ExpertMedium3 marksStructured
AQA GCSE · Question 05.5 · Chemical changes
A student investigated the electrolysis of green copper chromate solution. Figure 4 shows the apparatus and Figure 5 shows the results.
Copper chromate solution contains the ions Cu²⁺ and CrO₄²⁻.
Explain the results shown in Figure 5.
A student investigated the electrolysis of green copper chromate solution. Figure 4 shows the apparatus and Figure 5 shows the results.
Copper chromate solution contains the ions Cu²⁺ and CrO₄²⁻.
Explain the results shown in Figure 5.
How to approach this question
1. Identify the charge of the copper ion (Cu²⁺) and the chromate ion (CrO₄²⁻).
2. Recall the rule of attraction in electrolysis: which electrode do positive ions move to? Which electrode do negative ions move to?
3. Connect the ion to its colour (the question implies one is blue and one is yellow). The blue colour moves to the negative electrode, so which ion must be blue? The yellow colour moves to the positive electrode, so which ion must be yellow?
4. Combine these points into a clear explanation.
Full Answer
The experiment demonstrates the migration of ions during electrolysis.
1. The copper chromate solution initially appears green because it is a mixture of blue Cu²⁺ ions and yellow CrO₄²⁻ ions.
2. When the power supply is turned on, an electric field is created.
3. The copper ions (Cu²⁺) are positively charged. They are attracted to the negative electrode (cathode). This causes the blue colour to move towards the negative electrode.
4. The chromate ions (CrO₄²⁻) are negatively charged. They are attracted to the positive electrode (anode). This causes the yellow colour to move towards the positive electrode.
This separation of colours is direct evidence that ionic solutions contain mobile, charged particles (ions).
Common mistakes
✗ Just stating that opposites attract without identifying which ion is which charge and which colour.
✗ Confusing the anode and cathode.
✗ Not linking the ion to the observed colour movement.
Practice the full AQA GCSE Chemistry Higher Tier Paper 1
44 questions · hints · full answers · grading
More questions from this exam
Q01.1A student produced a salt by reacting copper carbonate with sulfuric acid.
This is the method use...EasyQ01.2Give one observation the student could make during Step 4 which shows that the copper carbonate i...EasyQ01.3Give one reason for filtering the mixture in Step 5.EasyQ01.4Name the equipment that can be used to warm the filtrate gently in Step 6.EasyQ01.5The maximum theoretical mass of the salt that could be produced using 50 cm³ of the sulfuric acid...Medium