1.4.10 Practical Skill: Controlling Variables & Calculating Uncertainty

Controlling Variables

1. Independent Variable: This is the variable you deliberately change, such as temperature, pH, enzyme concentration, or substrate concentration.
2. Dependent Variable: This is what you measure in the experiment, often the rate of reaction.
3. Control Variables: These are factors that must be kept constant to ensure a fair test. Common control variables in enzyme experiments include:

• Temperature (if not the independent variable): Use a water bath to maintain a constant temperature.
• pH: Use a buffer solution to keep the pH stable.
• Concentration of Substrate and Enzyme (if not the independent variable).
• Volume of Solutions: Keep volumes of enzyme and substrate consistent across all trials.

4. Control Experiments: Use control setups, such as a sample without the enzyme, to confirm that any observed changes are due to enzyme activity.

Calculating Uncertainty

Uncertainty is a measure of the accuracy of the equipment used and should be considered in experimental results:

5. Equipment Precision: The uncertainty for each piece of equipment depends on its precision. For example:

• Measuring Cylinder: ±0.5 cm³
• Balance: ±0.01 g
• Stopwatch: ±0.1 s

6. Calculating Percentage Uncertainty:

• Formula:

$$\text{Percentage Uncertainty} = \left( \frac{\text{Uncertainty of Equipment}}{\text{Measurement Taken}} \right) \times 100$$

• Example: If you measure 10 cm³ with a measuring cylinder that has an uncertainty of ±0.5 cm³:

$$\text{Percentage Uncertainty} = \left( \frac{0.5}{10} \right) \times 100 = :highlight[5$$

7. Reducing Uncertainty:

• Use more precise equipment if available.
• Increase the size of measurements, as this can reduce the percentage uncertainty.