Vidaara.orgClass 10 · Chemistry
CodeVID-C10-02-T2-01
pH Scale & its Importance — Assignment
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Give reasons for your answers wherever asked.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
A solution turns phenolphthalein pink. Its pH is most likely:
- A.2
- B.5
- C.7
- D.10
2.
Which of these is a strong acid?
- A.acetic acid
- B.carbonic acid
- C.hydrochloric acid
- D.citric acid
3.
The pH of a strongly basic solution is closest to:
- A.1
- B.5
- C.7
- D.13
4.
Methyl orange in an acidic solution is:
- A.yellow
- B.red
- C.pink
- D.colourless
5.
Acidic soil is best treated by adding:
- A.vinegar
- B.lime (CaO)
- C.common salt
- D.sugar
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Distinguish between a strong acid and a weak acid with one example each.
7.
Why is 'strong' not the same as 'concentrated'?
8.
State two ways pH is important in daily life.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
Explain how indicators help us identify acids and bases, naming three indicators and their colours.
10.
A bee sting causes pain because it injects an acid. Explain the cause and the remedy in terms of pH.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Describe the importance of pH in (a) the digestive system, (b) tooth decay, (c) plants and soil, and (d) survival of aquatic life, suggesting one corrective measure for each where relevant.
Answer Key
Section A — Multiple Choice Questions
- (D) 10
- (C) hydrochloric acid
- (D) 13
- (B) red
- (B) lime (CaO)
Section B — Short Answer (2 marks)
- A strong acid ionises completely in water and gives many H+ ions (e.g. HCl); a weak acid ionises only partly and gives fewer H+ ions (e.g. acetic acid).
- Strength refers to the degree of ionisation, while concentration refers to the amount dissolved per unit volume; a dilute strong acid can still ionise fully and a concentrated weak acid may ionise only partly.
- The stomach uses HCl at low pH for digestion (excess is treated with an antacid), and tooth enamel decays below pH 5.5 so a basic toothpaste neutralises mouth acid.
Section C — Short Answer (3 marks)
- Indicators change colour with pH. Litmus is red in acid and blue in base; methyl orange is red in acid and yellow in base; phenolphthalein is colourless in acid and pink in base. By matching the colour, we can tell whether a solution is acidic or basic.
- The bee injects an acidic liquid (containing methanoic acid), lowering the pH of the skin and causing irritation. Rubbing a mild base such as baking soda (sodium hydrogencarbonate) neutralises the acid, raises the pH and gives relief.
Section D — Long Answer (5 marks)
- (a) Digestion: the stomach secretes HCl (pH ~1.5-2) to digest food; excess acid causes acidity, treated with an antacid like Mg(OH)2. (b) Tooth decay: bacteria make acid from sugar; below pH 5.5 enamel dissolves, so a basic toothpaste neutralises the acid. (c) Plants and soil: crops grow best near neutral pH; acidic soil is treated with lime (CaO) and alkaline soil with gypsum or organic matter. (d) Aquatic life: river/pond water must stay near neutral; acid rain lowers the pH and harms fish, so pollution must be controlled. Maintaining the right pH range protects health, teeth, crops and aquatic organisms.
Generated by Vidaara.org · Assignment VID-C10-02-T2-01 · vidaara.org