Why do the water drops form?
5. Weigh the mini-lakes
Students record the date, day number, the weight, and any changes to the mini-lakes in the Data Table: Weight of mini-lake and on the Weight of mini-lake over time graph in their Science Notebooks. Remind them to include the cover in the weight.
How long do students continue to weigh mini-lakes?
It is critical that students continue to weigh their mini-lakes until all the water has evaporated. How will they know when all the water is gone? The most important source of evidence is the weight data: the weight stops decreasing and the line connecting the points on the graph levels off. This evidence is supported by observations that the lake looks and feels dry. If time permits, you may ask students to figure out how much the mini-lake will weigh when evaporation of water is complete. (Weight of mini-lake with salt added minus weight of water added to the mini-lake originally.)
Time for weighing the mini-lakes is not built into the rest of the investigations. You will need to make a plan that works in your situation. By now students are able to weigh their mini-lakes independently. Can they weigh their lakes before school? Or before you begin science class?
