Acids, Bases and Salts

Substances that taste sour, like lemon juice, vinegar and curd, contain acids; examples are citric acid in oranges, acetic acid in vinegar, lactic acid in curd, tartaric acid in tamarind and formic acid in an ant's sting. Substances that taste bitter and feel soapy, like baking soda, soap solution and lime water, are bases. Since tasting chemicals is dangerous, we use indicators, which change colour in acids and bases. Litmus, obtained from lichens, turns blue litmus red in an acid and red litmus blue in a base. Turmeric turns red in a base, and china rose solution turns acids dark pink and bases green. When an acid and a base are mixed, they neutralise each other, forming a salt and water and releasing heat: acid + base -> salt + water + heat. Neutralisation has many uses. An antacid such as milk of magnesia relieves indigestion, baking soda or calamine soothes an ant bite, acidic soil is treated with quicklime or slaked lime, and acidic factory wastes are neutralised before being released into water bodies.

Detailed Worksheet: Acids, Bases and Salts Section A - Definitions (10 marks) 1. Define acids and bases on the basis of taste and their effect on litmus. (2 marks) 2. What is an indicator? Name two natural indicators. (2 marks) 3. What is neutralisation? Write a word equation for it. (2 marks) 4. Name the acid present in (i) curd (ii) vinegar (iii) ant sting (iv) tamarind. (2 marks) 5. What is a neutral substance? Give two examples. (2 marks) Section B - Calculations and Applications (15 marks) 6. Name the base present in (i) lime water (ii) window cleaner (iii) soap (iv) milk of magnesia. State one use of any two. (3 marks) 7. Complete the word equations: (i) hydrochloric acid + sodium hydroxide -> ______ + water (ii) acid + base -> salt + water + ______ (iii) Name the salt formed in (i). (3 marks) 8. A student tested three solutions with red litmus, blue litmus and turmeric. Solution P turned blue litmus red; Q turned red litmus blue and turmeric red; R changed neither litmus. Identify each as acidic, basic or neutral and suggest one example of each. (3 marks) 9. Why is a person suffering from acidity advised to take an antacid? Name one antacid and the type of substance it is. (3 marks) 10. A farmer finds his soil is too acidic. (i) What should he add? (ii) What if it is too basic? (iii) Why is soil treatment needed? (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the activity to test a solution with china rose indicator in test tubes, showing the colour changes in an acid and a base. (4 marks) 12. Draw a flowchart showing the neutralisation of dilute hydrochloric acid with sodium hydroxide using phenolphthalein, including the colour change. (3 marks) 13. Draw a labelled diagram showing how turmeric paper can be made and used to test soap solution. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Paheli was stung by an ant. (i) Which acid did the ant inject? (ii) What should be rubbed on the skin? (iii) Why does it give relief? (iv) Why should we not taste unknown chemicals to identify them? (v) Name one more natural indicator she could use at home. (5 marks) 15. Factory wastes often contain acids. (i) Why should they not be dumped directly into rivers? (ii) What substances are added to neutralise them? (iii) What is formed in the process? (iv) Suggest one way the government can make factories follow this rule. (5 marks) 16. You are given three unlabelled bottles containing distilled water, lemon juice and soap solution, and only turmeric indicator. (i) Which bottle will turn turmeric red? (ii) How will you distinguish the other two using the identified bottle? (iii) Explain the reasoning. (iv) Why is a neutral solution needed to complete the identification? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Never taste chemicals in the laboratory, and write the colour change for every indicator test.