Learning focus
Explain chemical changes using particles, collisions, equilibrium and electron transfer; interpret graphs and industrial conditions; and apply the ideas to unfamiliar reactions.
Variables
State the independent variable with values or range, the dependent measurement with units, and the variables kept constant. A vague statement such as ‘keep everything else the same’ is weaker than naming concentration, volume, temperature and particle size.
Range and repeats
Use enough values to reveal a trend. Include repeated measurements and calculate a mean. Investigate anomalies rather than automatically deleting them. A preliminary experiment helps choose a safe range and suitable sampling interval.
Reliability, accuracy and validity
Repeats improve reliability. Higher-resolution apparatus can improve measurement precision. Validity requires that the dependent change is caused by the independent variable rather than uncontrolled factors.
Targeted evaluation
Identify a specific error, describe its likely effect and propose a matching improvement. ‘Human error’ is too vague. For example, gas escapes while fitting the bung, causing early volumes to be underestimated; use a dropping funnel through a fitted bung.
Practical or data skill
Write a full plan to investigate temperature or surface area. Include apparatus, method, variables, table headings, safety and how the rate will be calculated.
Examination tip
An improvement earns credit when it solves the limitation you identified.
Review questions and suggested answers
Question 1
Why repeat each condition?
Suggested answer
To identify anomalies, reduce random variation and calculate a mean.
Question 2
What makes an investigation valid?
Suggested answer
Only the intended independent variable significantly affects the dependent variable.
Question 3
Give a targeted improvement for gas leakage.
Suggested answer
Test and seal connections or add reactant through a fitted dropping device.
