Practical Skill: Control of Variables & Uncertainty

Question 1

Factors that can be changed include:
Temperature
pH
Enzyme concentration
Substrate concentration

The key thing with enzyme rate experiments is to ensure that only one

Answer 1

only one of these variables is changed during a particular experiment
This is known as the independent variable

All other variables must be controlled (they must stay the same)
These are known as the control variables (if not this experiment can be called unreliable

Question 2

Uncertainty is the

Answer 2

amount of error your measurements might contain

Question 3

There will always be a small degree of uncertainty in your readings or measurements
This is often because the sensitivity of the apparatus being used is limited
For example, you may want to measure reaction rate by measuring how much of a product is made in a given time period, e.g. using a gas syringe to measure the volume of oxygen produced from the breakdown of hydrogen peroxide by catalase
The gas syringe may give readings to the nearest 1 cm3

However, the real volume produced could be up to 0.5 cm3 smaller or larger
E.g. if the syringe were to collect 4 cm3 of oxygen, then the true value for the volume of oxygen would lie somewhere between 3.5 cm3 and 4.5 cm3

Question 4

The gas syringe may give readings to the nearest 1 cm3
However, the real volume produced could be up to 0.5 cm3 smaller or larger

E.g. if the syringe were to collect 4 cm3 of oxygen, then the true value for the volume of oxygen would lie somewhere between 3.5 cm3 and 4.5 cm3
The syringe has an uncertainty of ± 0.5 cm3

Question 5

A ‘±’ sign tells you the

Answer 5

range in which the true value lies; this range is called the margin of error
if done to nearest 1 cm cubed

E.g. for the example above, the syringe can be said to have measured 4 ± 0.5 cm3

Question 6

For enzyme rate experiments, you may need to calculate the percentage error of your measurements

As long as you know the uncertainty value of your measurements, the percentage error can be calculated using the following formula

percentage error formula

Answer 6

percentage error = (uncertainty ÷ measured value) x 100

Question 7

In an enzyme-controlled reaction involving the breakdown of hydrogen peroxide by catalase, 50 cm3 of oxygen was produced, with an uncertainty value of 0.5 cm3.

Calculate the percentage error of this measurement.

Answer 7

Percentage error = (uncertainty ÷ measured value) x 100

Percentage error = (0.5 ÷ 50) x 100

Percentage error = 0.01 x 100

Percentage error = 1 %

Question 8

In an enzyme rate experiment involving the breakdown of hydrogen peroxide by catalase, a student recorded that 10 cm3 of oxygen was produced in 5.245 seconds.

The student measured this using a stopwatch that counted in milliseconds. Calculate the percentage error of the stopwatch measurements.

Answer 8

Step 1: Calculate the uncertainty value

The stopwatch can measure to the nearest millisecond (0.001 second)

This means the actual time taken could be up to 0.0005 seconds shorter or longer than this

This means stopwatch measurements have an uncertainty of ± 0.0005 s

Step 2: Calculate the percentage error of the student’s measurement of 5.245 seconds

Percentage error = (uncertainty ÷ measured value) x 100

Percentage error = (0.0005 ÷ 5.245) x 100

Percentage error = 0.000095 x 100

Percentage error = 0.0095 % or 0.01 %