Experimental Skills » Planning Experiments and Variables

Hypotheses: a reason you can test

Science often starts with an observation. A hypothesis is a possible explanation for it, and it must be something you can test with an experiment. In the exam you may be given some observations or data and asked to suggest a hypothesis. You use what you already know about chemistry (a scientific theory) to explain what you see.

Example: a student notices that bikes left by the sea rust faster than bikes kept inland. A good hypothesis is: "Salt in sea air makes iron rust faster." It gives a reason, and we can test it by comparing iron nails in salty water and in pure water.

A hypothesis is not just a guess. It uses a theory or earlier evidence, and it makes a prediction that an experiment can support or disprove.

Key terms:

• Prediction: what you expect to happen in an experiment if your hypothesis is correct.

Planning an experiment

You may plan an experiment to make observations, to make or identify a substance, to test a hypothesis, to check data or to explore something new. Whatever the purpose, a good plan describes a procedure that someone else could follow. It says what to measure, which apparatus to use, how many readings to take and how to stay safe.

A good plan manipulates one thing and controls the rest. That is why we need three kinds of variable.

We keep control variables the same so that any change in the dependent variable must be caused by the independent variable. If two things change at once, you cannot tell which one caused the effect.

Examiners also ask you to explain why a procedure is well designed. Link each feature to its job. For example: "The same volume of water is used each time so that it does not affect how fast the sugar dissolves."

Choosing apparatus and techniques

Pick the apparatus that does the job well, and say why. The best choice depends on how accurately you need to measure.

A good answer to "select the apparatus and explain why" names the item and gives a reason, such as "a stopwatch, because the dissolving time needs to be measured in seconds".

Working safely

Before you start, identify the main hazards in the practical. Then suggest ways to reduce the risk of harm. Match each method to its hazard.

• Corrosive or irritant liquids: wear eye protection, use a dilute solution, and wipe up spills straight away.
• Hot liquids and flames: tie back long hair, keep bags away, and let apparatus cool before touching it.
• Harmful gases or fumes: work in a fume cupboard or a well-ventilated room, and use small amounts.
• Glassware: keep it away from the table edge and clear up breakages with a brush and pan.

Handling apparatus correctly also matters. Read volumes at eye level, zero the balance before weighing, and hold a test tube at an angle with the open end pointing away from people.

Sampling: making sure a sample is representative

Sometimes you cannot test everything, so you take a sample. A sample is representative if it reflects the whole thing. A poor sample gives misleading results.

Example: to test the water in a large pond, one cup from the edge is not enough. A better technique is to collect water from several places and depths, then test each one (or mix them). For a solid such as soil or a powder, take small amounts from different places, then mix them together before testing. Choosing the spots at random avoids picking only the places that give the answer you hope for.

Look at the context in the question. Say where you sample, how many samples you take and why.

Making and recording measurements

Look carefully at the scale. Work out what each small division is worth before you read it. Record the value to the same number of decimal places as the scale allows, and always include the unit. Write results in a table as you go, with the units in the column headings, not in every box.

Observations count too. Record what you see, such as colour changes, bubbles or a solid forming, using clear words.

Evaluating a method

After an experiment, judge how good it was. You should be able to assess whether the measurements were sufficient and precise, and whether the method is valid.

Key terms:

• Sufficient: enough readings, over a wide enough range, to spot a pattern.
• Precise: repeat readings that are very close together.
• Valid: the method measures what it is supposed to measure, and the variables are controlled properly.

To improve a method, you might:

• take more readings, or over a wider range of the independent variable;
• repeat each reading and calculate a mean;
• use apparatus that measures more finely, such as a pipette instead of a measuring cylinder;
• control a variable that was not controlled.

You can also suggest a further investigation. If sugar dissolves faster in hot water, you could ask whether stirring changes the time, or whether salt behaves the same way.

Exam-style question

A student investigates how the temperature of water affects the time for a fizzy vitamin tablet to stop fizzing. She drops one tablet into 100 cm³ of water at 10 °C and times it. She repeats this with a smaller tablet at 30 °C and then a crushed tablet at 60 °C. She uses a beaker to measure the water.

(a) Name the independent variable and the dependent variable. (2 marks)
(b) Give two problems with her method, apart from how she measures the water. (2 marks)
(c) Suggest one safety precaution and one way to improve the accuracy of the volume. (2 marks)