"Evaporation" Return to list of pracs

Aim: This activity is used to investigate the rate of evaporation of water in the drying of clothes.

Equipment:

• outside clothes line;
• indoor clothes drying rack;
• roll of Scot paper towel;
• clock;
• pegs.

Method:

Give 2 pieces of Scot towel to each student. Write their name on each piece. Wet each piece, squeeze out and hang out on each of the clothes lines (indoor and outdoor). Write the time you hang both up and then time to see how long it takes for each to dry. You could do it on a hot or cold day, windy or still day and see if there is any difference. Offer explanations as to why one might dry faster than the other.

Background Knowledge:

There are a number of different things occurring. The greater convection air currents during the drying period the faster the drying. The water molecules on the surface on the cloth in collision with fast moving air particles are "knocked away" ... ie. evaporated. When water molecules reach a certain speed they can then escape and break the bonds with other water molecules nearby. The energy of fast moving air particles is transferred to the slow moving water molecules. Radiant heat from the Sun also provides energy to the water molecules to again allow them to have enough energy to escape the surface of the cloth. The faster particles move the higher the temperature. When you rub your hands together they get warm. This is because faster particles on the skin of one hand collide with particles on the other particles transferring their movement energy to these slower particles. This faster speed is a direct measure of their overall temperature. That is why rubbing your hands together heat up your hands. The act of shivering is a natural way for the body to go into spasms to make your muscles work rapidly. Again, this constant motion causes friction and heat energy is formed ... helping you warm up.

If it is a humid day, the amount of water vapour in the air is large. This tends to hamper any more water molecules from escaping from the surface of the cloth into an already crowded atmosphere around it. If it is a cold day, the air particles around the cloth are relatively slow moving particles. They cannot provide the same amount of energy necessary to "knock" the water molecules away from the surface of the cloth. Obviously, the best drying conditions are hot, dry and windy days.