Why are the results like this?
It's because of SURFACE TENSION.
According to research, the addition of soluble impurities in water might result in changes in surface tension. Surface tension will affect the rate of evaporation because it is caused by the cohesive forces between the molecules at the surface of the liquid. Hence, the higher the surface tension, will result in stronger forces between the molecules, hence it is more difficult for these molecules to break apart and evaporate, resulting in a lower evaporation rate. Different soluble impurities might affect the surface tension in different ways. Impurities that have a negative surface excess when placed in water will increase the surface tension, whereas those with positive surface excess will decrease the surface tension. Salt in water has a negative surface excess, whence, more effort is needed to push the salt molecules away from the surface such that evaporation can take place, hence evaporation rate is slower. This can be shown through my investigation as the evaporation rate of the salt solution is lowest at 0.185ml/hour which is 77.08% of the evaporation rate of water. Sugar is not known to affect the surface tension because it has neither a positive surface excess or negative surface excess. However there might be a minimal increase in surface tension, like proved in the experiment where the evaporation rate of the sugar solution is 0.0231ml/hour which is 96.25% of the evaporation rate of water. Research has also proved that insoluble impurities do not affect the rate of evaporation. This is because it does not alter the vapour pressure or the evaporation rate in any way. So, water containing insoluble impurities should have the same evaporation rate as plain water. However, my result proved to be inaccurate, the evaporation rate of the water with sand is 0.0198ml/hour, which is 82.5%of the evaporation rate of plain water, which is 0.0240ml/hour. This is probably due to the fact that the sand sample I collected was not 100% sand and might have contained some soluble impurities that altered the surface tension and the evaporation rate. Hence the fact that the sand sample was not purely sand is one of the limitations of my experiment. However, if the sand sample was purely sand, the evaporation should be the same as that of the plain water.
discovery @ 9:11 PM