This should be an investigation involving developing a hypothesis.

The idea

A hypothesis is a testable prediction linked to scientific reasoning. For this practical, you might predict that as the concentration of a reactant increases, the time taken for a cross to disappear decreases, because more reactant particles in the same volume collide more frequently. The hypothesis must identify the independent variable, the dependent variable and the expected direction of change, and it should be justified using collision theory. You then design a method that produces evidence capable of supporting or refuting the prediction. This means choosing suitable concentrations, controlling variables such as temperature and volume, deciding how many readings and repeats to take, and selecting a measurement that genuinely reflects rate. Finally, you compare your results with the hypothesis and explain any anomalies.

What earns the marks

• Write a hypothesis that predicts a relationship between the independent variable, such as concentration, and the dependent variable, such as time or volume of gas.
• Justify the hypothesis using collision theory, for example more particles per unit volume lead to more frequent successful collisions.
• Design a method that changes only the independent variable and controls other variables such as temperature, volumes and reactant amounts.
• Select a suitable range and number of concentrations, and plan repeats so that a pattern can be identified and anomalies recognised.
• Collect and present data in a table and graph, then use the evidence to state whether the hypothesis is supported.
• Evaluate the investigation by discussing sources of error, such as timing reactions by eye, and suggest improvements.

Common mistakes and how to put them right

• Writing a hypothesis as a question or a vague statement.

  Put it right: Write a clear prediction such as increasing concentration will decrease the time taken for the cross to disappear.

• Changing more than one variable at a time.

  Put it right: Identify the independent variable and keep all other relevant variables constant.

• Ignoring anomalous results when drawing a conclusion.

  Put it right: Identify anomalies, consider repeating them and base the conclusion on the overall pattern.

Exam tips

• Use the structure if concentration increases, then rate increases because collision frequency increases when writing a hypothesis.
• Name the independent variable, dependent variable and control variables explicitly in your plan.
• Refer back to the hypothesis in your conclusion and state whether the evidence supports it, using data values as evidence.