Question 1 Medium
Copper sulfate solution is electrolysed with inert electrodes. Which gas forms at the positive electrode?
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Answer: oxygen
- Hydroxide ions lose electrons at the positive electrode.
- Oxygen gas forms.
Required practicals and working scientifically in chemistry: GCSE Combined Science Chemistry practice questions
A second set of questions on required practicals and working scientifically in chemistry. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
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Question 2 Medium
Copper sulfate solution is electrolysed with copper electrodes. Does the positive electrode gain or lose mass?
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Answer: lose mass
- Copper atoms at the positive electrode become ions in solution.
- So the positive electrode loses mass.
Question 3 Hard
Three repeat results are 24.0 s, 25.0 s and 23.0 s. Calculate the mean time. Give your answer in s to 3 significant figures.
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Answer: 24.0 s
- Add them: 24.0 + 25.0 + 23.0 = 72.0.
- 72.0 ÷ 3 = 24.0 s.
Question 4 Hard
Repeat results range from 23.0 s to 26.0 s. Estimate the uncertainty using half the range. Give your answer in s.
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Answer: 1.5 s
- Range = 26.0 − 23.0 = 3.0.
- Half the range is 1.5 s.
Question 5 Hard
A copper cathode has a mass of 3.42 g before and 3.90 g after electrolysis. Calculate its gain in mass in g.
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Answer: 0.48 g
- Gain = after − before.
- 3.90 − 3.42 = 0.48 g.
Question 6 Hard
A reaction gives 60 cm³ of gas in 30 s. Calculate the mean rate in cm³/s.
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Answer: 2.0 cm³/s
- Mean rate = volume ÷ time.
- 60 ÷ 30 = 2.0 cm³/s.
Question 7 Hard
The time for a cross to disappear halves when the temperature is raised. What happens to the rate of reaction?
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Answer: it doubles
- Rate is proportional to 1 ÷ time.
- Half the time means double the rate.
Question 8 Hard
Calcium hydroxide is added to dilute hydrochloric acid. Does the pH go up or down?
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Answer: up
- The alkali neutralises the acid.
- So the pH goes up.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
What is the independent variable?
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The variable you change on purpose.
Check 2
What is the dependent variable?
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The variable you measure.
Check 3
What is a control variable?
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A variable you keep the same to make the test fair.
Check 4
What is an anomalous result?
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A result that does not fit the pattern. Leave it out of the mean.
Check 5
What does accurate mean?
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Close to the true value.
Check 6
What does precise mean?
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Repeat results that are close to each other.
Check 7
How do you estimate the uncertainty from repeat results?
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Half the range.
Check 8
What is a hazard?
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Something that could cause harm.
Check 9
What is a risk?
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The chance of a hazard causing harm.
Check 10
What does a corrosion hazard symbol warn about?
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The substance can burn skin and damage eyes and materials.
Check 11
What does a flammable symbol warn about?
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The substance catches fire easily.
Check 12
Why is the solid added in excess when making a soluble salt from an acid?
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So that all the acid is used up.
Check 13
How do you remove the excess solid in salt making?
Show answer
Filter the mixture.
Check 14
Why not evaporate the salt solution to dryness?
Show answer
Crystals form best when part of the water is evaporated and the solution is cooled slowly.
Check 15
Why is a polystyrene cup used in a temperature change experiment?
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It is a good insulator, so less heat is lost.
Check 16
How do you find the temperature change?
Show answer
Highest temperature minus starting temperature.
Check 17
What does a temperature rise show?
Show answer
An exothermic reaction.
Check 18
Which electrode gains mass when copper sulfate solution is electrolysed?
Show answer
The negative electrode, as copper is deposited.
Check 19
What forms at the positive inert electrode in copper sulfate solution?
Show answer
Oxygen gas.
Check 20
What happens to a copper positive electrode?
Show answer
It loses mass as copper dissolves into the solution.
Check 21
Name three ways to follow the rate of a reaction.
Show answer
Gas volume, loss in mass, and time for a cross to disappear.
Check 22
What is the formula for the mean rate of a reaction?
Show answer
Amount of product formed (or reactant used) divided by time.
Check 23
What does a shorter time for a cross to disappear mean?
Show answer
A faster reaction.
Check 24
Name four things that can change the rate of a reaction.
Show answer
Concentration, temperature, surface area and a catalyst.
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