The Drying Time Trial
A student investigates whether adding table salt speeds up the evaporation of water. She places 50 mL of tap water in each of four identical shallow dishes. She adds 0 g, 2 g, 5 g, and 10 g of salt to dishes A, B, C, and D respectively and stirs each for 30 seconds. She leaves all four dishes on the same bench near a window and records the mass of each dish every 30 minutes for four hours. Dish D still contains visible liquid at the end, while dish A is dry after three hours. She concludes that salt slows evaporation and that this proves salt 'holds onto' water molecules. Her friend notes that the dishes were not covered and that the window was opened during the afternoon.
1. Identify the independent, dependent, and at least two controlled variables in this investigation, and state one variable that was NOT adequately controlled.
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Independent variable: the mass of salt added (0, 2, 5, 10 g). This is what the student deliberately changes.
Dependent variable: the rate of evaporation, measured indirectly as the change in mass of each dish over time. Mass lost is mostly water vapour leaving the dish.
Controlled variables (at least two): volume of water (50 mL), type and size of dish, starting temperature of the water, stirring time (30 s), position on the bench, and the timing interval of measurements.
Not adequately controlled: the dishes were uncovered and the window was opened, so air movement and temperature were not the same for all dishes at all times. A draught could increase evaporation from one dish more than another, confounding the comparison. A fairer design would cover the dishes with identical mesh or place them in a still-air location, and monitor room temperature.
2. Dish D still had liquid after four hours while dish A was dry. Does this support the conclusion that salt slows evaporation? Explain what else could explain the result and how the design could be improved.
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The observation is consistent with salt slowing evaporation, but it does not prove it, for two reasons.
First, the dishes contained different total masses of material. Dish D had 10 g of dissolved salt in addition to the water, so even if all the water evaporated, the dish would not become dry in the same way - the salt would remain as a residue. 'Still liquid' and 'dry' are therefore not directly comparable end points across treatments. Measuring mass change per unit time, rather than judging dryness by eye, is the better dependent variable.
Second, the open window introduced uncontrolled air movement and temperature changes that could affect dishes unequally.
Improvements:
- Use the same total volume in every dish and record mass loss at fixed intervals.
- Keep all dishes in still air, covered with identical mesh to exclude draughts but allow vapour escape.
- Repeat the whole experiment at least three times and average results.
- Include a control dish with no salt and compare rates rather than end points.
Only after these controls could the student claim that salt concentration affects evaporation rate.
3. The student claims the result 'proves salt holds onto water molecules'. Evaluate this as a scientific explanation.
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This is a hypothesis or interpretation, not a proven mechanism. The data show an association between added salt and slower mass loss; they do not show how or why.
A scientific explanation should:
- Be stated as a testable claim - for example, 'dissolved salt lowers the vapour pressure of the solution, so fewer water molecules escape per unit time'.
- Be supported by evidence that could have refuted it - for example, measuring evaporation rate at several salt concentrations and showing a consistent trend.
- Avoid untestable phrasing such as 'holds onto'. Molecules are not 'held' in a way that can be directly observed here.
The student could strengthen the claim by testing more concentrations, measuring temperature of each solution (dissolving salt can change temperature slightly), and comparing with a non-salt solute such as sugar to see whether the effect is specific to salt. Until then, the conclusion should be phrased cautiously: 'Under these conditions, adding salt was associated with slower water loss.'