Water cycle model

This activity shows one cycle of the water cycle: water evaporates from a salty 'ocean', condenses into 'clouds' then drips into a freshwater 'lake'.

Prepare before class:
Lay a piece of cling wrap over the top of the plastic tub (Glad brand works well; test cheaper brands first). Use masking tape to secure the cling wrap most of the way round, leaving it open the last 1/3 of the length of the tub.
Through the opening, add the salt and food colouring to the bottom of one end of the tub. (Alternatively, students can later add the salt as you set up the water cycle model.)

In class:
Boil the water, then immediately pour it into the tub on top of the salt/food dye, to a depth below the rim of the pot. (In my 6L plastic tub it is only just over 1cm deep.) This is the 'ocean'. The food dye will follow the salt, so will show us which water is salty in the system.
Note: the water cycle model will not be dramatic unless the water added to the tub is very hot i.e. only just boiled. When I set up more than one water cycle model, I reboil the water between adding to the tub at each table group.

Immediately after the boiling water is poured in a tub, place the pot in the centre of the 'ocean' (I do not add before I pour the water as sometimes food dye will splash into the pot). As fast as possible, fold the saran wrap over the tub and clip in place with the mini binder clips. The cling wrap will balloon up; push it back down slowly so that it dips slightly into the tub. Place the marble on the top of the cling wrap, directly over the little pot, and push gently in place so that it does not roll away.


The 'ocean' water evaporates (faster than real ocean water as it is warm) and water vapour fills the air space inside the container.
As the water vapour cools on the sides of the container or the underside of the cling wrap it condenses into droplets. This models 'cloud' formation.
After 5 mins or so, condensed water will flow down the underside of the saran wrap and drip from under the marble into the little pot. (The marble keeps the cling wrap at a low point so that the water runs to there.) I return to the marble pushing it down a little to help water drip into the pot, pointing out the drips to the students, and asking them what part of the water cycle is dripping water - rain!
After 10 or 15 minutes there will be enough 'rain' to have made a puddle (a 'lake') in the little pot.
Ask the students what colour the 'lake' is compared to the 'ocean'. The lake has no colour - the food dye stayed with the salt in the 'ocean' and did not evaporate with the water. Only pure water evaporates. Hence the lake is freshwater.

The students can optionally taste and compare the water in the 'lake' then the 'ocean': give them a Q tip to dip first in the lake water and taste, then use the other end of the Q tip to dip in the ocean and taste. The ocean water is salty and the lake water is fresh. Only the water evaporates from the ocean - the salt is left behind.

This activity can also be used to explain the usefulness of a model.
A model is often smaller/larger than the real thing and/or faster/slower than the real thing, and helps us visualize and understand the real thing.
This water cycle model is smaller and faster than the real water cycle.
It also only shows one cycle of the water cycle (ocean to water vapour to clouds to rain to lake). The water cycle keeps cycling around and around.