PYQs Acids, Bases and Salts - II

Also read: Mind Map: Acids, Bases & Salts

Previous Year Questions 2016

Q1: Answer the following questions: (CBSE 2016)
(i) State the colour of phenolphthalein in soap solution.
(ii) Name the by-product of chlor-alkali process which is used for the manufacture of bleaching powder.
(iii) Name one indicator which specifies the Various levels of H+ ion concentration.

Solution:

Ans:
(i) Phenolphthalein in soap solution turns pink.
(ii) Chlorine gas (Cl₂) is the by-product of the chlor-alkali process used to manufacture bleaching powder.
(iii) One indicator that shows various levels of H+ ion concentration is litmus.

Q2: On passing excess carbon dioxide gas through lime water, it first turns milky and then becomes colourless. Explain why? Write all the chemical equations of the reactions involved. (CBSE 2016)

Solution:

Ans:

When excess carbon dioxide is passed through lime water, the following occurs:
The lime water initially turns milky due to the formation of calcium carbonate (CaCO3).
With further addition of carbon dioxide, the solution becomes colourless as calcium bicarbonate (Ca(HCO3)2) is formed, which is soluble in water.
The chemical reactions involved are:
Ca(OH)2 + CO2 → CaCO3(s) + H2O
CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2

Q3: Name of type of chemical reaction represented by the following equation : (CBSE 2016)
(i) CaO + H2O → Ca(OH)2
(ii) 3BaCl2 + Al2 (SO4)3 → 2AlCl3 + 3BaSO4

Solution:

Ans:
(i) Combination reaction.
(ii) Double displacement reaction.
(iii) Decomposition reaction.

Q4: (a) Define olfactory indicators. Name two substances which can be used as olfactory indicators.
(b) Choose strong acids from the following :
CH3COOH, H2SO4, H2CO3, HNO3 (CBSE 2016)

Solution:

Ans: (a)

Indicators which work on the basis of odour are called olfactory indicators.
Onion and vanilla extracts can be used as olfactory indicators.

(b) H2SO4 and HNO3 are strong acids out of the four acids given.

Q5: (a) The soil in a field is highly acidic. List any two materials which can be added to this soil to reduce its acidity. Give the reason for your choice. (CBSE 2016)
(b) A gas produced in the laboratory is highly soluble in water. Its colourless solution turns pink when a few drops of phenolphthalein is added to it. What is the nature of this gas?

Solution:

Ans: (a) To reduce the acidity of soil, the following materials can be added:

Lime, (CaO) can be added to neutralize acidity.
Chalk, (CaCO3) can also be added to neutralise acidity.

It is because CaO and CaCO3are basic in nature which neutralize acid present in soil.

(b) The nature of gas is basic because it turns phenolphthalein pink. The following reaction takes place in aqueous solution,

Example: NH3 + H2O → NH4+ + OH-

Q6: (a) The pH of soil A is 7.5 while that of soil B is 4.5. Which of the two soils A or B should be treated with powdered chalk to adjust its pH and why? (CBSE 2016)
(b) Name the chemical which is injected into the skin of a person:
(i) During an ant's sting
(ii) During the nettle leaf sting.
How can the effect of these stings be neutralised?
(c) Explain how the pH change in the river water can endanger the lives of aquatic animals like fish?

Solution:

Ans:
(a) Soil B has a pH of 4.5, making it acidic. It should be treated with powdered chalk, which is basic, to raise its pH.

(b)The chemical injected during stings is:

(i) During an ant's sting: Formic acid (HCOOH).

(ii) During a nettle sting: Formic acid (HCOOH).

The effects of these stings can be neutralised by rubbing the area with a mild base, such as baking soda (NaHCO3).

It may reduce the amount of oxygen dissolved in the water.
Both acidic and basic water can harm the skin of aquatic animals, such as fish.

Q7: State reasons for the following statements:
(i) Stain of curry on a white cloth becomes reddish brown when soap is scrubbed on it and turns yellow again when the cloth in washed with plenty of water.
(ii) Curd should not be kept in copper or brass vessels. What is done to protect it? (CBSE 2016)

Solution:

Ans:
(i)

A stain of curry on a white cloth turns reddish-brown when soap is scrubbed on it because:
Turmeric in the curry reacts with sodium hydroxide in the soap, forming a red compound.
When the cloth is rinsed with plenty of water, the sodium hydroxide becomes very dilute, stopping the reaction and returning the stain to its original yellow colour.

(ii)

Curd should not be stored in copper or brass vessels because:
Curd contains lactic acid, which reacts with these metals, altering the taste.
To prevent this, curd should be kept in glass, steel, or ceramic containers that do not react with lactic acid.

Previous Year Questions 2015

Q1: (a) A student detected the pH of four unknown solutions A, B, C and D as follows: 11, 5, 7 and 2.
Predict the nature of the solution.
(b) Explain how an antacid works. (CBSE 2015)

Solution:

Ans:
(a) A is basic, 'B' is acidic 'C' is neutral and 'D' is strongly acidic.
(b) Hyperacidity is Caused by excess of hydrochloric acid in stomach. Antacid is basic in nature. It neutralises excess of acid and releases CO2 gas which gives relief from pain caused by hyperacidity.

Q2: Explain the action of dilute hydrochloric acid on the following with chemical equations: (CBSE 2015)
(a) Magnesium ribbon
(b) Sodium hydroxide
(c) Crushed egg shells

Solution:

Ans:
(a) Hydrogen gas will be formed, e.g.
Mg (s) + 2HCl (dil) → MgCl2 (aq) + H2 (s)
(b) Sodium chloride and water will be formed, e.g.
NaOH + HCl → NaCl + H2O (Neutralization reaction)
(c) Crushed egg shells are made up of CaCO3 which reacts with dil HCl to give brisk effervescence due to CO2, e.g.
CaCO3 (s) + 2HCl → CaCl2 + H2O + CO2

Q3: What is meant by water of crystallisation? How would you show that copper sulphate crystals contain water of crystallisation? (CBSE 2015)

Solution:

Ans:

Water which is present in the crystals of a compound is called water of crystallisation.
It is defined as the fixed number of water molecules present in one formula unit of a salt.
Copper sulphate crystals are blue in colour.
On heating solid copper sulphate crystals, they lose water of crystallisation and become white.
On adding a few drops of water to the white powder, blue colour of copper sulphate is restored.
This shows that copper sulphate crystals contain water of crystallisation.

Q4: (a) Three acidic solutions A, B and C have p H = 0, 3 and 5 respectively.
(i) Which solution has highest concentration of H+ ions?
(ii) Which solution has the lowest concentration of H+ ions?
(b) How concentrated sulphuric acid can be diluted? Describe the process. (CBSE 2015)

Solution:

Ans:
(a) (i) Solution A has the highest concentration of H+ ions.
(ii) Solution C has the lowest concentration of H+ ions.
(b) To dilute concentrated sulphuric acid:

Always add the acid to water, not the other way around.
Start by pouring some water into a beaker.
Gradually add concentrated sulphuric acid in small amounts to the water.
Stir the mixture continuously with a glass rod.
If the beaker gets hot, cool it by placing it in cold water.

Q5: Give the chemical name and formula of bleaching powder. What happens when it is exposed to air? Mention two uses of bleaching powder. (CBSE 2015)

Solution:

Ans: Chemical name of bleaching powder is calcium oxychloride.
Formula: CaOCl2 . On exposure to air, it absorbs moisture.
Uses:
(i) As an oxidising agent in chemical industries.
(ii) For disinfecting drinking water to make it free from germs.

Q6: (a) What is pH value of salt formed by a
(i) Weak acid and strong base?
(ii) Strong acid and strong base?
(b) 15 mL of water and 10 ml of sulphuric acid are to be mixed in a beaker
(i) State the method that should be followed with reason.
(ii) What is this process called? (CBSE 2015)
(c) What is observed when sulphur dioxide is passed through
(i) water (ii) lime water?
Also write chemical equations for the reactions that take place.

Solution:

Ans:
(a) (i) The pH value will be greater than 7.
(ii) The pH value will be less that than 7.
(b) (i) When mixing, the acid should be added slowly to water. This prevents splashing and allows for constant cooling since the reaction is highly exothermic.

(ii) This process is known as dilution.

(c) (i) Sulphurous acid is formed.
SO2 + H2O → H2SO3 (Sulphurous acid)

Lime water turned turns milky due to formation of calcium sulphite.

Q7: (a) State the chemical properties on which the following uses of baking soda are based:
(i) as an antacid
(ii) as soda-acid fire extinguisher
(iii) to make bread and cake soft and spongy.
(b) How washing soda is obtained from baking soda? Write balanced chemical equation. (CBSE 2015)

Solution:

Ans:
(a) (i) It is weakly basic in nature and neutralises hyperacidity.
(ii) It liberates CO2 with H2SO4 reacts with it, which extinguisher fire.
(iii) It liberates CO2 on heating which makes bread and cake soft and sponge.
(b) Baking soda on heating gives sodium carbonate which on crystallisation from aqueous solution gives washing soda, e.g.
Na2CO3 + 10H2O → Na2CO3.10H2O

Q8: State reason for the following statements: (CBSE 2015)
(a) Tap water conducts electricity whereas distilled water does not.
(b) Dry hydrogen chloride gas does not turn blue litmus red whereas dilute hydrochloric acid does.
(c) During summer season, a milk man usually adds a very small amount of baking soda to fresh milk.
(d) For a dilution of acid, acid is added into water and not water into acid.
(e) Ammonia is a base but does not contain hydroxyl group.

Solution:

Ans:

(a) Tap water conducts electricity because it contains ions that facilitate electrical flow, while distilled water lacks these ions.

(b) Dry hydrogen chloride (HCl) gas does not turn blue litmus red because it does not form ions. In contrast, dilute hydrochloric acid releases H and Cl- ions, which are responsible for its acidic properties.

(c) A milkman adds a small amount of baking soda to fresh milk to prevent it from souring. Baking soda inhibits the formation of lactic acid, which causes milk to spoil.

(d) When diluting an acid, it is crucial to add acid to water rather than the other way around. This is because adding water to acid can cause a violent reaction and splashing due to the heat generated.

(e) Ammonia is classified as a base despite not containing a hydroxyl group. When dissolved in water, it produces hydroxide ions (OH-), which gives it basic properties. NH3 + H2O → NH4+ + OH-

Q9: (a) Define universal indicator. Mention its one use.
(b) Solution A gives pink colour when a drop of phenolphthalein indicator is added to it. Solution B gives red colour when a drop of methyl orange is added to it. What type of solutions are A and B and which one of the solutions A and B will have a higher pH value?
(c) Name one salt whose solution has pH more than 7 and one salt whose solution has pH less than 7. (CBSE 2015)

Solution:

Ans: (a) A Universal indicator is a mixture of several indicators. It shows different colours at different concentrations of H+ ion concentrations.

(b) Solution A is alkaline in nature.
Solution B is acidic in nature.
Solution A has a higher pH value.
(c) Salts of weak acid and strong base give a solution which have pH more than 7. i.e. basic salt Sodium acetate is an example of such salts. Salts of strong acid and weak base give a solution which has pH less than 7. i.e. Acidic salt. Ammonium chloride is an example of such salts.