Unit Test (Solutions): Acid, Bases & Salts | Science Class 10 PDF Download

Time: 1 hour

M.M. 30

Attempt all questions.

• Question numbers 1 to 5 carry 1 mark each.
• Question numbers 6 to 8 carry 2 marks each.
• Question numbers  9 to 11 carry 3 marks each.
• Question number 12 & 13 carry 5 marks each

Q1: Which of the following substances is an acid?  (1 Mark)
(a) NaOH
(b) HCl
(c) NH₃
(d) KOH
Ans: (b)
Among the given options, HCl (Hydrochloric acid) is the only substance that is an acid. NaOH (Sodium hydroxide) and KOH (Potassium hydroxide) are both bases, while NH3 (Ammonia) is a weak base.

Q2: The pH value of a neutral solution is:  (1 Mark)
(a) 0
(b) 7
(c) 14
(d) 10
Ans: (b)
The pH value of a neutral solution is 7. A pH value of 7 indicates that the solution is neither acidic nor basic but is neutral.

Q3: Which of the following is a natural indicator?  (1 Mark)
(a) Phenolphthalein
(b) Litmus
(c) Methyl orange
(d) Universal indicator
Ans: (b)
Litmus is a natural indicator obtained from lichens. It turns red in acidic solutions and blue in basic solutions. Phenolphthalein, Methyl orange, and Universal indicator are synthetic indicators.

Q4: Write the chemical equation for the reaction between an acid and a metal.  (1 Mark)
Ans: The chemical equation for the reaction between an acid (HCl) and a metal (Zn) is:
HCl + Zn → ZnCl₂ + H₂

Q5: Name one common substance that can be used to neutralize the excessive acidity in the stomach.  (1 Mark)
Ans: One common substance that can be used to neutralize excessive acidity in the stomach is "Sodium bicarbonate" (baking soda).

Q6: Identify the salt and the acid used in the reaction to prepare the salt: Calcium chloride. Write the chemical equation for the reaction. (2 marks)
Ans: The salt is Calcium chloride, and the acid used in the reaction to prepare it is Hydrochloric acid (HCl).
Chemical equation: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O

Q7: A student added a few drops of phenolphthalein to a colourless solution taken in a test tube. The solution turned pink. Predict whether the solution is acidic or basic, and write a balanced chemical equation for the reaction that might have occurred. (2 marks)
Ans: The solution is basic.
The balanced chemical equation for the reaction that occurred can be:
NaOH + H₂O → Na+(aq) + OH-(aq) + H₂O

Q8: What happens when an antacid is taken after overeating? How does it provide relief? (2 marks)
Ans: When an antacid is taken after overeating, it neutralizes the excess acid present in the stomach.
Overeating can lead to an increase in stomach acid, causing discomfort and acidity. Antacids contain bases, such as aluminum hydroxide or magnesium hydroxide, which react with the excess acid in the stomach. This reaction produces salt and water, reducing the acidity of the stomach contents and providing relief from heartburn and indigestion.

Q9: Explain the process of neutralization. Why is the reaction mixture not acidic or basic at the end point of a neutralization reaction? (3 marks)
Ans: Neutralization is a chemical reaction that occurs between an acid and a base, resulting in the formation of salt and water. During this reaction, the H+ ions from the acid combine with the OH- ions from the base to form water, leaving behind the salt.
The general chemical equation for neutralization can be written as:
Acid + Base → Salt + Water
At the end point of a neutralization reaction, the reaction mixture is neither acidic nor basic. This is because the number of moles of H+ ions from the acid is equal to the number of moles of OH- ions from the base. As a result, all the acidic and basic properties get neutralized, leading to the formation of neutral water and a salt.

Q10: Why should we not touch the chemicals used for tasting or testing their reaction for acidity or basicity? Write the general precautions one should follow while performing a chemistry experiment in the laboratory.  (3 marks)
Ans: We should not touch the chemicals used for tasting or testing their reaction for acidity or basicity because some of these chemicals may be toxic or harmful to the skin. Additionally, they may cause skin irritation or allergic reactions.
General precautions to follow while performing a chemistry experiment in the laboratory:

• Wear appropriate safety gear, such as lab coats, safety goggles, and gloves, to protect against chemical spills and splashes.
• Do not taste or smell any chemicals directly, as some substances may be toxic or harmful.
• Handle chemicals with care, and follow the instructions given by the teacher or in the lab manual.
• Always use clean and dry glassware to avoid contamination of the chemicals.
• Dispose of chemicals and waste properly according to the guidelines provided by the teacher or lab supervisor.
• Keep the work area clean and organized to prevent accidents.
• Do not perform experiments without the presence of a teacher or lab supervisor.
• Keep emergency equipment, such as eyewash stations and fire extinguishers, accessible in the laboratory.

Q11: How can you distinguish between baking powder and washing soda by heating? Write the chemical reactions involved. (3 marks)
Ans: By heating, both baking powder and washing soda undergo decomposition reactions, but they produce different products.
When baking powder is heated, it undergoes thermal decomposition to produce carbon dioxide gas, water, and a residue.
Chemical reaction for heating baking powder:

2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O
On the other hand, when washing soda is heated, it loses its water of crystallization and forms anhydrous sodium carbonate.
Chemical reaction for heating washing soda:
Na₂CO₃ · 10H₂O → Na₂CO₃ + 10H₂O
Thus, by heating, baking powder produces carbon dioxide gas, while washing soda loses water of crystallization and becomes anhydrous sodium carbonate.

Q12: Explain the action of the following:
(i) Lime water on blue litmus paper
(ii) Dilute hydrochloric acid on a piece of blue litmus paper (5 marks)
Ans:  (i) Lime water on blue litmus paper:
Lime water is a solution of calcium hydroxide (Ca(OH)2). When lime water is added to blue litmus paper, the litmus paper remains blue, indicating that lime water is a base. Lime water reacts with carbon dioxide in the air to form calcium carbonate, which is slightly soluble in water. The formation of calcium carbonate gives a milky appearance to lime water.
Chemical reaction with carbon dioxide in the air:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
The formation of calcium carbonate (CaCO3) causes the milky appearance of lime water.
(ii) Dilute hydrochloric acid on a piece of blue litmus paper:
Dilute hydrochloric acid (HCl) is an acid. When a piece of blue litmus paper is dipped in dilute HCl, the litmus paper turns red. This color change indicates the acidic nature of the hydrochloric acid. The red coloration is due to the hydrogen ions (H+) present in the acid, which react with the blue litmus to produce the red color.

Q13: Describe the process of the formation of a curd from milk. Write the chemical equation for the reaction that takes place during the curdling of milk. (5 marks)
Ans: Process of the formation of curd from milk:

The formation of curd from milk is a result of the fermentation of lactose (milk sugar) present in milk. The process involves the action of bacteria, specifically Lactobacillus, on milk. Lactobacillus is a type of lactic acid bacteria.

Steps involved in the formation of curd:

• Milk is first heated to a specific temperature to kill any harmful bacteria present in it and to denature the proteins.
• The milk is then allowed to cool to a suitable temperature (around 30-40°C).
• A small amount of curd or curd culture containing Lactobacillus bacteria is added to the warm milk.
• The Lactobacillus bacteria ferment the lactose present in the milk into lactic acid.
• The lactic acid lowers the pH of the milk, causing it to coagulate or solidify.
• As a result, the milk proteins, mainly casein, undergo a conformational change and come together to form a network-like structure, trapping the liquid component within it. This solidified mass is called curd.

Chemical equation for the fermentation of lactose in milk:

C₁₂H₂₂O₁₁ (lactose) + H₂O → 4C₃H₆O₃ (lactic acid)
The lactic acid produced during the fermentation process curdles the milk and gives it a tangy taste.