Worlds of David Darling > Children's
Encyclopedia of Science > From Glasses to Gases > Chapter 2
FROM GLASSES TO GASES:
The Science of Matter
a book in the eXperiment! series by David Darling
2. Solids and Solutions
The next time you take a sip of cola, think about what you are drinking.
It's mostly water, with some sugar, carbon dioxide gas (to make it fizz),
and a few drops of flavorings and colorings. Cola is a solution –
a mixture of several things that look and behave like a single thing.
| Seawater is a solution of different kinds
of salts in water
A solution is made by dissolving one or more substances in a liquid such
as water or alcohol. Ordinary table salt, or sodium chloride, dissolves
well in water. It has a high solubility. Huge amounts of sodium chloride,
together with other kinds of salts, are dissolved in the world's oceans
Solutions also contain a mixture of liquids and gases. The water in swimming
pools, for example, contains a dissolved gas called chlorine. This kills
germs so that the water is safe for swimming.
Solubility, like hardness and density, is an important property of matter.
You will need:
- A measuring cup
- A glass
- A teaspoon
- A magnifying glass
- A low-power microscope
- A battery
- Other powdered or crystalline substances, such as baking soda
- A shallow dish
What to do:
Look at a few crystals of table salt under the microscope. What shape
are they? Sketch some of the crystals. Do the same with a few crystals
of sugar. What do you think the shape of the crystals tells you about
the arrangement of atoms in these substances?
Pour a cup of cold water into the glass. Add a teaspoonful of salt
to the water and stir. Watch what happens through the magnifying glass.
When all the salt has dissolved, add another teaspoonful and stir.
Repeat this until no more salt will dissolve. How many teaspoonfuls
did you use?
Empty the glass and rinse it out. Pour in another cup of cold water.
Find out how many teaspoonfuls of sugar you can dissolve. Which has
the higher solubility in cold water – salt or sugar? Repeat
the experiment using hot water. Does the solubility remain the same,
increase, or decrease? Invent a theory to explain your results.
Taking it further:
Test the solubility of other substances at home or in your school
laboratory. Ask permission first and remember that many chemicals
are poisonous if swallowed. What happens if you pour a solution into
a shallow dish and allow it to stand for several days? Can you explain
Dissolving and Melting
Sugar will dissolve and it will also melt, but the two are not the same.
When sugar is dissolved in water, it makes a sweet-tasting solution. When
sugar is melted in a heated pan, it turns into a brown, sticky liquid that
is used in making caramel.
There are several important differences between dissolving and melting:
- You start with a liquid and something else, often a solid.
- The solid mixes into the liquid to make a new liquid.
- You don't need heat.
- You start with one sold only.
- You turn that one solid into a liquid.
- You have to heat the solid to melt it.
All substances will melt, but not all will dissolve. A substance that will
not dissolve in a liquid is said to be INSOLUBLE in that liquid.
Most solids become more soluble as the temperature of the liquid increases.
Gases, on the other hand, become less soluble as the temperature rises.
Natural water in lakes and rivers contains dissolved gases from the air,
such as oxygen and carbon dioxide. More gas dissolves if the water is colder
or the water flows over waterfalls, where it mixes more freely with air.
This explains why fish caught in rivers cannot usually survive in indoor
fish tanks. The water in such tanks is warmer, so it cannot dissolve enough
oxygen for the fish to breathe.
Molecules That Fit Together
Molecules come in all shapes and sizes, like the pieces of jigsaw puzzles.
The shape of a molecule affects how well it can work with other molecules.
Neither water nor alcohol molecules, for instance, fit together well with
their own kind. But when the two liquids are combined, the alcohol molecules
fit into the gaps between the water molecules. Because of this, the space
the mixed molecules take up is less than the original spaces they occupied.
The Missing Liquid
You will need:
- A cup of water
- A cup of rubbing alcohol
- A two-cup measuring cup
What to do:
Pour all the water and rubbing alcohol into the measuring cup. How
much liquid is in the cup? You might expect that it would be two cups,
but is it? Try to explain your results.
In Search of Solutions
|Water is a common solvent
A liquid that dissolves something is called a SOLVENT. The most common,
and one of the best solvents, is water. But there are many substances that
will not dissolve in water. Oils and greases are insoluble in water but
very soluble in other liquids, such as trichloromethane, or cleaning fluid.
This solvent is used by dry cleaners to remove grease stains from clothes.
Nail polish is insoluble in water but dissolves easily in a liquid known
as acetone. Nail-polish remover is made of acetone.