dish temperature/°C air blown across? A 20 no B 40 no C 20 yes D 40 yes A student measures the mass of water lost by evaporation from four identical dishes ...
Question 1 Report
| dish | temperature/°C | air blown across? |
|---|---|---|
| A | 20 | no |
| B | 40 | no |
| C | 20 | yes |
| D | 40 | yes |
A student measures the mass of water lost by evaporation from four identical dishes in one hour. The conditions are shown in Table 1.1.
From which dish does the water evaporate fastest?
Answer Details
Evaporation rate increases with both higher temperature and greater air movement across the liquid surface. The dish at 40 °C with air blown across it combines both favourable conditions, so it loses water fastest.
A higher temperature means water molecules at the surface have more kinetic energy on average, so a greater fraction of them have enough energy to escape the liquid. Air blown across the surface carries away the escaped water vapour, preventing a saturated layer from building up just above the liquid. This keeps the concentration gradient steep and allows further molecules to leave.
The dish at 20 °C with no air movement has neither advantage, so it evaporates slowest. The other two dishes each have only one of the two factors, placing them between the extremes.
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