ExpertMedium2 marksMultiple Choice
AQA GCSE · Question 07.2 · The rate and extent of chemical change
Which two variables should the student keep the same to make the investigation a fair test? Select two options.
Which two variables should the student keep the same to make the investigation a fair test? Select two options.
Answer options:
A.
Concentration of hydrochloric acid
B.
Mass of magnesium powder
C.
Temperature of hydrochloric acid
D.
Time for reaction to end
E.
Volume of gas collected
How to approach this question
A fair test means only changing one variable (the independent variable) and measuring its effect on another variable (the dependent variable). All other factors that could affect the outcome must be kept constant (control variables).
1. **Independent Variable:** What is the student changing? (Concentration of acid). This should not be ticked.
2. **Dependent Variable:** What is the student measuring? (Volume of gas over time). This should not be ticked.
3. **Control Variables:** What other factors could change the rate of this reaction? (e.g., amount of reactants, temperature, surface area). These must be kept the same.
Full Answer
Mass of magnesium powder, Temperature of hydrochloric acid
In a scientific investigation, a fair test requires that only the independent variable is changed. All other variables that could affect the result (control variables) must be kept constant.
- **Independent variable** (what is changed): Concentration of hydrochloric acid.
- **Dependent variable** (what is measured): Volume of gas collected over time.
- **Control variables** (what must be kept the same):
- **Mass of magnesium powder:** The amount of reactant affects the rate.
- **Temperature of hydrochloric acid:** Temperature affects the rate (higher temperature means a faster rate).
- **Surface area of magnesium:** The question specifies magnesium powder, which helps control this, but it must be the same type of powder each time.
- **Volume of acid used.**
Therefore, the mass of magnesium and the temperature of the acid must be kept the same.
Common mistakes
✗ Selecting the independent variable (concentration).
✗ Selecting the dependent variable (volume of gas).
✗ Not understanding the concept of a fair test.
Practice the full AQA GCSE Chemistry Foundation Tier Paper 2
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