Why do puddles disappear overnight?
They certainly never reach the boiling point of water (100oC).
The answer is EVAPORATION.
Evaporation occurs below a liquids boiling temperature and it works because all of the particles in a liquid are at different temperatures. Their temperature decides how fast they are moving around and they change their speeds with every collision they have with another particle, randomly getting quicker or slower with each collision.
The key time though is when a particle randomly finds itself on the suface. It it reaches the surface with a lot of energy (at a high temperature) then it can escape and become a gas.
It not it just stays as a liquid.
So after time a liquid will lose its hotter particles and just keep hold of its cooler particles.
This causes the average temperature of the liquid to drop.
When we sweat, the sweat evaporates off our skin.
The hotter particles in the sweat escape leaving the cooler particles
These cooler particles steal our heat and cool us down.
Investigating evaporation
Any investigation needs us to decide on our variables.
Our Dependant Variable is the factor that we are measuring. We would typically measure how quickly a lioquid is evaporating.
Our Independant Variable is the factor that we will change. By changing it we would be trying to see if it affected our dependant variable.
Every other factor should be kept the same to have a valid experiment. Otherwise how would you know what was making your dependant variable change?
Factors we keep the same are Control Variables.
Choose your Independant Variable
There are 2 obvious factors that will affect how quickly a liquid evaporates.
1) The surrounding temperature.
This could be changed by having your liquids placed in rooms of different temperatures.
2) Wind or passing air flow.
This could be changed by having an electric fan blowing next to your liquid.
You could vary the strength of the wind by placing your liquid different distances from the fan.
e.g. 20cm, 40cm, 60cm, 80cm, 100cm.
Measuring the rate of evaporation
This is your dependant variable. There are a couple of ways that I could suggest to see how quickly a liquid is evaporating.
1) Measure out a volume of liquid in a container. (Keep the volume the same for each container, this is your control variable.)
Record the volume at the start of the experiment. Leave each for the same amount of time (another control variable) and record the volume after the time has passed. The less liquid that is left, the quicker the evaporation.
2) Wet cotton wool and attach it to a temperature sensor or a thermometer using an elastic band.
Record the temperature at the start of the experiment. Leave each for the same amount of time (control variable) and record the temperature after the time has passed.
This is using the knowledge that evaporation leaves the liquid that is left at a lower temperature.
The cotton wool that is the coolest would have undergone the most evaporation.
