6.1.3Inquiry Process
Controlling Variables in Biology — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 155 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Types of Variable
Independent variable
- Independent variable The one factor deliberately changed, for example the temperature of the water bath.
Dependent variable
- Dependent variable The factor measured as the response, such as the volume of gas collected per minute.
Controlled variables
- Controlled variables Other factors that could affect the result, such as pH, volume of solution, mass of tissue and time, which must be kept constant.
Uncontrolled variables
- Uncontrolled variables Factors that cannot be controlled completely, such as genetic differences between organisms, must be recorded and discussed.
Example
- Example In a catalase investigation the temperature is independent, oxygen volume per minute is dependent,
- and the mass of liver, volume of hydrogen peroxide and pH are controlled.
Section 2 of 4Methods of Control
Methods of Control
Water baths
- Water baths They keep temperature constant, and the temperature is checked with a thermometer during the trial.
Buffers
- Buffers Buffer solutions keep pH constant when pH may change during the experiment.
Standard samples
- Standard samples Using tissue from the same source, with the same size, mass or surface area, reduces differences between trials.
Same equipment
- Same equipment Using the same instrument and observer for each measurement reduces systematic variation.
Randomising
- Randomising Allocating organisms or positions to treatments at random reduces bias from conditions such as light or position.
Monitoring
- Monitoring Controlled variables are checked during the experiment, for example by reading the thermometer at the start and end of each trial, to show they stayed constant.
Section 3 of 4Control Experiments
1
A negative control has the independent variable removed or inactive, for example boiled enzyme, to show that the effect is not due to something else.
2
A positive control uses a treatment known to produce the effect, to show that the method works.
3
Each control is treated in exactly the same way as the experimental sample in all other respects.
4
The results of controls are compared with the experimental results to support the conclusion.
5
If a control gives an unexpected result, the cause is investigated before the conclusion is trusted.
| Control | Example in an enzyme experiment | Purpose |
|---|---|---|
| Negative | Boiled enzyme added | Shows no activity without a working enzyme |
| Positive | Enzyme at its optimum conditions | Shows the method detects activity |
Section 4 of 4Exam Checklist & Tips
6.1.3 – Key Points for the Exam
- Independent variable The variable changed by the investigator.
- Dependent variable The variable that is measured.
- Controlled variable A variable kept constant so that it does not affect the result.
- Negative control A treatment in which no effect is expected.
Common Mistakes
- Listing the dependent variable as a controlled variable.
- Writing 'keep everything the same' without naming the variables and the method of control.
- Forgetting that a control must be treated identically to the experimental sample except for the factor being tested.
- Treating an uncontrolled variable as if it did not matter.
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