How to write a hypothesis (SPM Chemistry)

Writing a hypothesis is one of the most reliable marks in SPM Chemistry Paper 3, the practical paper (Paper 3 is a practical test assessing science process skills) of SPM Chemistry, but only if you write it the way the marking scheme expects. Many students lose the mark by writing something vague or by stating a fact rather than a prediction. This guide explains exactly what a hypothesis is, gives a formula you can use every time, works through several examples, and lists the errors that cost marks.

What a hypothesis is

A hypothesis is a testable statement that predicts the relationship between two variables: the one you change and the one you measure. It is not a question, and it is not a general fact. It is a prediction you can put to the test in the experiment described. A good hypothesis does three things: it names the manipulated variable, it names the responding variable, and it states the direction of the relationship between them.

The two variables a hypothesis must name

Before you can write a hypothesis you must identify the variables. The manipulated variable is the quantity you deliberately change. The responding variable is the quantity you measure because it responds to that change. Every hypothesis must connect these two. If your statement mentions only one of them, it cannot score full marks, because a relationship needs two things to relate.

A formula that works every time

The most reliable way to phrase a hypothesis is:

“The higher/greater the [manipulated variable], the higher/greater (or the lower/smaller) the [responding variable].”

For example: “The higher the temperature of the reactants, the shorter the time taken for the reaction to complete.” This single sentence names both variables and states the direction, which is exactly what the marking scheme looks for. You can also write it as “When the [manipulated variable] increases, the [responding variable] increases/decreases.” Either form scores, as long as both variables and the direction are present.

Worked examples

Rate of reaction and concentration. Manipulated variable: concentration of acid. Responding variable: rate of reaction (or time for a fixed change). Hypothesis: “The higher the concentration of the acid, the faster the rate of reaction.”

Effect of alloying on hardness. Manipulated variable: type of metal (pure metal or alloy). Responding variable: size of the dent. Hypothesis: “An alloy is harder than the pure metal, so the alloy is dented less than the pure metal under the same load.” Here the manipulated variable is categorical (pure versus alloy), so the hypothesis compares the two categories rather than using “the higher the”.

Electrolysis and current. Manipulated variable: the current passed. Responding variable: the mass of metal deposited. Hypothesis: “The larger the current, the greater the mass of metal deposited.”

Solubility and temperature. Manipulated variable: temperature of the water. Responding variable: mass of solute that dissolves. Hypothesis: “The higher the temperature, the greater the mass of solute that dissolves.”

Notice that when the manipulated variable is a number you can increase, the “the higher the … the higher/lower the …” form fits perfectly. When the manipulated variable is a type or category, you compare the categories directly instead.

Making sure it is testable

A hypothesis must be something the experiment can actually check. If the described experiment measures the time for a reaction, your responding variable should be the time (or the rate derived from it), not some quantity that is never measured. Always match the responding variable in your hypothesis to what the experiment measures, and the manipulated variable to what the experiment changes. A prediction the experiment cannot test does not score.

Common errors

• Naming only one variable. “Temperature affects the rate” mentions the manipulated variable but not what happens to the responding one, and gives no direction. Name both and state the direction.
• Writing a fact, not a prediction. “Magnesium is more reactive than copper” may be true, but if it is not the relationship the experiment tests, it is not a hypothesis for that experiment.
• Writing a question. “Does concentration affect the rate?” is a question, not a hypothesis. Turn it into a prediction.
• Predicting the wrong direction. Think about the chemistry: higher concentration means a faster rate, so the time taken is shorter, not longer. Getting the direction wrong loses the mark even though the form is right.
• Being too vague. “The rate will change” gives no direction and no clear relationship. Always commit to a direction.

Tips for full marks

Read the experiment and find the quantity being changed and the quantity being measured first; those become your two variables. Then drop them into the formula “the higher the [changed quantity], the higher/lower the [measured quantity]” and check the direction makes chemical sense. Keep the sentence to one clear statement, you do not need to explain the reason in the hypothesis itself, though the reason may be asked separately. Use the exact terms manipulated and responding when the question asks you to identify the variables alongside the hypothesis.

How our teachers train this

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we practise turning any described experiment into a correct hypothesis in seconds, identify the two variables, apply the formula, check the direction, because this is one of the most predictable marks in the Paper 3 practical of SPM Chemistry. Once the pattern is automatic, students stop losing this mark, and the same clear thinking about variables and direction feeds straight into identifying variables and drawing conclusions elsewhere in the paper.