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Chemistry · Practice book · Senior 1 · Unit 8 of 11

Acids, bases and pH

Senior 1 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 8 Acids, bases and pH

Key unit competence: Analyse the properties, uses and effects of acids and bases to explain their role in daily life and ensure safe handling.

Key ideas

  • An acid is a substance that produces hydrogen ions, H+, in water. A base is a substance that neutralises an acid; a base that dissolves in water is an alkali and produces hydroxide ions, OH−.
  • Acids taste sour, turn blue litmus red and react with metals (giving hydrogen) and carbonates (giving carbon dioxide). Alkalis feel soapy, taste bitter and turn red litmus blue.
  • Indicators change colour with acids and alkalis: litmus, phenolphthalein (colourless in acid, pink in alkali), methyl orange (red in acid, yellow in alkali) and natural ones such as red cabbage, hibiscus flowers and beetroot.
  • The pH scale runs from 0 to 14: below 7 is acidic, 7 is neutral, above 7 is alkaline. The lower the pH, the stronger the acid; the higher the pH, the stronger the alkali. Universal indicator gives a colour for each pH.
  • Neutralisation: acid + base → salt + water. Uses: antacids for indigestion, lime (CaO or Ca(OH)2) for acidic soils, toothpaste against acids in the mouth, treating bee stings with a weak alkali (bicarbonate).
  • Common acids: hydrochloric (HCl), sulfuric (H2SO4), nitric (HNO3), ethanoic (vinegar), citric (lemons). Common alkalis: sodium hydroxide (NaOH), calcium hydroxide (limewater), ammonia solution. Concentrated acids and alkalis are corrosive.

Common misconceptions

  • "All acids are dangerous." No: Weak, dilute acids like citric acid in oranges and ethanoic acid in vinegar are eaten safely.
  • "pH 2 is a weaker acid than pH 5 because 2 is smaller." No: A lower pH means a stronger acid.
  • "All bases are alkalis." No: Only bases that dissolve in water are alkalis; copper(II) oxide is a base but not an alkali.

Worked examples

Example 1. Write the word and balanced chemical equations for the neutralisation of nitric acid by potassium hydroxide.

Solution

Word: nitric acid + potassium hydroxide → potassium nitrate + water.

Formulae: HNO3 + KOH → KNO3 + H2O.

Check: K 1, N 1, O 4, H 2 on each side: balanced.

Example 2. Explain how a farmer can find out whether a soil is acidic and how to correct it.

Solution

Mix a small soil sample with distilled water, let it settle and test the liquid with universal indicator or a pH meter.

Compare the colour with the pH chart; pH below 6 is acidic.

Correct it by adding lime (CaO or Ca(OH)2), which neutralises the acid.

Practice questions

  1. Define an acid.

    Show answer

    A substance that produces hydrogen ions (H+) when dissolved in water.

    Common mistake: Saying 'a substance that burns'.

  2. State two properties of alkalis.

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    Soapy feel, bitter taste, turn red litmus blue, pH above 7 (any two).

    Common mistake: Saying alkalis turn blue litmus red.

  3. Name one acid and one alkali found at home.

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    Acid: vinegar (ethanoic acid) or lemon juice (citric acid). Alkali: soap, toothpaste, baking soda, wood-ash water.

    Common mistake: Naming sulfuric acid as a home acid.

  4. What is the pH of pure water?

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    7.

    Common mistake: Saying 0.

  5. Write the word equation for the reaction between magnesium and hydrochloric acid.

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    Magnesium + hydrochloric acid → magnesium chloride + hydrogen.

    Common mistake: Writing water as a product.

  6. What colour is universal indicator in a strong acid?

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    Red.

    Common mistake: Saying blue or purple.

  7. Explain why lime is added to acidic soils.

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    Lime is a base; it neutralises soil acid and raises the pH so crops grow well.

    Common mistake: Saying lime adds acid.

  8. Write the balanced equation for HCl reacting with Ca(OH)2.

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    2HCl + Ca(OH)2 → CaCl2 + 2H2O.

    Common mistake: Writing CaCl as the salt.

  9. Give two dangers of concentrated acids and one precaution.

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    Corrosive to skin and eyes; damage clothes and metals. Precaution: wear goggles and gloves.

    Common mistake: Saying they are only dangerous when hot.

  10. A solution turns methyl orange yellow and phenolphthalein pink. Is it acidic or alkaline? Explain.

    Show answer

    Alkaline: both indicators show the alkaline colour.

    Common mistake: Saying neutral because the two colours differ.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. What gas is produced when an acid reacts with zinc?a. Baking soda (sodium hydrogen carbonate) paste
    2. What gas is produced when an acid reacts with a carbonate?b. Hydrogen
    3. Which substance has a pH greater than 7?c. Carbon dioxide
    4. A bee sting is acidic. Which can be used to relieve it?d. Soap solution
    Show answer

    1 → b (Hydrogen) · 2 → c (Carbon dioxide) · 3 → d (Soap solution) · 4 → a (Baking soda (sodium hydrogen carbonate) paste)

    • Metal + acid → salt + hydrogen.
    • Carbonate + acid → salt + water + carbon dioxide.
    • Soap is alkaline.
    • A weak alkali neutralises the acid.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    neutral  ·  H+  ·  OH−  ·  neutralise soil acidity
    1. Farmers add lime to soil to .
    2. A solution with pH 7 is .
    3. The ion produced by all acids in water is .
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    1: neutralise soil acidity · 2: neutral · 3: H+

    • Lime is a base that raises the pH of acidic soils.
    • pH 7 is neutral, like pure water.
    • Hydrogen ions make solutions acidic.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which pH value shows the strongest acid?a. dissolves in water
    2. Which is a natural indicator?b. 1
    3. Phenolphthalein in an alkaline solution is:c. Red cabbage extract
    4. An alkali is a base that:d. pink
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    1 → b (1) · 2 → c (Red cabbage extract) · 3 → d (pink) · 4 → a (dissolves in water)

    • The lower the pH, the stronger the acid.
    • Red cabbage contains a pigment that changes colour with pH.
    • Phenolphthalein is pink in alkali and colourless in acid.
    • Soluble bases are called alkalis.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    an antacid such as magnesium hydroxide  ·  red  ·  lemon juice  ·  sodium chloride
    1. Methyl orange in a strong acid is .
    2. The salt formed when hydrochloric acid reacts with sodium hydroxide is .
    3. Indigestion caused by excess stomach acid is treated with .
    Show answer

    1: red · 2: sodium chloride · 3: an antacid such as magnesium hydroxide

    • Methyl orange is red in acid and yellow in alkali.
    • HCl + NaOH → NaCl + H2O.
    • An antacid is a weak base that neutralises excess acid.

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