Water cooling

The water level in the tube drops.

The Erlmeyer flask is filled to the brim with water dyed with food coloring or ink. It is then sealed with the rubber stopper so that there is no air left in the flask. Now push the glass tube through the hole in the stopper into the flask until the water in the tube is about ¾ full. The flask is then placed in the container filled with crushed ice so that the flask is almost completely surrounded by ice.

• 1 Erlmeyer flask,
• 1 glass tube,
• 1 rubber stopper with hole (suitable for the Erlmeyer flask),
• water,
• food coloring or ink,
• 1 container with crushed ice,

What is technically behind the experiment

Temperature characterizes the thermal state of a substance. According to the kinetic theory of heat, it is based on the movement of molecules at different speeds. Temperature is a measure of the average speed of the particles. The temperature is not attributed to a single molecule, but always to the entire molecule collective. Heat, on the other hand, refers to the energy that flows between bodies of different temperatures. Heat triggers physical and chemical effects as well as physiological sensations. The following example illustrates that heat and temperature mean different things in physics: If you heat ice from zero degrees, the ice-water mixture retains the temperature of zero degrees for quite a while. In this example, heat is constantly being added without the temperature changing. If the thermal state of a substance is changed, this has an effect on its volume. Almost all substances expand when heated and contract when cooled. This property of most substances is used as the basis for the various thermometers. Liquids are usually used here, as the change in volume can be visualized particularly well. The volume expansion of liquids depends on the following factors: the initial volume V0, a material constant, the specific expansion coefficient b, and the temperature difference between the initial and final state DT. The volume difference D V is then calculated as follows: A heat exchange between two substances always takes place when one of the two substances has a higher temperature than the other. A certain amount of heat is transferred from the warmer to the colder substance. In the experiment above, a heat exchange takes place between the ice and the liquid in the Erlmeyer flask (via the glass of the Erlmeyer flask). Heat is extracted from the water. Just as water expands when heated and thus increases its volume, it contracts when cooled and reduces its volume (this applies to water until it cools down to approx. 4° C.), which is why a drop in the water level can be observed.

Monika Abhar