Your friend says that road salt makes ice melt faster. Another friend says sugar works just as well. How could you find out who is right? You could sprinkle some salt on one ice cube and sugar on another and watch. But what if one cube was bigger, or one sat in the sun? Then you would not know whether the salt or the sunshine made the difference. Science gives you a way to answer questions like this fairly: change only one thing, measure what happens, and keep everything else the same. This lesson walks through that process, step by step, so you can plan and judge experiments in every unit of this course.
The scientific experimentation process
- Ask a testable question. A testable question can be answered by measuring something, for example "How does the amount of salt affect how long an ice cube takes to melt?"
- Write a hypothesis. A hypothesis is a possible answer to the question, with a reason, that the experiment can support or not support.
- Plan the procedure. Choose the variables, list the materials and write numbered steps that someone else could follow exactly.
- Carry out the experiment and record observations. Write down what you see and measure, in a data table, as it happens.
- Analyse the data. Calculate averages, make a graph and look for a pattern or trend.
- Draw a conclusion. Say whether the data support the hypothesis, and describe sources of error.
- Communicate and repeat. Share the results so others can check them. New questions often lead back to step 1.
Testable questions and hypotheses
A question like "Is salt good?" cannot be tested, because "good" is not something you can measure. A testable question names what you will change and what you will measure: "How does the mass of salt added affect the time for a 20 g ice cube to melt?" A common way to write a hypothesis is If … then … because …: "If more salt is added to an ice cube, then it will melt faster, because salt lowers the temperature at which water freezes." The "if … then" part is a prediction (what you expect to see); the "because" part is the scientific reason. A hypothesis does not have to turn out right. A hypothesis that the data do not support still teaches you something.
Variables: change one, measure one, control the rest
| Kind of variable | What it means | In the salt and ice test |
|---|---|---|
| Independent variable | The one thing you change on purpose | Mass of salt sprinkled on the ice (0 g, 2 g, 4 g, 6 g) |
| Dependent variable | What you measure to see the effect; it depends on the independent variable | Time for the ice cube to melt completely (in minutes) |
| Controlled variables | Everything else you keep the same so the test is fair | Size of ice cube, starting temperature, room temperature, type of dish, type of salt |
A control group (or control) is a trial where the independent variable is left out or kept at its normal level, such as an ice cube with no salt. It gives you something to compare against. Do not mix it up with controlled variables, which are the conditions kept the same in every trial. Scientists also run repeated trials: doing each test several times (often at least three) and averaging the results. One odd result, from a cube that was slightly bigger or a timer started late, has less effect on an average of several trials. Testing more samples, such as several plants per pot size instead of one, does the same job.
- Describe with words, not numbers
- Colour, smell, texture, shape, sound
- "The salty ice became slushy and cloudy."
- "Bubbles formed on the nail."
- Use numbers and units from a measurement
- Mass, length, time, temperature, volume
- "The ice melted in 12.5 min."
- "The plant grew 3.2 cm in a week."
Conclusions and sources of error
A conclusion answers the question using your data: "As the mass of salt increased from 0 g to 6 g, the average melting time decreased from 31 min to 14 min, so the data support the hypothesis." Then name specific sources of error: things that could have made the results less accurate. "Human error" is too vague. Good examples: "the ice cubes were not all exactly the same mass", "the timer was started a few seconds after the salt was added", "the room warmed up during the afternoon". For each source of error, suggest an improvement, such as weighing each ice cube before starting.