Designing a Scientific Investigation

Aim

A scientific investigation starts with an observation or a problem statement, not directly with an aim. For example, the observation that a reaction speeds up when warmed becomes the problem statement: how does temperature affect the rate of an enzyme reaction?

The aim then states exactly what the investigation will find out, in a form that can be tested by an experiment. One worked example is used throughout this page: to investigate the effect of temperature on the rate at which amylase breaks down starch.

A hypothesis goes further, predicting the relationship between the two main variables so the results can show it true or false, for example: the higher the temperature up to an optimum, the faster amylase breaks down starch; beyond the optimum, the rate decreases. This predicts how the reaction behaves, not what will be asked in an examination.

Variables

• Manipulated variable (independent variable), the one factor deliberately changed between trials. Here, it is the temperature of the reaction mixture, set at values such as 25°C, 35°C, 45°C, 55°C and 65°C.
• Responding variable (dependent variable), the factor measured because it may change as a result. Here, it is the time taken for a sample to give a negative iodine test, showing how quickly the starch has been broken down.
• Controlled variables, every other factor that could affect the result, kept the same in every trial so a change in the responding variable can be attributed only to the manipulated variable. These include the concentration and volume of amylase and starch solution, and the pH of the mixture.

Materials and Apparatus

Apparatus should set and measure the manipulated variable accurately, measure the responding variable precisely, and hold every controlled variable constant. For the amylase-and-temperature investigation, this includes:

• amylase solution and starch solution of a stated, equal concentration and volume for every trial
• water baths (or beakers of water heated with a Bunsen burner, or cooled with ice) set at the chosen range of temperatures
• a thermometer, to check that each water bath has reached and is holding its intended temperature
• test tubes, a dropper, and a white tile with iodine solution, to sample and test the mixture at regular intervals
• a stopwatch, to time from mixing to a negative iodine result
• a measuring cylinder, to measure equal volumes of amylase solution and starch solution each time

Procedure

A good procedure states exact quantities and concentrations, uses a suitable range of the manipulated variable, keeps every controlled variable the same, and is repeated at each value so a reliable pattern can be seen. A suitable procedure for the amylase investigation is:

1. Set up five water baths at 25°C, 35°C, 45°C, 55°C and 65°C, using a thermometer to check each temperature.
2. Measure 5 cm³ of starch solution and 5 cm³ of amylase solution into two separate test tubes and place both in the same water bath for 5 minutes to reach that temperature.
3. Mix the two solutions together in one test tube and immediately start the stopwatch.
4. At 30-second intervals, use a dropper to place one drop of the mixture onto a white tile containing a drop of iodine solution.
5. Record the time at which the iodine solution no longer turns blue-black, showing that all the starch has been broken down.
6. Repeat steps 1 to 5 at each of the other four temperatures, keeping the concentration and volume of amylase and starch solution the same each time.

Expected Results

Results should be recorded in a table with a clear heading and unit for each column, such as temperature in °C and time for a negative iodine test in seconds. Based on the hypothesis, the time is expected to fall as temperature rises towards an optimum (commonly 35–45°C for most enzymes), then rise again as the enzyme denatures and stops working.

This predicts how the experiment should behave, checked against the actual results, not what will appear on an examination paper.

Conclusion

A good conclusion restates the aim, describes the pattern actually shown by the results, and states whether the hypothesis is supported. For the amylase investigation: as temperature increases from 25°C towards an optimum, the time for a negative iodine test decreases, showing the rate of starch breakdown increases; above the optimum, the time increases again as the enzyme is denatured by heat.

This pattern supports the hypothesis that temperature affects enzyme activity up to an optimum, beyond which the reaction slows.

Common Mistakes

Paper 3-style questions

Scenario. A student notices that pondweed produces more bubbles in bright light than in dim light and wants to investigate how light intensity affects the rate of photosynthesis. Use this scenario for the questions below.

Question 1 (making a hypothesis). Write a suitable hypothesis for the investigation.

Model answer. The higher the light intensity, the higher the rate of photosynthesis (more bubbles produced per minute), up to a point. This links the manipulated variable, light intensity, to the responding variable, the number of bubbles per minute.

Question 2 (controlling variables). State the manipulated variable, the responding variable and two controlled variables.

Model answer. Manipulated: the light intensity, changed by moving a lamp to set distances. Responding: the number of bubbles of gas released per minute.

Controlled: the temperature of the water and the concentration of carbon dioxide, for example from added sodium hydrogencarbonate, as well as the species and length of pondweed.

Question 3 (tabulating and displaying data). Describe how the results should be recorded and displayed.

Model answer. Record the light intensity and the mean number of bubbles per minute in a table with clear headings and units. Plot mean rate against light intensity as a line graph, because light intensity is a continuous variable, and draw a line of best fit.

Question 4 (making an inference). At high light intensities the rate stops rising. What can you infer?

Model answer. You can infer that another factor has become limiting, such as the carbon dioxide concentration or the temperature. Increasing the light intensity further no longer increases the rate, because the reaction is now limited by that other factor.

Safety

• When you design any investigation, plan to wear eye protection wherever chemicals, heating or glassware are used.
• Include steps to handle hot water baths, Bunsen burners and glassware safely, and to switch off heat sources when they are not in use.
• Where living organisms are used, plan to treat them humanely and to return or dispose of them responsibly after the investigation.
• Assess the hazards of the chemicals you choose, use the lowest suitable concentrations, and follow the safety information on the labels.

Source:SRC-DSKP-EN

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