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InkFlaskCambridge IGCSE

Pupil sheet · Reversible reactions and equilibrium

Water out, water back — questions

Name ______________________ Date ____________

Try every question before you use a nudge. Write in full sentences where you are asked to explain.

1. identify

Which equation shows a reversible reaction?

Circle one answer.

  • hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water
  • calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
  • methane + oxygen → carbon dioxide + water
  • magnesium + oxygen → magnesium oxide

Nudge (only if you are stuck): Look at the arrow.

2. describe / name

Blue copper(II) sulfate crystals are heated gently in a test tube. (a) What colour does the solid turn? (b) What collects near the cool mouth of the tube? (c) Name the solid left in the tube.

(a) Colour of the solid

(b) What collects

(c) The solid left

Nudge (only if you are stuck): Heating drives out the water of crystallisation.

3. identify

Match each compound to its colour.

Write the number of the matching option.

  1. pink
  2. blue
  3. white
  • 1. Hydrated copper(II) sulfate: ______
  • 2. Anhydrous copper(II) sulfate: ______
  • 3. Hydrated cobalt(II) chloride: ______
  • 4. Anhydrous cobalt(II) chloride: ______

Nudge (only if you are stuck): Copper(II) sulfate goes from blue to white when heated; cobalt(II) chloride from pink to blue.

4. describe / explain

Water is added drop by drop to white anhydrous copper(II) sulfate in a basin. (a) Describe two things you would observe. (b) Which way does the reversible reaction go, and what is made?

(a) Two observations

(b) Direction and product

Nudge (only if you are stuck): One observation is a colour; the other you can feel.

5. evaluate

A pupil says: "Heating blue copper(II) sulfate is a physical change, because adding water turns it back." Which reply is correct?

Circle one answer.

  • Yes: any change that can be reversed is a physical change
  • No: a new substance forms each way, so both directions are chemical changes
  • Yes: only water leaves the crystals, and water is not a new substance
  • No: a heated substance can never be turned back

Nudge (only if you are stuck): Ask whether a new substance forms.

6. state / describe

Hydrated cobalt(II) chloride is CoCl2·6H2O. (a) Give the symbol equation for the reversible reaction when it is heated. (b) Give the colour of the solid on each side of the equation.

(a) Symbol equation

(b) Colours

Nudge (only if you are stuck): The products are the anhydrous compound and water.

7. describe

Describe how to use cobalt chloride paper to test whether a liquid contains water, and give the result if it does.

Method

Result if water is present

Nudge (only if you are stuck): Dry cobalt chloride paper is blue.

8. explain / suggest

A box of new shoes holds a small sachet of drying crystals. Some of the crystals contain anhydrous cobalt(II) chloride and are blue. (a) After a few months in a damp cupboard, these crystals are pink. What has happened? (b) Suggest how the sachet could be made to work again.

(a) What has happened

(b) How to make it work again

Nudge (only if you are stuck): Which form of cobalt(II) chloride is pink?