Category Chemical World

One way of separating chemicals is to use a ‘filter’. A face mask is a type of filter. It is made of material that is full of tiny holes. Air can pass through these holes but particles of dust or paint are too big and get trapped on the outside of the mask.

A filter cannot separate salt from a salt solution. The sodium and chlorine atoms and the water molecules which form a salt solution are small enough to pass through the holes of any filter. If the solution is warmed, the water begins to change into vapour and eventually only salt is left behind. We call this process ‘evaporation’. Stalagmites form by evaporation.

Chromatography

There are three primary colours — red, blue and yellow. Most inks are made by mixing two or more of these colours. To separate ink, a process called chromatography is used. To try this yourself, draw a small circle on blotting paper using a water-based pen. Then put a drop of water on top of the circle. As the water spreads through the blotting paper, it carries the chemicals at different speeds and separates the colours.

Electrolysis

Electrolysis is a process used to separate elements from compounds. An electrical current from a battery is passed through a liquid — called the electrolyte. Some molecules in the electrolyte are positively charged and others are negatively charged. Positively charged particles are attracted to the negative electrode. Negatively charged particles are attracted to the positive electrode. The liquid begins to separate. Electrolysis can be used to purify metals.

A Face mask separates chemicals by preventing large particles from passing through the holes.

We have seen how to separate salt from a salt solution. If you look at salt under a microscope, you can see that each grain of salt is a perfect cube. The cubes form naturally as the salt comes out of solution. They are called salt ‘crystals’. All salt crystals are the same shape, but they may be different sizes.

Many chemicals form crystals. Sugar makes crystals and so does water when it turns to frost or snow. Each type of crystal has its own shape. Most rocks are made of crystals of chemical compounds called ‘minerals’. Granite is made up of crystals of ‘quartz’, ‘feldspar’ and ‘mica’. Crystals of diamond or emerald may be made into jewellery.

Salt crystals

Salt is made up of an equal number of sodium and chlorine atoms. When salt is in solution, these atoms are far apart. But as the water evaporates, the atoms get closer together. They always arrange themselves in the same way. This arrangement, called a ‘crystal lattice’, gives the salt its cubic shape.

Carbon

Carbon is an element: it is made up only of carbon atoms. But these atoms can arrange themselves in a number of ways to form several different types of carbon crystals. For example, charcoal is a soft black substance which can be used for drawing or may be burnt as a fuel; graphite is harder and is used to make the ‘lead’ in pencils. Surprisingly, diamonds are also pure carbon! Unlike charcoal and graphite, diamonds are extremely rare. They are used to make highly priced jewellery, and they also have industrial uses. Diamond is the hardest substance known and can be shaped to make cutting tools. Diamonds are also crystals of carbon. They are cut in a particular way to make them sparkle.

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Salt ‘disappears’ in water. But a taste of the liquid tells you that the salt is still there. So what has happened to the salt? When salt is mixed with water, the sodium and chlorine atoms break away from each other and move freely in the water. Gradually, the sodium and chlorine atoms and the water molecules are mixed up evenly. The liquid is called a salt ‘solution’. The salt has ‘dissolved’.

Many substances dissolve in water. Water is a good ‘solvent’. Carbon dioxide gas dissolves in water and makes it fizzy. Some substances dissolve in different solvents — paint dissolves in white spirit. Detergents ‘help’ water to dissolve oil.

Drops of oil floating on water will rush away from a drop of detergent added to the water. This is because detergent causes the surface of the water to change and the oil starts to dissolve.

Diffusion

If orange juice is poured down a straw into a glass of water, an orange patch forms in the middle of the water. The molecules of the water and the orange juice are moving all the time. Eventually, the orange molecules spread evenly throughout the water and the solution looks pale orange all over. This spreading of molecules is called ‘diffusion’.

Sea water is a massive solution of salt and water.

Oil-based paint does not dissolve in water, but it will dissolve in a solvent called white spirit.

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When water changes its state, its molecules are still made of hydrogen and oxygen — the chemical itself has not changed. When elements combine to make a compound, new molecules are formed. This is a ‘chemical change’.

We see examples of compounds forming every day. Most metals combine with chemicals in the air. Copper roofs slowly turn green as the copper combines with water and oxygen to form a new compound. An iron nail left outside soon starts to turn brown. The iron has reacted with water and oxygen in the air to form ‘rust’. Cars are made of several metals including iron. They have to be coated in paint to try to prevent them from rusting.

Baking a cake

When you bake a sponge cake, one of the ingredients used is baking powder. As it is heated in the oven, the baking powder breaks down into different chemicals. One of these is a gas — carbon dioxide. The bubbles of carbon dioxide throughout the sponge make it light and fluffy. The baked sponge is now a new mixture of many different compounds.

Copper roofs slowly turn green as the copper combines with oxygen to form a new compound.

Iron bridges need to be coated with chemicals to prevent them from rusting.

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SOLIDS, LIQUIDS AND GASES

Chemicals may be solids, liquids or gases. Water is a liquid. When water is cooled to below 0°C it freezes and forms ice, a solid. When it is heated to 100°C it boils and changes to steam, a gas. We say that water can change its ‘state’.

A chemical’s state depends on its temperature. Solids may turn to liquids and gases and then back to solids again, as the temperature rises and falls. We usually see metals and rocks in their solid state. When they’re heated, they become softer. If they’re heated to a high enough temperature, they melt and become liquid.

Ice

Because the molecules in a solid, such as ice, are held firmly together and can only move about a fixed point, they have a definite shape.

Water

As a liquid, water molecules can move around more freely. The ‘shape’ of the water depends on the container.

Water vapour

The molecules in steam can move freely in all directions, spreading further and further apart until they fill their container.

In the Arctic, the temperature is so cold that sea water freezes into huge solid glaciers.

Steam is given off by cooling towers. The water vapour molecules move freely and spread apart.

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Everything that exists — from the Sun in the sky to the centre of the Earth, and from animals to vegetables —is made up of chemicals. Many of these are familiar to us, such as water, salt, sugar, iron and oxygen. Chemicals can be different from each other in many ways. They have different tastes, like sugar and salt, or different appearances, like gold and silver. Chemicals come in many different forms; some are solids, others are liquids or gases.

Our world is composed of thousands of different substances. We call these substances ‘chemicals’. Chemicals make up the air we breathe, the ground we walk on and the food we eat. Even our bodies are a collection of chemicals!

Chemicals are often put into groups. Water, salt, sugar and oxygen are all chemicals. We call them ‘natural’ chemicals. Plastics, detergents and cosmetics are everyday chemicals too. But these do not occur naturally — they are ‘man-made’. Both types of chemicals are useful. Man-made cleaning agents remove dirt from our clothes and natural dyes from plants are used to colour fabrics.

Our food contains many different chemicals such as vitamins, proteins, fats, carbohydrates and sugars. Chemicals give fruit and vegetables their colour.

Water is made of chemicals and without it there would be no life on Earth.

Wool and cotton can be dyed with man-made chemicals or with natural chemicals from plants.

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