Speeding up dissolving

When people put sugar in their tea, why do they put the spoon in it too and stir it round? Almost everyone does it but how many people really know what they are doing? If they left their sugar alone it would collect as a pile at the bottom of the teacup. The particles of sugar next to the water would separate and slip between the water particles as the sugar began to dissolve.

In a short time the spaces between the water particles around the sugar would soon be full. As the spaces filled up there would be fewer and fewer places for other particles in the crystal to go. This would slow down the speed of dissolving but dissolving would not stop completely. This is due to the action of the water particles. They jostle the dissolved sugar particles just as football supporters sometimes jostle each other as they leave the stadium after a game. This jostling of the particles gradually moves some of the dissolved sugar particles away so that more spaces become available next to the undissolved sugar. As the spaces become available, a small number of particles keep separating and the dissolving process continues slowly.

When someone stirs their tea, they make currents of water in their drink. These currents move the water about the cup. Water with a large number of sugar particles dissolved in it is whisked away from the pile of sugar and water packed with empty spaces is brought near. This allows more sugar particles in the crystal to separate and enter the water. A moment later the water has moved on and is replaced by yet more water with empty spaces so the speed of dissolving continues to increase. Eventually all the crystals will have dissolved and a cup of sweet tea is ready to be drunk.

If someone is making gravy they do not just drop a stock cube into water. They break it up and drop in the bits. This may seem a messy thing to do but it makes the gravy more quickly. A stock cube has six large sides or faces. Most of the substances in the stock cube are hidden away from the water. Only the stock cube particles on the six faces can separate and dissolve and it takes time for them to enter the water. This means that it takes a long time before the water reaches the centre of the stock cube for the process of dissolving to be completed.

When a stock cube is crumbled it is made into hundreds of small pieces. Each one may have many small sides or faces. The area covered by these faces is much larger than the area covered by the six faces of the original cube. This means that more water can get in contact with the substances in the stock cube straightaway and dissolving can take place quickly. As the pieces of stock cube are only small it does not take long for the water to reach their centres and completely dissolve them. If you are going to be a speedy gravy maker remember to crumble your stock cube.

Heat is a form of energy and when particles of substance receive it they move faster. This gives a clue to why sugar dissolves faster in hot water than cold water. In the cold water the particles are moving slowly. They separate the sugar particles slowly and jostle them away slowly. This makes the speed of dissolving slow. In hot water the particles are more active and separate the sugar particles more quickly. They also jostle them away more quickly so the speed of dissolving is fast.

If you are helping an adult to cook remind them of the importance of stirring, crumbling and using hot water. Try and explain how these actions affect the particles in the food.

When you stir up tea after adding sugar don't the sugar crystals separate too?

Yes, they do. When they are in a pile some of the crystals are so close together that the water cannot get to all their surfaces and this slows down the dissolving process. If you stir hard the pile is destroyed and the crystals swirl round with the water. Each crystal is separate from all the others and water can attack all its sides. This also makes the sugar dissolve faster.

Do dissolved substances move through a liquid like a scent moves through the air?

Yes, they do. Gas particles are free to move. They crash into each other then bounce off and keep on moving. When a scent evaporates into the air the scent particles are hit by other particles in the air and slowly moved along. In a liquid the particles are close together but they still move about and dissolved substances can move through them. This movement of particles is called diffusion. It occurs both in gases and liquids.

When you make tea the water goes brown but the tea leaves do not dissolve. Why is this?

Some of the substances in the tea leaf dissolve and give the tea its brown colour. Parts of the tea leaf are insoluble and simply do not dissolve no matter how long you stir your cup of tea.

Is dissolving a widely used process?

Yes, it is. It is used in the making or manufacture of many things. Great care is taken to make dissolving take place as fast as possible. This helps the product to be made cheaply. The solvent and soluble substances are often mixed in large containers and a machine does the stirring.

When a saucer of warm salt water is left on a sunny windowsill you get salt forming round the edges. Why is this?

You may expect salt always to remain dissolved in warm water. The reason it comes out of the water is that the water is evaporating. When water evaporates there are fewer spaces in which the salt particles can stay. Eventually the salt solution becomes saturated. This means that all the spaces are full of salt particles. If any more water evaporates, the salt particles join together and form salt crystals again.

Do strong acids dissolve metals more quickly than weak acids do?

Yes, they do, but remember that an irreversible change is taking place to dissolve the metal. Dissolving is a reversible change. It is important to remember that strong acids are more hazardous than weak acids. There are also substances called alkalis which dissolve grease. Strong alkalis dissolve grease more easily than weak acids do. Strong alkalis are hazardous substances like strong acids. They should only be handled carefully by responsible adults.