Class 10 Science Chapter 2 Case Based Questions - Acids, Bases and Salts

Case Study - 1

A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for ‘potenz’ in German, meaning power. On the pH scale we can measure pH generally from 0 (very acidic) to 14 (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution. Higher the hydronium ion concentration, lower is the pH value.

Answer the following on the basis of above paragraph:
Q1: What does the scale represent when pH value increases from 7 to 14?
Ans: It represents an increase in OH- ions concentration in the solution i.e. the increment in the strength of alkali.

Q2: What is the pH value of milk of magnesia?
Ans: It is 10.

Q3: What are the important of pH in everyday life?
Ans: We human beings, plants and animals all are sensitive to pH i.e. their body  work on normal pH such as plants grow between the pH range of 6 to 8. Our human body work within the pH range of 7 to 7.8.

Q4: What happens when the pH of mouth is lower than 5.5?
Ans: Tooth decay starts in which the enamel gets corroded due to the much production of acids in mouth by bacteria.

Q5: Two solutions X&Y. The pH of X is 4 and the pH of Y is 7. What is the nature of two solution?
Ans: Solution X is acidic in nature and the solution Y is neutral in nature.

Case Study - 2

Salts of a strong acid and a strong base are neutral with pH value of 7. On the other hand, salts of a strong acid and weak base are acidic with pH value less than 7 and those of a strong base and weak acid are basic in nature, with pH value more than 7.

Answer the following in reference to the above paragraph:
Q1: Classify the following as strong bases and weak bases: KOH, NaOH, CsOH, NH4OH
Ans: Strong bases: KOH, NaOH, CsOH
Weak base: NH₄OH

Q2: Write a reaction of a strong acid and a weak base?
Ans: HCl(aq) + NH₄OH(aq) → NH₄Cl(aq) + H₂O(aq)

Q3: What happens when strong acids and bases react to each other? Explain by giving example.
Ans: HCl + NaOH → NaCl + H₂O i.e. neutral salt is formed.

Q4: Identify the following as strong acid: CH₃COOH, H₂SO₄, HNO₃, H₃PO₄, H₂CO₃, HCl.
Ans: Strong acid: HCl, HNO₃,   H₂SO₄.

Q5: Classify the following acis as strong or weak acid: acetic acid, citric acid, tartaric acid, oxalic acid.
Ans: All are weak acid present in  fruits and vegetables.

Case Study - 3

Sodium hydroxide When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because of the products formed– chlor for chlorine and alkali for sodium hydroxide.

Based on the above given information, answer the following questions:
Q1: Write the chemical equation involved in this process?
Ans: 2NaCl (aq)+ 2H₂O(l) → 2 NaOH(aq) + Cl₂ (g) + H₂(g)

Q2: What are the substance that are formed at anode and cathode on chlor- alkali process?
Ans: At anode Chlorine gas & at cathode hydrogen gas are formed.

Q3: What are the uses of chlorine?
Ans: 

• Used for water treatment
• Disinfectants
• PVC
• pesticides.

Q4: Where does the sodium hydroxide solution is formed?
Ans: It is formed near the cathode.

Q5: What are the uses of Sodium hydroxide?
Ans:

• uses in making soaps and detergents
• artificial fibres
• paper making

Case Study - 4

Plaster of Paris
On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate ( CaSO₄ .½ H₂O ). This is called Plaster of Paris. Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass. Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. Five water molecules are present in one formula unit of copper sulphate. Chemical formula for hydrated copper sulphate is Cu SO₄ . 5H₂O. Now you would be able to answer the question whether the molecule of Na2CO3 .10H2O is wet.

Answer the following questions on the basis of the above paragraph:
Q1: What is the molecular formula of gypsum?
Ans: CaSO₄. 2H₂O

Q2: Write the equation of formation of plaster of paris by heating gypsum?
Ans: CaSO₄. 2H₂O + heat ⇒ CaSO₄. 1/2 H₂O + 1^1/2 H₂O

Q3: What are the uses of Plaster of Paris?
Ans: It is used by doctor for supporting of fractured bones, to make toys etc.

Q4: Give the equation when POP is mixed with water?
Ans: CaSO4. 1/2H2O + 1^1/2 H2O ⇒ CaSO4. 2H2O

Q5: What does this 2 denotes in CaSO₄. 2 H₂O?
Ans: 2 denotes the two water molecules as water of crystallisation.

Case Study - 5

Take solutions of glucose, alcohol, hydrochloric acid, sulphuric acid, etc. n Fix two nails on a cork, and place the cork in a 100 mL beaker. Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in. Now pour some dilute HCl in the beaker and switch on the current. Repeat with dilute sulphuric acid. What do you observe?  Repeat the experiment separately with glucose and alcohol solutions. What do you observe now? Does the bulb glow in all cases?

Following the above paragraph, answer the following questions;
Q1: What was the changes occur in case of acids i.e HCl, H2SO4?
Ans: The bulbs will start glowing as it contains hydrogen ions H+ ions (aq) as cation and Cl- or SO4^2- as anion.

Q2: Why do glucose and alcohol do not conduct electricity?
Ans: They do not contains free ions neither cation nor anion. To conduct electricity, free ions are required.

Q3: Why do acids do not show acidic behaviour in absence of water?
Ans: Acidic behaviour are shown by releasing of H+ ions from acids. To dissociate into H+ ions, the acids need medium i.e water.

Q4: Does rain water or distilled water will conduct electricity?
Ans: Rain water will conduct electricity as it contains both positive and negative ions of different salts in it.

Q5: Why do aqueous solution of acids conduct electricity?
Ans: The acid contains Hydrogen ions in solutions as well as anions. Due to the presence of free ions they conduct electricity.

Case Study - 6

The reaction between carbon dioxide and calcium hydroxide (lime water), Calcium hydroxide, which is a base, reacts with carbon dioxide to produce a salt and water. Since this is similar to the reaction between a base and an acid, we can conclude that nonmetallic oxides are acidic in nature.

Based on the above paragraph answer the following questions:
Q1: What is the nature of Carbon dioxide?
Ans: It is a non- metallic oxide as carbon belongs to non- metals group i.e P – Block elements group 6.

Q2: Give another reaction of non- metallic oxide and a base?
Ans: CO₂(g) + 2NaOH(aq)→ Na₂CO₃ (aq) + H₂O(aq)

Q3: Arrange the following bases in increasing order: NaOH, Ca(OH)₂ & Mg(OH)₂.
Ans: Mg(OH)₂ < Ca(OH)₂ < NaOH.

Q4: Write the complete reaction between calcium hydroxide and carbon dioxide with physical states?
Ans: Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O(l)

Q5: What is the nature of non- metallic oxide?
Ans: The non- metallic oxide are acidic in nature because when they dissolved in water, they form acidic substance turning blue litmus into red.