

When an acid is diluted, the concentration of hydronium ions (H₃O⁺) per unit volume decreases. This means that the strength of the acid decreases.

The concentration of hydroxide ions (OH⁻) would increase when excess base is dissolved in a solution of sodium hydroxide.

A pH value of less than 7 indicates an acidic solution, while greater than 7 indicates a basic solution. Therefore, the solution with pH = 6 is acidic and has more hydrogen ion concentration than the solution of pH = 8 which is basic.

If the concentration of H⁺ (aq) ions is increased (>10-7) then the solution become acidic and if the concentration of H⁺ (aq) ions is decreased (<10-7) then the solution become basic in nature.

Yes, basic solution also has H⁺ ions. However, their concentration is less as compared to the concentration of OH^– ions that makes the solution basic.

If the soil is acidic and improper for cultivation, then to increase the basicity of soil, the farmer would treat the soil with quick lime or slaked lime or chalk.

Bleaching Powder

Calcium hydroxide [Ca(OH)₂], on treatment with chlorine, yields bleaching powder.

Washing soda (Na₂CO₃.10H₂O)

When sodium hydrogen carbonate is heated then sodium carbonate and water is formed along with the evolution of carbon dioxide gas.

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 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

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 of HCl solution (the same solution as before) required to neutralise it will be
(a) 4 mL
(b) 8mL
(c) 12 mL
(d) 16 mL

Answer (d) 16 mL

Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic

Answer (c) Antacid

 Write word equations and then balanced equations for the reaction taking place when −
(a) dilute sulphuric acid reacts with zinc granules.
(b) dilute hydrochloric acid reacts with magnesium ribbon.
(c) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.

Answer

(a) H₂SO₄ (aq) + Zn (s) → ZnSO₄ (aq) + H₂ (g)

(b) 2HCl (aq) + Mg (s) → MgCl (aq) + H₂ (g)

(c) 3H₂SO₄ (aq) + 2Al (s) → Al₂SO₄ (aq) + 3H₂ (g)

(d) 6HCl (aq) + 2Fe (s) → 2FeCl₃ (aq) + 3H₂ (g)

Two nails are fitted on a cork and are kept it in a 100 mL beaker. The nails are then connected to the two terminals of a 6-volt battery through a bulb and a switch. Some dilute HCl is poured in the beaker and the current is switched on. The same experiment is then performed with glucose solution and alcohol solution.

Observations:
It will be observed that the bulb glows in the HCl solution and does not glow in the glucose solution.



Result:
HCl dissociates into H+and Cl−ions. These ions conduct electricity in the solution resulting in the glowing of the bulb. On the other hand, the glucose solution does not dissociate into ions. Therefore, it does not conduct electricity.
Conclusion:
From this activity, it can be concluded that all acids contain hydrogen but not all compounds containing hydrogen are acids.
That is why, though alcohols and glucose contain hydrogen, they are not categorised as acids.

Distilled water cannot conduct electricity because it does not contain ions while rain water conducts electricity as it contains ions due presence of dissolved salts in it.

Acids do not show acidic behaviour in the absence of water because the dissociation of hydrogen ions from an acid occurs in the presence of water only.

Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is
(a) neutral?
(b) strongly alkaline?
(c) strongly acidic?
(d) weakly acidic?
(e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.

Answer

(a) Neutral →Solution D with pH 7
(b) Strongly alkaline →Solution C with pH 11
(c) Strongly acidic →Solution B with pH 1
(d) Weakly acidic →Solution A with pH 4
(e) Weakly alkaline →Solution E with pH 9
The pH can be arranged in the increasing order of the concentration of hydrogen ions as: 11 < 9 < 7 < 4 < 1.

Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH₃COOH) is added to test tube B. In which test tube will the fizzing occur more vigorously and why?

Answer

The fizzing will occur strongly in test tube A, in which hydrochloric acid (HCl) is added. This is because HCl is a stronger acid than CH₃COOH and therefore produces hydrogen gas at a faster speed due to which fizzing occurs.

The pH of milk is 6. As it changes to curd, the pH will reduce because curd is acidic in nature. The acids present in it decrease the pH.

A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?

Answer

(a) The milkman shifts the pH of the fresh milk from 6 to slightly alkaline because in alkaline condition, milk does not set as curd easily.

(b) Since this milk is slightly basic than usual milk, acids produced to set the curd are neutralized by the base. Therefore, it takes a longer time for the curd to set.

The Plaster of Paris should be stored in a moisture-proof container as it absorbs water from moisture and turn into hard substance (Gypsum) as shown in following chemical equation.

A reaction in which an acid and base react with each other to give a salt and water is termed as neutralization reaction.For Example:
(i) NaOH + HCl → NaCl + H₂O
(ii) HNO₃ + KOH → KNO₃ + H₂O

Answer

Two important uses of washing soda are:
→ It is used in glass, soap, and paper industries.
→ It is used to remove permanent hardness of water.

Two important uses of baking soda are:
→ It is used as baking powder. Baking powder is a mixture of baking soda and a mild acid known as tartaric acid. When it is heated or mixed in water, it releases CO₂ that makes bread or cake fluffy.
→ It is used in soda-acid fire extinguishers.

1. What happens when a solution of an acid is mixed with a solution of a base in a test tube?
(i) The temperature of the solution increases
(ii) The temperature of the solution decreases
(iii)The temperature of the solution remains the same
(iv) Salt formation takes place
(a) (i) only
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (i) and (iv)
Ans. (d) (i) and (iv)
Explanation: When an acid is mixed with a base; it results in neutralization reaction.
Neutralization reaction results in formation of salt. Neutralization reaction is exothermic reaction.

2. 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
Ans. (d) Hydrochloric acid
Explanation: If an aqueous solution turns red litmus solution to blue, it means that the given solution is basic. Its effect can be neutralized by an acidic solution.

3. During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to
(a) absorb the evolved gas
(b) moisten the gas
(c) absorb moisture from the gas
(d) absorb Cl-
ions from the evolved gas
Ans. (c) absorb moisture from the gas

4. Which of the following salts does not contain water of crystallisation?
(a) Blue vitriol
(b) Baking soda
(c) Washing soda
(d) Gypsum
Ans. (b) Baking soda

Explanation: Other salts in the options usually appear as crystalline salts, but baking soda appears as white powder. Crystalline salts contain water of crystallization.


5. Sodium carbonate is a basic salt because it is a salt of
(a) strong acid and strong base
(b) weak acid and weak base
(c) strong acid and weak base
(d) weak acid and strong base
Ans. (d) weak acid and strong base
Explanation: A salt of strong base and weak acid produces basic salt.

6. Calcium phosphate is present in tooth enamel. Its nature is
(a) basic
(b) acidic
(c) neutral
(d) amphoteric
Ans. (a) basic
Explanation: Phosphate ion comes from phosphoric acid; which is a strong base. Hence, calcium phosphate is basic salt. Moreover, the fact that tooth enamel gets damaged by acid shows that calcium phosphate is basic in nature.


7. 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
Ans. (d) An antacid
Explanation: Since soil mixture is turning the pH paper yellowish-orange, hence soil is acidic in nature. So, adding an antacid would change the colour to greenish-blue


8. Which of the following gives the correct increasing order of acidic strength?
(a) Water <Acetic acid <Hydrochloric acid
(b) Water <Hydrochloric acid <Acetic acid
(c) Acetic acid <Water <Hydrochloric acid
(d) Hydrochloric acid <Water <Acetic acid
Ans. (a) Water <Acetic acid <Hydrochloric acid
Explanation: Distilled water is neutral. Acetic acid is an organic acid so it is less acidic than hydrochloric acid which is an inorganic acid.

9. If a few drops of a concentrated acid accidentally spills over the hand of a student, what should be done?
(a) Wash the hand with saline solution
(b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogen carbonate
(c) After washing with plenty of water apply solution of sodium hydroxide on the hand
(d) Neutralise the acid with a strong alkali

Ans. (b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogen carbonate
Explanation: Washing the hand with plenty of water will minimize the presence of acid. Further, application of sodium hydrogen carbonate will neutralize any remaining acid. Sodium hydroxide is corrosive in nature, so it should not be used.


10. 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)
Ans. (a) (i) and (ii)
Explanation: This reaction produces carbon dioxide gas. Carbon dioxide turns lime water milky and extinguishes a burning splinter.


11. Common salt besides being used in kitchen can also be used as the raw material for making
(i) washing soda
(ii) bleaching powder
(iii) baking soda
(iv) slaked lime
(a) (i) and (ii)
(b) (i), (ii) and (iv),
(c) (i) and (iii)
(d) (i), (iii) and (iv)
Ans. (a) (i) and (ii)
Explanation: Slaked time is made from calcium carbonate which is not common salt. Common salt is a raw material for making


12. One of the constituents of baking powder is sodium hydrogen carbonate, the other constituent is
(a) hydrochloric acid
(b) tartaric acid
(c) acetic acid
(d) sulphuric acid
Ans. (b) tartaric acid

Explanation: A milk edible acid is used along with sodium hydrogen carbonate to make baking soda. Usually, tartaric acid is used but acetic acid can also be used.

13. To protect tooth decay we are advised to brush our teeth regularly. The nature of the tooth paste commonly used is
(a) acidic
(b) neutral
(c) basic
(d) corrosive
Ans. (c) basic
Explanation: Basic toothpaste neutralizes any acid which may be present on teeth. Acid comes because of bacterial activity in mouth.

14. Which of the following statements is correct about an aqueous solution of an acid and of a base?
(i) Higher the pH, stronger the acid
(ii) Higher the pH, weaker the acid
(iii) Lower the pH, stronger the base
(iv) Lower the pH, weaker the base
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) (ii) and (iv)
Ans. (d) (ii) and (iv)
Explanation: pH of acids is below 7 and as we go down the pH scale, acids become stronger. When we go up the pH scale beyond 7, bases become stronger.

15. The pH of the gastric juices released during digestion is
(a) less than 7
(b) more than 7
(c) equal to 7
(d) equal to 0
Ans. (a) less than 7
Explanation: Refer to explanation for previous question


16. Which of the following phenomena occur, when a small amount of acid is added to water?
(i) Ionisation
(ii) Neutralisation
(iii) Dilution
(iv) Salt formation
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (ii) and (iv)

Ans. (b) (i) and (iii)
Explanation: No base is being added, so (ii) and (iii) cannot happen. When acid is dissolved in water, it breaks into its ions.

17. Which one of the following can be used as an acid�base indicator by a visually impaired student?
(a) Litmus
(b) Turmeric
(c) Vanilla essence
(d) Petunia leaves
Ans. (c) Vanilla essence
Explanation: Vanilla works as olfactory indicator, hence it can be used by visually impaired student.

18. Which of the following substance will not give carbon dioxide on treatment with dilute acid?
(a) Marble
(b) Limestone
(c) Baking soda
(d) Lime
Ans. (d) Lime
Explanation: Other options show carbonate or hydrogen carbonate. Hence, they will give carbon dioxide on reaction with acid. But lime contains hydroxide and hence will not give carbon dioxide on reaction with acid

19. Which of the following is acidic in nature?
(a) Lime juice
(b) Human blood
(c) Lime water
(d) Antacid
Ans. (a) Lime juice
Explanation: Other are alkaline.

20. In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus (Figure 2.1) was set up. Which among the following statement(s) is(are) correct?
(i) Bulb will not glow because electrolyte is not acidic
(ii) Bulb will glow because NaOH is a strong base and furnishes ions for conduction.
(iii) Bulb will not glow because circuit is incomplete
(iv) Bulb will not glow because it depends upon the type of electrolytic solution
(a) (i) and (iii)
(b) (ii) and (iv)
(c) (ii) only
(d) (iv) only

Ans. (c) (ii) only


21. Which of the following is used for dissolution of gold?
(a) Hydrochloric acid
(b) Sulphuric acid
(c) Nitric acid
(d) Aqua regia
Ans. (d) Aqua regia
Explanation: Gold is a noble metal and does not react with even concentrated acids.
Aqua regia is made by mixing nitric acid and hydrochloric acid in 1:3 ratio. It can dissolve even gold and platinum.

22. Which of the following is not a mineral acid?
(a) Hydrochloric acid
(b) Citric acid
(c) Sulphuric acid
(d) Nitric acid
Ans. (b) Citric acid
Explanation: It is an organic acid.

23. Which among the following is not a base?
(a) NaOH
(b) KOH
(c) NH₄OH
(d) C₂H₅OH
Ans. (d)C₂H₅OH
Explanation: It is alcohol

24. Which of the following statements is not correct?
(a) All metal carbonates react with acid to give a salt, water and carbon dioxide
(b) All metal oxides react with water to give salt and acid
(c) Some metals react with acids to give salt and hydrogen
(d) Some non-metal oxides react with water to form an acid
Ans. (b) All metal oxides react with water to give salt and acid
Explanation: Metal oxides are basic in nature. They give alkaline solution when dissolved in water.

25. Match the chemical substances given in Column (A) with their appropriate application given in Column (B)


(a) A�(ii), B�(i), C�(iv), D�(iii)
(b) A�(iii), B�(ii), C�(iv), D�(i)
(c) A�(iii), B�(iv), C�(i), D�(ii)
(d) A�(ii), B�(iv), C�(i), D�(iii)
Ans. (c) A-(iii), B-(iv), C-(i), D-(ii)
Explanation: Bleaching powder has got its name because of bleaching (decolourizing) property. Baking soda is used as antacid (as common household remedy). Sodium chloride (common salt) is used for making various chemicals through chloralkali process.



26. Equal volumes of hydrochloric acid and sodium hydroxide solutions of same concentration are mixed and the pH of the resulting solution is checked with a pH paper. What would be the colour obtained? (You may use colour guide given in Figure 2.2)

(a) Red
(b) Yellow
(c) Yellowish green
(d) Blue
Ans. (c) Yellowish green
Explanation: Neutralisation reaction takes place when hydrochloric acid and sodium hydroxide are mixed


27. Which of the following is(are) true when HCl (g) is passed through water?
(i) It does not ionise in the solution as it is a covalent compound.
(ii) It ionises in the solution.
(iii) It gives both hydrogen and hydroxyl ion in the solution.
(iv) It forms hydronium ion in the solution due to the combination of hydrogen ion with water molecule
(a) (i) only
(b) (iii) only
(c) (ii) and (iv)
(d) (iii) and (iv)
Ans. (c) (ii) and (iv)
Explanation: Any acid produces hydrogen ion (H+) which is present as hydronium ion(H₃O⁺) because of combination with water molecule.

28. Which of the following statements is true for acids?
(a) Bitter and change red litmus to blue
(b) Sour and change red litmus to blue
(c) Sour and change blue litmus to red
(d) Bitter and change blue litmus to red
Ans. (c) Sour and change blue litmus to red

29. Which of the following are present in a dilute aqueous solution of hydrochloric acid?
(a) H₃O⁺ + Cl⁻
(b) H₃O⁺ + OH⁻
(c) Cl⁻  + OH⁻
(d) unionised HCl
Ans. (a) H₃O⁺ + Cl

Explanation: Any acid produces hydrogen ion (H+) which is present as hydronium ion(H₃O⁺) because of combination with water molecule.

Column (A)	Column (B)
(a) Lactic acid	(i) Tomato
(b) Acetic acid	(ii) Lemon
(c) Citric acid	(iii) Vinegar
(d) Oxalic acid	(iv) Curd

Ans. (a)→ (iv)
(b) → (iii)
(c)→ (ii)
(d)→ (i)

Column (A)	Column (B)
(a) Plaster of Paris	(i) Ca(OH)2
(b) Gypsum	(ii) CaSO .1/2 H O
(c) Bleaching Powder	(iii) CaSO4.2H2O
(d) Slaked Lime	(iv) CaOC12

Ans. (a)→ (ii)
(b)→ (iii)
(c)→ (iv)
(d)→ (i)

Substance	Action on Litmus paper
Dry HCl gas	No change
Moistened NH3 gas	Turns red to blue
Lemon juice	Turns blue to red
Carbonated soft drink	Turns blue to red
Curd	Turns blue to red
Soap solution	Turns red to blue

Methanoic acid is present in ant sting. Its chemical formula is HCOOH. When baking soda is rubbed on the affected part, it gives relief from pain

When nitric acid is added to egg shell, it dissolves egg shell. Egg shell is composed of calcium carbonate which dissolves in nitric acid. The reaction between nitric acid and calcium carbonate yields calcium nitrate and carbon dioxide gas. 

CaCO₃ + HNO₃ → Ca(NO₃)₂ + CO₂ + H₂O

The student can use other indicator; like phenolphthalein or methyl orange to check the acidity or alkalinity of a particular solution. The student can also use turmeric powder to check the solutions. It is mentioned that litmus paper is not available. So, we can assume that the laboratory may have other indicators.

When baking powder is heated, carbon dioxide is produced. This can be shown by following equation.

2NaHCO₃ +  Heat → Na₂CO₃ + CO₂ + H₂O 

Evolution of carbon dioxide can be confirmed by passing the gas into lime water. if lime water turns milky, it indicates evolution of carbon dioxide gas. This reaction does not happen when washing soda is heated.

Salt A is sodium hydrogen carbonate (baking soda). When it is heated, it turns into sodium carbonate; which is salt B. In this reaction, carbon dioxide gas is also produced; which is the gas C.

2NaHCO₃ +  Heat → Na₂CO₃ + CO₂ + H₂O 

Sodium hydrogen carbonate is used in bakery products. Sodium carbonate is used for removing hardness of water.

Sodium chloride is subjected to chloralkali process to manufacture sodium hydroxide. Chlorine and hydrogen gas are formed during this process.

2NaCl + 2H₂O → 2NaOH +Cl₂ +H₂

Chlorine reacts with lime water to give bleaching powder.

Ca(OH)₂ + Cl₂ → CaOCl₂ +H₂O 

X is chlorine. Y is bleaching powder.

ANS

Acids which get completely ionized in aqueous solution are called strong acids, e.g. hydrochloric acid, nitric acid and sulphuric acid. Acids which do not get completely ionized in aqueous solution are called weak acids, e.g. acetic acid, citric acid, and formic acid.

Following reaction takes place when zinc is treated with a dilute solution of a strong acid.

Zn +H₂SO₄→ ZnSO₄ + H₂ 

In this reaction, hydrogen gas is produced; which is utilized in the hydrogenation of oil. When a burning matchstick is brought near hydrogen gas, the matchstick burns with a pop sound. This confirms the evolution of hydrogen gas.