The scientific method in chemistry

This page covers a single Form 4 content standard: the scientific method in chemistry. It is the first idea in the whole syllabus for a reason, every experiment you meet in Form 4 and Form 5 is planned and marked against this same framework. Get it secure now and you will pick up marks in every practical question that follows.

What the scientific method is

The scientific method is the ordered, logical way a chemist investigates a problem so that the answer can be trusted and repeated. You are expected to know the steps in sequence and to apply them to a described experiment. The usual order is:

1. Observation, you notice something in the world (for example, sugar seems to dissolve faster in hot drinks).
2. Question / problem, you turn the observation into a clear question (“Does temperature affect how fast sugar dissolves?”).
3. Hypothesis, a testable statement that predicts a relationship (“The higher the temperature of the water, the shorter the time taken for the sugar to dissolve”).
4. Experiment, a fair test that changes one thing at a time and controls the rest.
5. Collecting and analysing data, recording measurements with the correct units and, where useful, drawing a graph.
6. Conclusion, a statement that says whether the data support or reject the hypothesis, and why.

The three kinds of variable

Almost every question on this standard tests whether you can identify variables. Learn these three by heart:

• The manipulated variable (independent variable) is the one you deliberately change.
• The responding variable (dependent variable) is the one you measure to see the effect.
• The controlled variables are the quantities you keep constant so that the test is fair and only the manipulated variable can be responsible for the change.

Key definition. A hypothesis is a testable statement that predicts a relationship between the manipulated variable and the responding variable. It must be something an experiment could support or reject, never a vague opinion.

Notice that a good hypothesis already names both variables. If you can write the hypothesis, you can almost always read the two variables straight out of it, and the controlled variables are then everything else that must stay the same.

Worked example

Question. A student investigates how the temperature of water affects how quickly a 5.0 g sugar cube dissolves. She uses 100 cm³ of water at 30 °C, 40 °C, 50 °C, 60 °C and 70 °C, stirs at the same rate each time, and records the time taken for the cube to disappear completely. Identify the manipulated, responding and controlled variables, and state a suitable hypothesis.

Step 1, Find what is deliberately changed. The student sets the water to five different temperatures (30–70 °C). Temperature is chosen on purpose, so the manipulated variable is the temperature of the water, measured in °C.

Step 2, Find what is measured as a result. She records how long the cube takes to dissolve. That result is measured, so the responding variable is the time taken for the sugar to dissolve, measured in seconds (s).

Step 3, Find what must stay the same. For the test to be fair, everything else must be constant: the mass of the sugar (5.0 g), the volume of water (100 cm³), the rate of stirring, and the size of the sugar cube. These are the controlled variables.

Step 4, Write the hypothesis. State a testable relationship: “The higher the temperature of the water, the shorter the time taken for the sugar to dissolve.” This names both variables and predicts the direction of the effect.

Answer. Manipulated: temperature of water (°C). Responding: time to dissolve (s). Controlled: mass of sugar (5.0 g), volume of water (100 cm³), rate of stirring, size of cube. Hypothesis: as temperature increases, dissolving time decreases.

Practice question

Question. A student wants to find out whether the concentration of hydrochloric acid affects how fast it reacts with marble chips. He measures the volume of carbon dioxide gas collected in the first 30 seconds using acid at 0.5 mol dm⁻³, 1.0 mol dm⁻³ and 2.0 mol dm⁻³, keeping the mass of marble chips and the temperature the same. Name the manipulated variable, the responding variable and two controlled variables, and write a suitable hypothesis.

Answer. Manipulated variable: the concentration of the hydrochloric acid (mol dm⁻³). Responding variable: the volume of carbon dioxide gas collected in 30 s (cm³). Controlled variables (any two): the mass of marble chips, the temperature, the volume of acid, and the size of the marble chips. Hypothesis: “The higher the concentration of the acid, the greater the volume of gas collected in 30 seconds”, that is, the faster the reaction.

Exam tip

When a question describes an experiment, underline the quantity that appears in a list of different values, that is almost always the manipulated variable. Then find the quantity that is measured or timed, that is the responding variable. Everything else that is stated as “the same” or “kept constant” is a controlled variable. Write your hypothesis as “The higher/greater the [manipulated variable], the higher/lower the [responding variable]”, because a hypothesis phrased this way clearly names both variables and predicts a direction, exactly what the marking scheme rewards. Never confuse the two: the variable you change is not the one you measure.

Where this fits

This standard opens the Introduction to Chemistry chapter and underpins every experiment in SPM Chemistry. To revise it alongside the rest of the chapter, use the revision notes; to see more variable-identification questions solved step by step, work through the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers check that your variable identification is precise and your hypotheses always name both variables, because these are the easy marks students most often drop. Master fair testing here and the practical work in later chapters, such as Rate of Reaction and Acids, Bases and Salts, becomes far more straightforward.

Quick recap

• The scientific method runs: observation → question → hypothesis → experiment → data → conclusion.
• A hypothesis is a testable prediction that names both variables.
• Manipulated = changed on purpose; responding = measured; controlled = kept constant for a fair test.
• Record data with correct units and write a conclusion that refers back to the hypothesis.