Take small but equal amounts of water and milk in two different but similar
containers. Freeze them by putting in a refrigerator. Now melt them in the sunshine,
boil them on the same stove under the same conditions, and evaporate them
completely. Note their temperature and time during changing of their states. What
similarities and differences you find while changing the states of both of the
materials. Support your investigation results with time and temperature graph.
(5 marks)
Answers
Explanation:
All liquids evaporate if exposed to certain elements. The rate at which a liquid evaporates depends on its molecular structure. The other factors that affect evaporation are surface area, temperature and air movement. You can do a few fairly simple experiments to demonstrate the effect that the various factors have on the rate of evaporation.
Testing the Effect of Surface Area
Molecules contained in a liquid evaporate from the surface area. This means that the larger the surface area, the faster the rate of evaporation. Test this by putting water into two different containers. Use one that has a diameter of a 3 or 4 inches, such as a glass, and another that has a diameter of 8 to 10 inches, such as a bowl. Put 2oz of water into a measuring jug then transfer it to the glass. Do the same for the bowl and then place the containers next to each other. This means all the other factors that affect the rate of evaporation are identical. Leave the containers for one hour. Pour the water from each container into the measuring jug and write down how much water is left. The amount of water left in the bowl is much less than is left in the glass, due to the difference in surface area.
Testing the Effect of Temperature
Temperature affects the rate of evaporation. The higher the temperature, the more molecules move, enabling them to escape from the surface of a liquid. Fill two same-size glasses with 2oz of water. Put one glass in the refrigerator and the other in a warm place, perhaps near a heater, or on a sunny window sill. Leave the water for an hour, then pour the water from each container into a measuring jug. You find that practically no water has evaporated from the glass in the refrigerator. However, the water in the warm-placed glass has reduced. This proves that the rate of evaporation is affected by temperature.