My Science School

ACIDS AND BASES

Some substances and chemicals are very reactive. This means they easily combine or join with other substances in chemical reactions, to form new substances. Two types of reactive chemicals are acids and bases. Many bases dissolve in water: these are called alkalis. Strong or concentrated acids and alkalis are so reactive that they are corrosive. This means they break down and dissolve substances, including human skin, to cause severe chemical burns. Examples are the sulphuric acid in a car battery and the alkali sodium hydroxide, which is used as a drain-cleaner. Strong alkalis may also be slippery or slimy, almost like thin jelly.

A weak acid usually has a sharp or sour taste, like the natural citric acid in citrus fruits such as lemons and grapefruits. A weak alkali has a bitter taste, such as the caffeine in coffee. Caffeine is an example of an alkaloid - a natural alkali found in certain plants. Many plants make alkaloids in their leaves and stems. These are poisonous so that animals avoid eating them. Some animals have bites or stings that inject poisonous acid into prey or enemies.

Acids are substances with hydrogen in their molecules. For example, sulphuric acid is H2SO4 and hydrochloric acid is HCl. In solution with water, the hydrogen forms a positive ion, H+. This hydrogen ion is, in fact, a hydrogen atom without its electron - that is, it is just a proton. An acid is reactive because it is always ready to give up, or donate, this proton in a chemical change, in order to rid itself of the positive charge and become neutral. Alternatively, the acid can accept an electron, which is negative, to achieve the same result. This is why acids are known as proton donors or electron receptors. An alkali does the opposite and so is a proton receptor or electron donor.

            The fumes from vehicles exhausts, factories and power stations contain nitrogen and sulphur chemicals. These dissolve in the drops of water in clouds to form nitric and sulphuric acids which fall as acid rain.

            The human body contains a strong acid. This is hydrochloric acid, found in the stomach. It is made in the stomach lining in an inactive form, and released when food enters the stomach during a meal. The acid is very corrosive and helps to break down the nutrients in food as part of digestion. It also helps to kill any dangerous microbes (germs) that might be in the food. The stomach lining protect itself from being dissolved by its own acid by making a thick layer of slimy mucus.

A bee’s sting contains a mix of poisonous substances including acidic apitoxin. The bee leaves its barb and poison sack in the skin. The sting can be treated by rubbing on a weak solution of bicarbonate of soda, an alkali that counteracts the acid.

Like a bee, a wasp has a painful sting – also acidic, rather than alkaline as is popularly believed. The pain can be treated with a weak solution of ammonia, which is an alkali that neutralizes the acid in the sting.

Acids and alkalis are widely used in industry. Millions of tonnes of sulphuric acid are produced every year, not only for vehicle batteries but for processes such as making detergents, explosives, fertilizers and dyes for colouring. Sulphuric acid is also used in the splitting or refining of crude oil (petroleum) to make petrol, diesel, plastics, paints and other petroleum products.

ACID OR ALKALI?

Touching or tasting an unknown liquid to find out if it is an acid or alkali is far too dangerous - even deadly. The usual way is to use an indicator. This is a substance which changes its colour when added to an acid or alkali. One of the best-known indicators is litmus. It can be used as a liquid or on a dry paper strip. Normally litmus is a pinky colour. When it is added to an acid it turns red. Added to an alkali it becomes blue. If litmus does not change colour when added to a substance then the substance is neither acid nor alkali but neutral. Normal rainwater is a very weak natural acid because it contains tiny amounts of dissolved carbon dioxide, one of the gases in air, which forms carbonic acid. Acid rain from pollution is much stronger.

Acids and alkalis are very important in farming and forestry. Some soils are naturally slightly acidic, others are slightly alkaline. Each type of plant grows best in a certain type of soil, acidic or neutral or alkaline. To grow a certain plant the soil can be changed using additives. Adding alkaline lime to an acidic soil makes it more neutral.

            In medieval times the terrible disease plague killed millions. The dead bodies were sprinkled with lime, a strong and corrosive base. It helped to kill the germs in the bodies and make them rot faster.

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CRYSTALS

In many solid substances, the atoms or molecules are fixed in place but they are not positioned at random. They are arranged in an orderly or regular pattern known as a crystalline framework. The result is that the substance forms crystals. These are not irregular lumps but orderly, geometric shapes with sharp edges and flat sides at certain angles to each other. Many pure metals have a crystalline structure. So do minerals in the rocks, and sugar and salt.

There are seven basic shapes or systems of crystals. Simplest is the cubic shape which is like a box. Diamonds are cubic crystals. The monoclinic system is like a matchbox which has been squashed slightly flat. The calcium-rich mineral gypsum has this shape. Some natural minerals like ruby and emerald form large, shiny crystals with beautiful colours. They are cut and polished as gemstones.

Crystals of quartz in sand grains have a triangular shape. Ice crystals form snowflakes and are six-sided.

            Tiny crystals of minerals in many kinds of rocks fit together like miniature building bricks to form much bigger regular shapes. The hexagonal (six-sided) pillars are made of the rock basalt.

SOLUTIONS

Stir a teaspoon of table salt into a glass of water - and the salt disappears. However, tasting the water shows the salt is still there. It has dissolved. The large grains or crystals of salt have broken down into their individual atoms. These are too small to see and float about freely among the molecules of water. The substance which dissolves, which is usually a crystalline solid, is the solute. The substance it dissolves in, usually a liquid is the solvent. The solute in the solvent is known as a solution.

When substances dissolve, their atoms or molecules usually gain or lose electrons. For example table salt, sodium chloride (NaCl), dissolves and breaks apart into its atoms of sodium (Na) and chlorine (Cl). Sodium loses an electron and becomes positive (Na+), while chlorine gains an electron and becomes negative (Cl-). Atoms which are positive or negative are known as ions. Many solutes form ions.

Shaking salt and sand together produces a mixture, which is different from a solution. In a mixture two or more substances intermingle but they do not dissolve and their molecules do not alter by becoming ions. Add water to the mixture and the salt dissolves. It is soluble. The sand grains do not dissolve. They are insoluble.

            The sea contains dissolved salt. The Dead Sea has so much that no more can dissolve: a saturated solution.

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OXYGEN

We cannot see, smell or taste oxygen. Yet it forms one-fifth of air and is vital for life. We must breathe oxygen to stay alive. So must all animals and plants.

Oxygen is one of the commonest chemical elements on Earth. It forms about half the weight of the planet’s hard outer rock layer or crust. It makes up one-fifth of the air. Joined with hydrogen it forms the water which covers more than two-thirds of Earth’s surface.

Oxygen is a vital part of chemical changes inside each microscopic living cell, which break apart food substances to obtain the energy for life. This is why oxygen is essential for all living things (except for a few specialized types of microbes).

Oxygen is also needed for burning. A substance such as coal or wood burns by splitting apart and joining with oxygen to form new substances. It quickly gives out lots of heat and often light in the process. This is known as combustion (burning).

            Substances burn by joining with oxygen and giving out energy as heat and light. The welding torch burns acetylene gas.

HYDROGEN

The most abundant element in the Universe is hydrogen. It forms the bulk of most stars. On Earth, most hydrogen (chemical symbol H) is joined to oxygen (O) to form water (H2O). Hydrogen is also the simplest and lightest chemical element because each of its atoms has only two sub-atomic particles, one proton and one electron.

Hydrogen joins with carbon to form the substances known as hydrocarbons. Many of the fuel gases obtained from natural gas or crude oil, such as propane and butane, are hydrocarbons. Hydrogen also joins with carbon and oxygen to form carbohydrates. Starches in foods like potatoes and rice, and sugars in cane or beet, are carbohydrates.

            Pure hydrogen gas is much lighter or less dense than air. It filled the great airships of the early 20th century to keep them aloft. However hydrogen also burns very easily. After several disasters where airships caught fire, hydrogen was no longer used. Today airships use another light gas, helium, which does not burn.

            The Sun and other stars are mostly made of hydrogen. In the star’s centre, tremendous temperatures and pressures squash hydrogen atoms together to form atoms of the gas helium. As this happens, huge amounts of energy are released as heat and light. The energy travels up to the Sun’s glowing surface and then passes through space to Earth. This is known as nuclear fusion because the centres, or nuclei, of the hydrogen atoms join or fuse together. Nuclear energy on Earth is obtained by splitting nuclei apart, known as nuclear fission.

MOLECULES

Atoms make up all the objects and substances in our world. But they are rarely single atoms, alone or unattached. They are usually attached or joined to other atoms. For example, the oxygen gas that makes up one-fifth of the air does not float about as single atoms of oxygen, O. It is in the form of oxygen atoms joined together in pairs, O2. Two or more atoms linked or joined together make a molecule. O2 is a molecule of oxygen.

If atoms of one chemical element join or combine with atoms from other elements, this forms a compound. O2 is a molecule of oxygen but not a compound. Two atoms of hydrogen and one of oxygen form H2O, which is a molecule and a compound. Some compounds, like minerals in rocks, have 50 or 100 atoms in each molecule from 15 or 20 different elements. Other compounds, like certain plastics, have millions of atoms in each molecule but usually from only a few elements, mainly carbon, hydrogen, oxygen and nitrogen. The links between atoms are called bonds. There are different types of bonds depending on the atom’s structure and the conditions such as temperature and pressure.

One of the main features of the chemical element carbon is that it joins or bonds easily with many other types of atoms and also with itself. Carbon atoms can join like links in a chain to form enormously long molecules. Often the chain is made of the same groups of atoms, called sub-units, which are repeated hundreds or thousands of times along its length. This type of molecule is called a polymer and the repeated sub-units are monomers. Many types of plastics and artificial fibres like rayon, acrylic and nylon are polymers. So are molecules in living things like cellulose in plants, chitin in insect body casings and the carrier of genetic information, DNA.

The molecule known as DNA, found in our genes, is based on a group of atoms called ribose sugar, which is repeated millions of times in a long, coiled chain.

PRECIOUS METALS

Certain types of metals are precious or valuable. This may be because they are rare and difficult to obtain from their ores, so owning them has become a symbol of power and wealth. Some metals are prized for their beautiful colours and lustre or sheen. Some are valuable because they are easy to hammer or cast into detailed, intricate shapes such as thin wires or leaves.

Two of the main precious metals are gold and silver. People have fought wars and killed for them. These metals have become even more valuable recently because they are excellent carriers of electricity. They are used in switches, circuits and other devices in electrical equipment. Silver is also mixed with another metal, mercury, to make tooth fillings.

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METALS

Chemical elements can be divided into several groups. The largest group, forming about three-quarters of all elements, is the metals. Metals have several features that the other elements or non-metals lack. They carry heat and electricity very well compared to non-metals. They are solid at normal or room temperature. They are strong, hard and tough, and they can be polished to give a smooth, shiny surface. When they are squeezed under great pressure, they change shape or deform and become squashed, rather than splinter apart or shatter. These features are true of most metals, but not all. The metal sodium is very soft, while the metal mercury is a silvery liquid at normal temperature.

Metals are very important in the modern world. Because of their strength and hardness they are used to make all kinds of buildings, structures, machines and engines. The most widely-used metal in industry is iron - but not usually in its pure form. Iron is mixed with small amounts of other substances, especially carbon, to form steel. A metal mixed with other metals and substances like this is known as an alloy. There are hundreds of different kinds of alloy steels, each with slightly different amounts of carbon and other elements, and each designed to do a different task. Stainless steel is used for sinks and cutlery. Titanium and vanadium steels resist very high temperatures without melting.

            A modern plane like the Boeing 747 Jumbo has more than four million parts, and many of these are made of metals. The main panels of the fuselage, wings and tail are aluminium. This metal is strong but very lightweight.

            Not all metals stay hard and solid. Some can burn very brightly, especially in powdered form. Fireworks contain mixtures of powdered metals such as magnesium as well as other substances, to make them flare up with bright colours.

MAKING METALS

Very rarely a lump of pure metal is found lying on the ground, such as a gold nugget. But most metals occur inside rocks. Rock especially rich in a certain metal is called a metal ore. The metal is separated from its ore by various means. Iron is obtained from ores by heating them until they melt, a process called smelting.

Aluminium ore is known as bauxite. To separate the aluminium, it is treated with chemicals and electricity is passed through it, a process called electrolysis.

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CARBON

One of the most important chemical elements is carbon - partly because it makes up one-fifth of the human body. It is also the main element in all living things and the sixth most common element in the Universe.

Atoms of carbon can join or bond easily with each other and also with numerous other atoms. This allows carbon to be the basis of a vast variety of substances, from wood to plastics. Indeed carbon is such a common and adaptable element that it has its own branch of science, known as organic chemistry.

The structures and substances in all living things are based on carbon. This includes our own skin, hair, blood, muscles, bones and brain, as well as the body parts of birds, fish, insects and worms, and all the parts of plants. Even the chemicals which form our genes, known as DNA, have carbon as their main element. This is why the chemistry of carbon is often called “the chemistry of life itself”.

            Atoms of carbon easily join with a variety of other chemical elements such as oxygen, nitrogen, and hydrogen. In different combinations they form substances such as the sugars and starches found in living things.

            One of the carbon atoms main features is that it joins to itself in long chains. Oil (crude petroleum) is a complicated mixture of many hundreds of substances like this. They include one carbon atom linked to four hydrogens as the gas methane, and eight carbons in a row forming the gas octane.

            Unlike most elements, pure carbon can exist in different forms. If its atoms join as six-sided rings they form the soft, black, slippery powder graphite, used as “lead” in pencils. If they join in a box-like pattern they form the hardest substance of all, diamond.

            Atoms of carbon can even join to each other to form necklace-like rings. One type of ring has six carbon atoms. This is known as the benzene ring. If each carbon in the ring is also joined to a hydrogen atom, the result is the substance known as benzene. It is a very important chemical in many industries.

            The entire living world is based on carbon. It joins with other substances to form snail shells, spider’s legs, ants’ eggs, plant roots and countless other parts.

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MATTER

Everything is made of matter. Every object, substance, chemical and material is matter. This includes not only things you can see easily, like the paper of page and the ink that forms the words and pictures. It also includes specks of dust too small to notice, houses and cars, living things like trees and your own body, the rocks of the Earth, the clouds in the sky and the invisible air around you. And not only objects and substances on Earth are made of matter. All of the planets and stars in deep space contain matter. In fact everything in the entire Universe is made of matter. All matter is made of tiny building blocks called atoms.

There are places where there is no matter. If there is no matter then there is nothing at all. The total or complete absence of matter is called a vacuum. However a total vacuum is very unusual. “Space” is named because it is supposed to be just empty space, with no matter. But even in the depths of space, a few micro-particles of dust or some wispy bits of gas are floating about. These tiny bits of matter may be several metres apart, instead of crammed together like they are on Earth. But they are still present. Here on Earth, powerful vacuum pumps can suck most of the matter out of a container, but never quite all of it.

            Why is a butterfly like a lump of rock whizzing through space? Both are made of atoms. The butterfly is living matter, the rock non-living matter.

STATES OF MATTER

Matter exists in three main forms, called the states of matter. These are solid, liquid and gas. In a solid such as ice, the molecules are very close together and joined in a rigid pattern. They can hardly move. So a solid object stays the same volume and does not change its shape. In a liquid such as water, the molecules are still quite close together but they are not joined to each other. They can move about, which means the whole liquid can change shape and flow, although, like the solid, it still takes up the same volume. In a gas like water vapour, the molecules can move nearer together or farther apart. So a gas can also get bigger or smaller, to fill the container it is in.

            A hot-air balloon contains matter in the form of gas – air. Heat from the burner causes the air’s molecules to rush farther apart, so taking up more room. Soon there are fewer molecules in the hot air inside the balloon than in the normal air outside. The balloon is lighter or less dense and rises.

CHANGING STATES

Matter or substance can change state from solid to liquid, or liquid to gas. This usually happens by adding heat. Matter can also change state the other way from gas to liquid or liquid to solid. This usually happens by cooling (taking away heat). A common example which is all around us is water. The world’s water is always on the move and changing state in a never-ending process, the water cycle.

In the water cycle, the Sun warms the sea. The heat makes liquid water turn into a gas, invisible water vapour, which rises into the sky. It is cooler high up so water vapour changes state back into a liquid, forming tiny droplets. These are so light that they float as clouds. Wind blows the droplets over the land. Some clumps together, become too heavy to float and fall as rain. Some droplets blow even higher, up over a mountain, and become even colder. They change state again, freezing solid into snowflakes. The snow falls to the ground and melts into liquid water. With the rain, it flows into streams and rivers, and finally into the sea and so the cycle continues.

PROPERTIES OF MATTER

Matter has many features, or properties. One of the main properties is its state -solid, liquid or gas. Another property is the type of atoms it is made of. Each kind of pure substance, like iron, carbon, oxygen or sulphur, has a different kind of atom. It is known as a chemical element.

A third property of matter is density. This is the amount of matter in a certain place or volume. The more matter within a certain volume, the denser or heavier the substance or object. Dense substances like iron have lots of large atoms packed close together. Density is important because it determines whether things float or sink. If an object is less dense than water, such as a lump of wood, it floats. A lump of iron is more dense than water and so it sinks. But if the iron is made into a boat’s hull its shape contains lots of air, which has a very low density and is extremely light. The overall density of the iron-plus-air is less than the density of water, and makes the boat float.

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ATOMS

All matter is made of atoms. An individual atom is far too small to see, even with the most powerful microscope. But atoms joined together make up every solid object, substance, chemical and material in the Universe. A pinhead, for example, contains about one billion billion atoms.

Atoms able to move about make up liquids and gases. Atoms which are more fixed and unable to move much make up solids. Not all atoms are the same. There are about 92 different kinds of atoms that occur naturally. Scientists have made another 17 or so artificial kinds in laboratories. Each kind of atom has individual properties that distinguish it in some way from another kind. A substance made from just one kind of atom is known as a chemical element. Examples of six elements are:

Atoms of different chemical elements have different numbers of particles. Simplest is Hydrogen (1), a very light gas that makes up most of the Sun. Neon (2) is a gas used in coloured strip-lights. Egg yolks are rich in sulphur (3). Calcium (4) is needed for healthy bones. Silver (5) is a shiny valuable metal. Lead (6) has many particles and so is very heavy. It is used to make small weights or shot.

Atoms are not solid, like marbles. In fact, they are mostly empty space. But this space contains even smaller pieces of matter known as subatomic particles. There are three main kinds of subatomic particles - protons, neutrons and electrons. The protons and neutrons are gathered together in the middle of the atom, forming its central part or nucleus. The electrons are much smaller and move at speed around the nucleus. They do not move at random, but stay in certain layers known as shells.

Elements differ in their numbers of subatomic particles. Hydrogen is the simplest because its atom has just two particles, one proton and one electron. In most atoms there are the same numbers of protons as electrons. This is because a proton has a tiny positive electrical charge and an electron has the same amount of negative charge. The two sets of opposite charges balance each other out so the whole atom has no charge. This makes it stable or unlikely to break up.

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RADIOACTIVITY

Most atoms are stable. They remain the same through time. Others are unstable - they are likely to break up. As they do so they give off some of their particles or energy in the form of rays. These particles or rays are known as radioactivity. Examples of chemical elements with radioactive atoms include uranium, plutonium and radium. As atoms give out particles or rays they change into the atoms of simpler elements. For example, uranium changes into lead. This change is called radioactive decay. It happens at different speeds or rates for different radioactive elements. The time taken for half of a number of atoms to decay is known as the half-life of that element. Radioactivity can be dangerous since it harms living things. But under controlled conditions it is very useful in medicine and scientific research.

            The name “radioactivity” was invented by Polish-born scientist Marie Curie (1867-1934). She studied various rocks and minerals from the Earth and gave the name to the invisible rays or particles that some of them gave off, which affected photographic paper and various electrical equipment. In particular Marie worked with the substance pitchblende, a raw material used to obtain the metal uranium. Pitchblende, gave off more radioactivity than expected from uranium alone. Marie purified the substances which gave off this extra radiation and so discovered two new elements, polonium and radium.

            How do we know the age of mummies from Ancient Egypt? By measuring the tiny amounts of radioactive substances they contain. This is known as radiocarbon dating.

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