Recovering dissolved substances

Do you use sea salt in your home? If you do, you may wonder how it is taken from the sea. The key process in taking salt out of the sea is evaporation.

This process takes place at the surface of every liquid. When a liquid evaporates part of it turns into a gas. This reversible change takes place at the liquid surface. In a liquid the particles slide around each other. They keep hold of each other as they move so they do not fly apart. Deep in a liquid, a particle is surrounded by other particles but at the surface it only has particles below it and at the sides. There is nothing above it to hold it in place. If the particle has enough energy it can move so fast that the other particles around it lose their grip. When this happens, the particle flies off into the air and becomes a particle of gas.

Three things help evaporation to take place. They are dry air, wind and warmth. When liquid particles evaporate, they move about in the air and take up space. If there are a large number of water vapour particles, as occurs in humid air, there is not enough room for other particles in the liquid to join them. When this happens, the speed of evaporation is very slow. If the air is dry there is plenty of room for evaporating water particles and the speed of evaporation is fast.

When water particles evaporate they fill the spaces in the air just above the liquid and gradually spread out. When the particles spread out, they make space near the liquid surface into which other water particles can evaporate. You can think of the water particles as being in a queue. Those at the head of the queue are the ones that are spreading out. Those at the back of the queue are those at the water surface trying to get into the air. In still air the water particles spread out slowly. This means that only a few of the particles at the back of the queue, on the liquid surface, can become particles of water vapour and evaporation is slow. If a current of air blows over the surface it whisks away particles of water vapour and cuts short the queue. It also brings in air with plenty of space so evaporation can speed up tremendously.

Heat is a form of energy. If water is warmed its particles receive more energy and they use it to move faster.

At the surface of a liquid more particles reach a speed at which they can shoot out of the liquid surface and the speed of evaporation increases.

You could make sea salt in Britain but you would need at least a bucket and fire on a beach. When you had heated the water the particles would probably still have a problem pushing themselves into cold damp air. If you were going to make a living making and selling sea salt it would cost you a lot of money for firewood. People do make a living from taking salt from the sea but they do not live in Britain. They live in countries with hot, dry climates. The sea water is allowed to flow into larger shallow lagoons and the weather does the rest. The Sun warms the water and dries the air, while breezes blow across the shore and the lagoons. All three requirements for rapid evaporation occur and as the water disappears, tonnes of white crystals appear as if by magic.

Some people used to think that evaporation was an example of magic. Now we know it is the movement of water particles - if they get the chance.

Where does sea salt come from?

It comes from the land. The water in the sea takes part in the water cycle. This is a path of water between the sea, the air and the land. Water evaporates from the surface of the sea, not just lagoons on the shore. The water vapour rises and forms clouds. When the clouds go over hills and mountains they release water as rain. This flows through the soil and some kinds of rock and dissolves minerals. Salt is a very common mineral and, over time, large amounts of it have been taken from the land and carried into the sea by streams and rivers.

Why doesn't salt evaporate?

The salt is present in the water as a dissolved solid. Very few solids can change directly from the solid state to the gaseous state and salt is not one of them. When the water particles leave because of evaporation, the salt particles just join together and make a solid - a salt crystal.

When a salt crystal dissolves does it form again when the water evaporates?

No. The same crystal does not reform. A salt crystal is made from thousands of particles. When the crystal dissolves, they all spread out though the water and mix with the particles from other dissolved crystals. When the water evaporates, the particles of salt join with other particles close by. They cannot seek out other particles from the same crystal. Although the salt crystals that appear when the water evaporates look the same as the original crystals they contain a different group of particles.

Can boiling be used to separate salt from water?

Yes, it can. Boiling is a rapid form of evaporation. In boiling, the water particles have so much energy that some of them turn to water vapour and steam inside the liquid. When this happens, bubbles form. The bubbles rise through the water, pop at the surface and release the gases into the air.

Does not boiling damage the salt crystals?

No. All that happens is that the water particles leave the salt particles behind much faster so the salt forms more quickly. You may think burnt or melted crystals would form but they do not.

Is it possible to collect the water from a boiling salt solution?

Yes, it is. Once the steam has left the surface of the boiling solution, it begins to cool. When this happens, the steam condenses on dust particles in the air. A water droplet forms on each dust particle and, as there are thousands of dust particles above the boiling solution, thousands of water particles form. They form a white cloud which we wrongly call steam. The real steam is the hot invisible gas above the boiling solution. If a cool plate is held at an angle over the boiling solution the steam will condense on it instead of forming a cloud. Large water drops will form that flow down the plate and drip into a jug. When the salt solution has boiled away, you will have a pan of salt and a jug of water.