Solar Still

Safety Note

This is an outdoor activity and you will spend time exposed to the sun, apply the current measures like applying sunscreen and wearing sunglasses, hat and long sleeve shirt.

Activity Description

1. Dig a hole about 30 cm (about 12 inches) deep and 60 cm (about 24 inches) in diameter in a sunny spot. The hole can be smaller in diameter, if needed.
2. Collect fresh vegetation, like weeds or vegetable waste, and fill the hole with it. Grass clippings work well in suburban settings.
3. Place the jar or glass in the centre of the hole, ensuring it has a firm base supported by the ground, not the plants.
4. Cover the hole with plastic foil, ensuring it is well sealed. Use stones on the edges to stretch and tense the plastic foil.
5. Place the pebble or the small object in the centre of the plastic foil, so that it makes a dip there, which must be exactly above the jar or glass in the hole.
6. Wait at least 2 hours while the sun is shining on the plastic foil and observe from time to time what happens. The accelerated greenhouse effect inside the covered hole will generate condensation. The water will evaporate, condense on the plastic foil, and drip into the container.

Evaluation

Could you observe the collected moisture in the glass container? The amount of water collected will depend on factors such as sunlight exposure, soil type, and vegetation used.

Exploring the Science Behind

• In this experiment, we see the evaporation of water from two different “water reservoirs”, the plants and the soil.
  • One of the many important functions of soil is storing water. When it rains, one part of the water seeps into the ground, while the excess flows off on the surface. Depending on the type of soil, more or less water is retained and stored, like in a sponge. For example sand stores almost nothing, while humus or clay retains quite a lot. This is the water that the plants need to grow, that avoids that the soil to dry out and that provides one part of the groundwater supply.
  • The plants absorb water from the soil with their roots. From there, it is “pumped” up to the stems, leaves etc., where one part of it evaporates, which means it changes from a liquid state to gas. If the stems, leaves etc. are cut, they still contain some water.

• One of the effects that we can study in this experiment is the water cycle. From the soil and the fresh vegetation, the water evaporates. This water vapour then rises towards the plastic foil, where it cools down and condenses. The small water particles join, slide along the lower side of the foil, become too heavy and fall like raindrops into the jar.

   

• This compares to the water cycle in large scale; On world level, the sunlight evaporates water - mainly from the sea, the rivers, the lakes, but in smaller amounts also from the plants, the animals and us, the human beings. The water vapour, heated by the sun, is warmer and has less density than the air around, which makes it rise. Warm air can absorb more humidity than cold air. This means that as the warm air rises and cools down, the water vapour starts condensing. These small water particles form clouds. And when the particles keep joining and become heavy enough, they fall down to earth as rain drops. The rain is absorbed by the soil or flows back to the rivers, lakes and seas etc., where the whole cycle starts again.