Summary Demonstrate how freshwater lakes are made from salty ocean water, using a closed container with a pot to collect evaporated then re-condensed water. Science content Earth/Space: Water cycle, Water conservation (2) Science competencies (+ questioning + manipulation + others that are in every activity) Questioning/predicting: predicting (1 up), hypothesizing (7) Evaluating: inferring (3 up) Lessons activity is in Water cycle Materials For each model (e.g. one per table group) sturdy plastic storage tub ~6L (roughly 30cm long x 15cm wide x 15cm tall) small pot or bowl (that is less than half the height of the storage tub) 2 tablespoons salt 5 drips food colouring cling film to cover the top of the tub with at least 5cm overlap (I found Glad wrap worked way better than Titan brand) masking tape 4 mini binder clips kettle and water a marble optional for tasting water: Q-tips, one per student Procedure Prepare the activity: Lay a piece of cling wrap over the top of the plastic tub (Glad brand works well; test cheap brands first). Use masking tape to secure the cling wrap most of the way round, leaving it open the last 1/4 of the length of the tub. Through the opening, add the salt and food colouring to one end of the plastic tub. Alternatively, students can later add the salt as you visit each of their tables to 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'. Note: it will not work unless the water is only just boiled. The food dye will follow the salt, so will show us where the salt is in the system. Immediately place the pot in the centre of the 'ocean' (I do not add before 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 in the middle of the cling wrap, and push gently in place so that it does not roll away, and so that it is directly over the small pot. The 'ocean' water will evaporate (faster than real ocean water as it is warm). As it 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 pot. After 10 or 15 minutes there will be enough drips of water in the pot to have made a puddle (a 'lake'). The lake has no colour - the food dye The food dye does not evaporate, only the water molecules, so the food dye does not end up in the lake - hence the lake is not salty but freshwater. The students can optionally taste the water in the pot ('the lake') and the tray ('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. The food colouring also stays in the ocean, but does not appear in the lake. Grades taught Gr 1 Gr 2 Gr 3
