Acids, Bases and Salts

Acids taste sour and turn blue litmus red, while bases taste bitter, feel soapy and turn red litmus blue. In this chapter you will test substances with natural indicators such as litmus and turmeric, synthetic indicators such as methyl orange and phenolphthalein, and olfactory indicators such as onion and clove oil. You will study how acids react with metals to release hydrogen gas, with metal carbonates and hydrogen carbonates to release carbon dioxide, and with bases in neutralisation reactions that form a salt and water. You will learn that acids produce H+ (H3O+) ions in water and bases produce OH- ions. The pH scale, from 0 to 14, measures the strength of acidic and basic solutions, and pH matters in digestion, tooth decay, soil and insect stings. Finally, you will study important salts: common salt, and the chemicals made from it, namely sodium hydroxide by the chlor-alkali process, bleaching powder, baking soda and washing soda, along with plaster of Paris and the water of crystallisation in salts.

Detailed Worksheet: Acids, Bases and Salts Section A - Definitions (10 marks) 1. Define acids and bases in terms of the ions they produce in aqueous solution. Give two examples of each. (2 marks) 2. What are indicators? Name one natural, one synthetic and one olfactory indicator. (2 marks) 3. What is a neutralisation reaction? Write a balanced equation for one such reaction. (2 marks) 4. Define pH. What does a pH of 7, less than 7 and more than 7 indicate? (2 marks) 5. What is meant by water of crystallisation? Write the chemical formula of washing soda and of hydrated copper sulphate. (2 marks) Section B - Calculations and Applications (15 marks) 6. Five solutions A, B, C, D and E show pH values of 4, 1, 11, 7 and 9 respectively. Which solution is (i) neutral (ii) strongly alkaline (iii) strongly acidic (iv) weakly acidic (v) weakly alkaline? Arrange them in increasing order of hydrogen ion concentration. (3 marks) 7. Write balanced chemical equations for the reactions of: (i) zinc with dilute sulphuric acid (ii) sodium carbonate with dilute hydrochloric acid (iii) carbon dioxide with lime water. (3 marks) 8. When 8.4 g of sodium hydrogencarbonate is heated strongly, it decomposes into sodium carbonate, water and carbon dioxide. Write the balanced equation and calculate the mass of sodium carbonate and of carbon dioxide formed. (Na = 23, H = 1, C = 12, O = 16) (3 marks) 9. Calculate the percentage of water of crystallisation in washing soda, Na2CO3.10H2O. (Na = 23, C = 12, O = 16, H = 1) (3 marks) 10. How many grams of hydrochloric acid are needed to neutralise 4 g of sodium hydroxide completely? Write the balanced equation. (Na = 23, O = 16, H = 1, Cl = 35.5) (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the activity in which zinc granules react with dilute sulphuric acid in a test tube, the gas evolved passes through a delivery tube into soap solution, and the bubbles are tested with a burning candle. State the observation. (4 marks) 12. Draw a labelled diagram of the setup used to show that acid solutions conduct electricity: a beaker with dilute HCl, two nails fixed in a cork, a bulb and a 6 V battery. State why the bulb glows. (3 marks) 13. Draw a labelled diagram to show the action of dilute hydrochloric acid on sodium carbonate, with the gas passed through lime water. Write the equation for the change in lime water. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. A milkman adds a very small amount of baking soda to fresh milk. (i) Why does he shift the pH of fresh milk from 6 to slightly alkaline? (ii) Why does this milk take longer to set as curd? Then explain why tooth decay starts when the pH of the mouth falls below 5.5, and how toothpaste prevents it. (5 marks) 15. Explain why one should always add acid to water and not water to acid while diluting a concentrated acid. What happens to the concentration of H3O+ ions on dilution? Why do HCl and alcohol solutions differ in conducting electricity, even though both contain hydrogen? (5 marks) 16. A compound X of calcium is used for plastering fractured bones. When mixed with water it sets into a hard solid Y. (i) Identify X and Y and write their formulae. (ii) Write the equation for the preparation of X from Y and the condition required. (iii) Why should X be stored in a moisture-proof container? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Write balanced chemical equations with state symbols wherever possible. Draw all diagrams neatly in pencil and label every part.