What are variables

Variables are the factors that can change during an experiment.
To make your investigation a fair test, you must change only one variable at a time, while keeping all other factors constant.

Identifying variables correctly is essential to design a valid chemistry investigation and to write a strong scientific hypothesis.

Analyzing Your Hypothesis

Remember that your hypothesis must have three parts (clauses):

if – then – because

Before identifying your variables, you must analyze your hypothesis carefully. This will help you check if your investigation is clear, scientific, and testable.

Independent Variable

The independent variable is the variable that you change on purpose.

It represents the cause in the experiment.
It goes in the “if” part of the hypothesis.
Only the researcher decides its values.

Hypothesis example:
If the concentration of hydrochloric acid increases, then…

In this investigation, the independent variable is:

The concentration of hydrochloric acid (mol/L)

This is the variable the student deliberately changes.

Dependent Variable

The dependent variable is the variable that you measure or observe.

It represents the effect.
It goes in the “then” part of the hypothesis.
Its value depends on changes in the independent variable.

Hypothesis example:
…then the rate of reaction will increase.

In this investigation, the dependent variable is:

The rate of reaction, measured by:
- Volume of gas produced
- Time taken for a solid to disappear
- Temperature change

The dependent variable changes as a result of modifying the independent variable.

Controlled Variables

Controlled variables are all the factors that must stay the same throughout the experiment.

You do not change them.
They are controlled to ensure that the results are caused only by the independent variable.
If they change, the investigation becomes unreliable.

For the reaction between an acid and a carbonate, controlled variables may include:

Volume of acid used
Type of carbonate (e.g. calcium carbonate)
Size or surface area of the solid
Temperature of the reactants
Type of container used

These variables are kept constant to ensure a fair test.

Important Check Before You Continue

If your hypothesis does not clearly include an independent variable and a dependent variable, you must stop and rethink it.

Your hypothesis is the foundation of your investigation.
If it is not clear or scientific, the rest of your experiment will not be valid.

👉 Rewrite your hypothesis before continuing if needed.
You may return to the Hypothesis section to revise and improve it.

When Should You Redesign Your Hypothesis?

You should redesign your hypothesis if:

The independent variable cannot be measured
The dependent variable cannot be measured
The variables are too vague (for example: “more”, “better”, “faster” without numbers)
The hypothesis does not clearly explain cause and effect

In these cases, your investigation will not be a fair test, and your results will not be reliable.

How to Describe the Manipulation of Variables

Describe how you will be changing your variables in order to obtain different results, focusing specifically on the independent variable.
In the chemistry example above, what changes could you make? Instead of using a single value, could you use a lower or higher concentration of a solution? For example, could you use 0.5 mol/L, 1.0 mol/L, or 1.5 mol/L of an acid?

Describe how you think this manipulation of the independent variable could affect the final outcome. In the hypothesis example above, think about this: if you use a 0.5 mol/L acid instead of a 1.0 mol/L acid, how do you think the rate of reaction will change? What would you expect to observe or measure?

In a more general way, how will increasing or decreasing the concentration of a reactant affect the outcome of the reaction? How might this influence the amount of gas produced, the time taken for the reaction, or the temperature change observed?

You may want to use a table like this:

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