Q1.
If a hot, saturated solution of copper sulfate is cooled rapidly in ice-cold water, smaller and less well-formed crystals will form than if it is cooled slowly at room temperature. How would you design and perform an experiment to test this hypothesis?
Answer
Design a controlled experiment in which the rate of cooling is the only thing you change.
Hypothesis being tested: faster cooling of a saturated copper sulfate solution gives smaller and less well-formed crystals.
Variables
| Type | In this experiment |
|---|---|
| Independent variable (what you change) | Rate of cooling — ice-cold water bath versus still air at room temperature |
| Dependent variable (what you measure) | Size and shape of the crystals formed |
| Controlled variables (kept the same) | Same stock solution, equal volumes, same starting temperature, same type of beaker, both left undisturbed for the same time |
Procedure
- Prepare a hot saturated solution of copper sulfate as in Activity 5.3 and filter it while hot to remove insoluble impurities.
- Divide the hot filtrate into two equal parts in two identical beakers, and note the starting temperature of both with a thermometer.
- Stand beaker 1 in a bath of ice-cold water. Leave beaker 2 on the bench at room temperature. Cover both with watch glasses and do not disturb them.
- Record the temperature of each beaker every two minutes, so that you have evidence of the two different cooling rates.
- After the same fixed time, filter out the crystals from each beaker, rinse them with a little cold water and dry them on a watch glass.
- Compare them: measure the longest edge of about ten crystals from each beaker with a scale or a magnifying glass, and note how sharp and regular the faces look.
Expected result: beaker 1 gives many small crystals with rough, poorly formed faces; beaker 2 gives fewer but larger, shiny crystals with clear flat faces. That supports the hypothesis.
Why it happens: Crystals grow in two competing steps — new nuclei appear, and existing nuclei grow. Rapid cooling makes the solution strongly supersaturated all at once, so a very large number of nuclei appear together and share the same limited solute; each therefore stays small. Slow cooling keeps the supersaturation gentle, so few nuclei form and each has time to add particles in an orderly way, producing large well-shaped crystals.
Tip: A single trial is not proof. Repeat the whole comparison at least three times; if the same difference appears every time, the evidence is far stronger. Safety first: copper sulfate is toxic — handle it under adult supervision and never with bare hands.