Task 1: Melting of ice in a solution with salt

At least one day before the beginning of the laboratory practical, the teacher must prepare three 1.5 Litre PET bottles, which he/she fills up with water and leave them in the freezer to freeze until the next day. He can, of course, use other containers, which are also suitable for freezer use.

On the day of the laboratory practical the teacher prepares for all the students in the classroom 3 hammers, 3 tea towels or any other rugs that will the students use for wrapping the ice. He/she takes out the PET bottles from the freezer, cuts them open and takes out the ice. It is advisable to have the ice ready in the room so that its temperature rises to 0 °C. The temperature in the freezer is usually around –17 °C.

For each working group, the teacher prepares the following equipment on the table: 2 big bowls of various size or a bowl and a beaker, table salt (NaCl), thermometer for measuring lower temperatures (at least around –20 °C), a teaspoon, time gauge (pupils can use their own devices for measuring the time, for example clocks, mobile phones), juice or water with syrup, a plastic bag.

Pupils crush the ice wrapped up in the tea towel or a piece of rug into small pieces by the hammer. They put three units of weight of ice and 1 unit of weight of table salt into the bigger bowl. They stir the mixture of ice and table salt with the spoon and observe the change in temperature in the bowl. Pupils record the temperature change of the ice and table salt mixture depending on time in the table. They read the values every 30 seconds.

In the next step, pupils put the plastic bag into the smaller bowl or the beaker and fill it at least up to a half with some juice or water with syrup. They will tie up the plastic bag and while in the beaker, they put it into the bowl with the ice and table salt mixture. They wrap up the bowl in the tea towel or a piece of rug and leave it to stand in the corner of a desk, for at least 20 minutes. Meanwhile, they work on other set tasks.