Edexcel GCSE Combined Science: Chemistry: Chemical Changes
6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
A student placed small pieces of four different metals — magnesium, zinc, copper and iron — into separate test tubes of dilute hydrochloric acid and recorded how vigorously each reacted.
- Magnesium fizzed very rapidly.
- Zinc and iron each fizzed steadily, iron more slowly than zinc.
- Copper showed no reaction at all.
Explain how these observations can be used to place the four metals in order of reactivity, and explain why copper does not react with the acid. In your answer, refer to what is happening to the metal atoms. (6 marks)
In a core practical, a student prepared pure, dry crystals of the soluble salt copper sulfate by reacting excess insoluble copper oxide with warm dilute sulfuric acid.
The steps below are listed out of order:
| Letter | Step |
|---|---|
| P | Filter the mixture to remove the excess solid copper oxide. |
| Q | Add copper oxide to the warm acid, a little at a time, until no more dissolves (excess). |
| R | Leave the solution so that water evaporates slowly and crystals form. |
| S | Gently heat the filtered solution to evaporate some of the water and concentrate it. |
(a) Write the letters P, Q, R and S in the correct order to carry out the preparation. (2 marks)
(b) Explain why the copper oxide is added in excess. (2 marks)
When magnesium is added to copper sulfate solution, a displacement reaction occurs: the more reactive magnesium displaces copper, and the blue solution fades as a reddish-brown solid forms.
(a) Write a balanced symbol equation, including state symbols, for the reaction between magnesium and copper sulfate solution. (2 marks)
(b) Explain why this is described as a displacement reaction. (1 mark)
Iron is extracted from iron oxide in a blast furnace by heating it with carbon. The carbon removes the oxygen from the iron oxide:
iron oxide + carbon → iron + carbon dioxide
(a) In this reaction, the iron oxide is reduced. State, in terms of oxygen, what is meant by reduction. (1 mark)
(b) Explain why carbon can be used to extract iron from its oxide, but cannot be used to extract a more reactive metal such as aluminium. (2 marks)
Molten lead bromide (PbBr₂) can be broken down by electrolysis. During electrolysis, lead metal forms at the negative electrode (cathode).
Write the half-equation, including the electrons, for the formation of lead at the negative electrode. (2 marks)
The pH scale is used to measure how acidic or alkaline a solution is.
State the pH value of a neutral solution. (1 mark)