How scientists find answers
Does sugar dissolve faster in hot water? Does stirring help? Answers based on a hunch are often wrong. Scientists use a method for finding answers called scientific inquiry科学探究.
Stages of an investigation
- Ask a question: “Does water temperature affect how fast sugar dissolves?”
- Make a hypothesis: use what you know to propose something testable: “The hotter the water, the faster sugar dissolves.”
- Plan the experiment: decide what to change, what to measure, and what to keep the same.
- Run it and collect evidence: observe carefully, record the data honestly, and repeat runs.
- Analyze and conclude: do the data support your hypothesis or not?
- Reflect and share: what were the weaknesses? Share the results so others can check them.
You don’t always go through these stages once in order. Often you loop back, revise, and try again.
Three kinds of variables
| Variable | What it is | In the “water temperature and dissolving” experiment |
|---|---|---|
| Independent variable自变量 | The factor you deliberately change | Water temperature |
| Dependent variable因变量 | The result that responds and that you measure | Time for the sugar to dissolve completely |
| Controlled variables控制变量 | Everything else you keep the same | Volume of water, mass and form of sugar, stirring |
Change only one variable at a time and keep everything else the same. That makes the comparison fair, and it’s called controlling variables控制变量法.
Try an investigation
Here’s a good plan: do one run as your baseline. Then change one thing at a time (say, only raise the water from 20 °C to 60 °C) and see how the time changes. Finally, try changing two things at once and see what feedback you get.
Every run uses 100 mL of water and 5 g of sugar
| # | Water temperature | Stirring | Form of sugar | Time to dissolve |
|---|
Example: designing an experiment
Design an experiment to find out whether stirring affects how fast sugar dissolves.
- Question. Does stirring make sugar dissolve faster?
- Hypothesis. Stirring makes sugar dissolve faster.
- Variables. Independent: whether you stir. Dependent: the time for the sugar to dissolve completely. Controlled: water temperature, volume of water, and the mass and form of sugar.
- Method. Take two cups of 100 mL of water at 20 °C and add 5 g of granulated sugar to each. Stir one and leave the other. Time how long each takes to dissolve completely, and repeat three times.
- Conclusion. If the stirred cup is clearly faster, stirring speeds up dissolving.
- Water temperature
It's the factor you deliberately change.
- Time for the sugar to dissolve completely
It's the result you measure.
- Volume of water
Keep it the same, or the comparison isn't fair.
- Mass of sugar
Keep it the same, or the comparison isn't fair.
- Whether you stir
This experiment is only about temperature, so stirring must stay the same.
- Form of sugar (cube or granulated)
The form also affects dissolving speed, so keep it the same.
Common mistakes
Mistake 1: Changing several variables at once. If you switch to hot water and start stirring, the sugar dissolves faster, but you can’t tell which change did it.
Mistake 2: Drawing a conclusion from one run. A single result can be off by chance. Repeat the experiment and check that the results agree.
Mistake 3: Changing data that don’t match your hypothesis. Record what really happened. Data that don’t support your hypothesis are a valuable result too.
Quick check
Sam wants to find out whether light affects plant growth. Which plan is correct?
Only the light changes. The kind of plant, the amount of water, and everything else stay the same.
In the experiment above, what is the dependent variable?
The dependent variable is the result that responds and that you measure: how well the plants grow.
Summary
- Scientific inquiry: ask a question, make a hypothesis, plan, experiment, analyze and conclude, reflect and share.
- The independent variable is what you change, the dependent variable is what you measure, and controlled variables stay the same.
- Change one variable at a time and repeat your runs so your conclusion is reliable.
On to AP (for later)
AP Chemistry free-response lab questions often ask you to design an experiment, name the independent and dependent variables, say which conditions to control, and explain how an error affects the result. That’s AP Science Practice 2, Question and Method.