This post was inspired by Emily and Stephanie, who last week presented a lesson on the solar still to their peers.
Without clean water to drink, humans do not last very long at all. So when Pi is adrift on the Pacific Ocean, one of his first challenges (aside from cohabitation in a confined area with a tiger) is to find drinking water to sustain him (Martel, 2002). Thankfully, he discovers a set of solar stills on his boat, and promptly sets them up upon the sea water.
Two sets of instructions, with explanations, are available from the CSIRO and the Surfing Scientist (ABC). Both explanations are sufficient, but I don’t think they do the best job they could! Warning: I’m a primary school science teacher, so my explanations tend to be fairly basic. But that doesn’t mean they’re unsophisticated…
Setting up a solar still
You will need:
- an ice cream container or bucket
- a clean cup
- a permanent marker
- some tack
- some plastic wrap or plastic sheeting
- a weight (such as a marble, stone, or nut)
- (optional) items to make your water “dirty”, such as salt*, food colouring*, and sand.
- (optional) sticky tape
How to set it up:
- Mark off 10 ml, 20 ml, etc up the inside of the cup using a marker. Afterwards, wash the cup or make it clean without removing the permanent ink.
- Use the tack to fix the cup in the centre of the bucket. It should stay there even when water is added to the bucket.
- Pour water around the cup. You don’t need much; if you want a cup of water, use a cup of water. (There could be an investigation here for a curious student, into the relationship between the amount of water added to the bucket and the amount of water extracted from the still over time.)
- (Optional) If you want the water to be “dirty”, add those things that will make it dirty. I particularly like to use food colouring and/or salt, because it adds a further dimension to the explanation later.
- Cover the top of the bucket in plastic wrap. The edges should be sealed as best as possible. If the wrap isn’t sticking to the bucket, you might like to tape it down.
- Put the weight on top of the plastic wrap, over the top of the cup. It should force the plastic wrap into a sort of almost-flat cone, where the lowest point of the wrap is where the weight it.
- (Optional) Weigh the solar still using kitchen scales.
- Make a prediction. What do you think will happen? What will end up in the cup? Does the solar still weigh more, less, or just the same?
- Put your solar still in the sun. Wait a few hours. Make some observations. A particularly attentive child might like to take photos every half hour or so, and keep a record of how much water fills the cup.
- (Optional) Weigh the solar still using kitchen scales, again.
- (Optional, at the discretion of the parent, as I take no responsibility for this part!) Drink the water! Mmm, tastes fresh, right?
So what happens?
Evidence is about what we observe, without interpretation. This is what you should observe as your solar still sits in the sun:
After the solar still has been sitting in the sun for half an hour or so, it starts to look foggy inside.
Soon, the foggy bits on the plastic join and become droplets that stick to the plastic.
The droplets grow larger as they join and then travel down the plastic to its lowest point.
At the lowest point of the plastic, the droplets combine and then fall into the cup.
The droplets are transparent, as is the liquid in the cup.
Over time, the amount of liquid in the bucket goes down, but the amount of liquid in the cup goes up. If you weighed the solar still before and after sitting in the sun for a few hours, you *should* find that the whole thing weighs the same at the end as it did at the beginning.
The first part is really cool, and something I think many of us don’t fully appreciate. First, light from the sun (some visible to our eyes, some not)that has travelled an amazingly large distance at astonishing speeds hits the bucket. This transfer of light energy is called radiation. Light from the sun is absorbed by the bucket and everything within it, and the energy is transformed into kinetic energy by the molecules that make up the bucket and everything in it, causing them to vibrate faster. This is thermal energy, and when we touch the bucket it is transferred to our skin as heat.
People don’t often think about why it’s warmer in the sun than in the shade. But it’s because when the sun’s light hits you, it gives its energy to you and that extra energy is felt as heat. Isn’t that cool? It’s just the same when a strong lamp, such as one with a halogen bulb, shines on you. That’s not convection happening – hot air rises above cooler air, so it’s not the air bringing you the warmth – that’s the light energy that transforms when it is absorbed by your skin!
Next, the extra energy in the water causes some of it to evaporate. It turns into the gaseous form of water, which we call water vapour. Water vapour is invisible to our eyes; we just can’t see something that small. However, as it rises, the water vapour loses some energy and condenses; it clumps together, particularly when it hits the plastic wrap and gives off some of its energy to the plastic. We can see these clumps of condensed water. Initially the droplets are very very small and look like fog. Over time, they grow bigger because water is a particularly sticky molecule; it bonds easily with other molecules. So the droplets become bigger and bigger and heavier and heavier until they slide along the plastic wrap until they reach the lowest point, where they fall into the cup in the centre.
This is a similar process to the water cycle, in which water in its liquid form is evaporated by the sun’s rays, and rises as water vapour into the atmosphere. Along the way it loses energy and encounters particles of dust, etc, which cause it to clump so that we can see it, in the form of clouds. The droplets in the cloud become bigger and bigger and heavier and heavier, until eventually they can no longer be supported by the wind currents and other factors in the atmosphere, so they fall as precipitation, hitting the land and (eventually) returning to the rivers, lakes, seas and oceans, or else evaporating along the way. These phase changes are physical changes: the substance (water) changes in physical properties but not chemical make-up.
Finally, let’s look at how the liquid in the cup is different to the liquid in the bucket.
The liquid in the bucket, if you took the optional route of making it dirty, is a mixture of sand, salt, and food colouring. The sand is always visible and the food colouring changes the colour of the water, but the salt dissolves in the water and becomes invisible. Is this a physical change, where the two substances are mixed but do not react to make a new product, or a chemical change, where the two substances react to make at least one new product?
It’s difficult to argue that the salty water is just water with some salt in it; how can you tell, Miss Pezaro? How do you know? It is also difficult to convince students that adding food colouring to the water is merely a physical change; after all, Miss Pezaro, there’s been a colour change and you told us that was evidence for a chemical change (sometimes it is, kids, sometimes).
But the solar still separates this mixture, with what can be taste-tested as water sitting clear in the cup, and leaving behind the sand, salt and food colouring and just a small amount of water. It’s a nice way of showing primary-aged students that no new products were formed, and that food colouring and salt are merely dissolved into the water in the first place.
As for a chemical explanation for dissolving, we might leave that until we have some atomic ideas and a basic understanding of polarity, shall we?
The design of Pi’s still is a little bit different. His looks like an inverted plastic dome. Water from the ocean is evaporated and hits the plastic cover, where it condenses and collects, running down the sides into a channel that goes all the way around the still. The water runs down the little tube into the plastic bag, which I presume Pi can detach and drink from. The mechanism by which Pi’s still works is the same as ours; his simply has a different design. I wonder which is more efficient, or which works better in different conditions? What other designs might work?
Have you tried making a solar still before?
Is there anything you would improve about my explanation for the solar still?