P.05 Standardising or Changing Variables

Importance of Variable Control

• Goal: Keep all significant variables, other than the independent and dependent variables, constant to ensure a fair test.
• Standardised Variables: Essential for isolating the effect of the independent variable on the dependent variable.

Common Standardised Variables and How to Control Them

1. Temperature

• Controlling Temperature:
  • Water Bath: Use to maintain a constant temperature or to produce a range if temperature is the independent variable.
  • Types:
    • Electric water bath: Set to a specific temperature, but verify with a thermometer.
    • Homemade water bath: Large beaker of water heated to the desired temperature.
• Temperature Tips:
  • Always use a thermometer to confirm the water bath temperature.
    • Hold the thermometer in the water without touching the sides or base.
  • Check Sample Temperature: Place tubes of liquid in the water bath and allow time for contents to reach equilibrium with the water bath temperature.
  • Simultaneous Heating for Enzyme Experiments:
    • Heat enzyme and substrate solutions separately to the target temperature before mixing, as the reaction will start immediately upon combining.
  • Animal Safety: For live subjects, avoid temperatures that may cause distress or harm (e.g., max of 35°C).

1. pH

• Controlling pH:
  • Buffer Solutions: Add a buffer solution to maintain a constant pH, even if the reaction produces acidic or alkaline by-products.
    • Use buffers specific to the desired pH level to stabilize the environment for enzyme reactions or other pH-sensitive processes.
• Measuring pH:
  • Universal Indicator: Provides a color range across the pH scale (0-14). Use a color chart to interpret results.
  • pH Meter: Offers a precise digital reading of pH and is ideal for accurate measurements.

1. Biological Material

• Consistency in Biological Samples:
  • Goal: Ensure samples are as similar as possible to reduce variation due to natural differences.
  • Standardisation Factors:
    • Age: Younger or older samples may have different characteristics.
    • Storage Conditions: Store all samples in identical conditions (e.g., temperature, humidity).
    • Genotype/Sex: Variability can be minimized by using samples with the same genetic background.
    • Mass and Volume: Consistent amounts ensure the independent variable’s effect isn’t obscured by sample size.
    • Source Location: For plant or animal parts, ensure samples are taken from the same part of the organism.

Key Terms

Practical Tips for Controlling Variables

1. Temperature Control:

• Use a water bath for consistency and always verify with a thermometer.
• Allow sufficient time for samples to reach equilibrium within the water bath.

1. pH Control:

• Use buffer solutions to keep the reaction environment stable.
• Double-check pH with universal indicators or a pH meter if precise measurements are necessary.

1. Consistency in Biological Samples:

• Match samples as closely as possible by controlling for age, size, and genetic factors.

This careful standardisation of variables ensures that experimental results are reliable and accurately reflect the effect of the independent variable on the dependent variable.