When a substance burns, there is a very rapid chemical reaction with oxygen in the air. This reaction generates heat and light, producing a flame. There are other forms of burning too. Some metals burn in corrosive gases, for example. Explosions are a form of burning in which the reaction takes place extremely rapidly, releasing very large amounts of heat.
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When sugar is stirred into water, it dissolves and forms a solution. This means that the solid sugar molecules have split apart and become distributed equally throughout the water, so that no particles can be seen. The warmer the water, the more sugar can be dissolved into the solution. There are some molecules that do not behave in this way. Fats and oils, for example, will not dissolve in water — they can be seen floating on it —although they will dissolve in some other types of liquid.
Solutions are not always liquids. Brass is a yellowish metal that is actually a solid solution containing copper and tin.
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Crystals are solid substances that have their atoms arranged in regular patterns. Most naturally occurring substances form crystals under the right conditions, although they are not always apparent. Crystals form into regular geometric shapes, and crystals of the same substance or mixture of substances will always have the same regular appearance.
Crystals are formed from solutions or melted materials. As the solutions evaporate or the melted materials cool, their atoms are forced closer together, producing a crystal. This crystal gradually grows as the process continues. Some crystals grow into complicated and beautiful shapes, which are often brightly coloured.
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Glass forms when melted solid materials are cooled quickly, so that they do not produce crystals. There are many types of glass, but the transparent glass that is most familiar to us is a very useful material. It is resistant to most corrosive substances, and it is a very good electrical insulator and heat insulator. Glass is stronger than most metals, but because it is brittle it shatters easily.
The most common forms of glass are made from a melted mixture of silica (sand), sodium oxide and calcium oxide. Lead crystal is a form of glass in which the calcium oxide is replaced by lead oxide. The most heat-resistant glass can be made from pure silica.
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Almost 80 percent of all elements are metals. They can be distinguished from other elements because when cut, they reflect light and appear shiny. Metals also conduct heat and electricity. Most metals are malleable, which means that they can be beaten or moulded into different shapes.
Most metals react with oxygen in the air, or with other kinds of elements, to form compounds. They are rarely found in the natural metallic state. However, gold is an exception, because it does not react easily with other elements and is often found in its natural state as grains or nuggets. Mixtures of metals are called alloys.
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Mercury is the only metal that is a liquid at ordinary room temperatures. It is a bright shiny colour, and flows rapidly when poured out of a container (this is why it used to be called ‘quicksilver’). Mercury does not stick to glass, so it is used in thermometers to indicate the temperature.
Mercury compounds are often brightly coloured and were once widely used in coloured paints. However, mercury is an extremely poisonous substance and its use is now strictly controlled. Some other uses of mercury are in the manufacture of batteries and in medicine. Dentists still use a mixture of mercury and silver, in the form of amalgam, to fill cavities in teeth.
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Mercury is the only metal that is a liquid at ordinary room temperatures. It is a bright shiny colour, and flows rapidly when poured out of a container (this is why it used to be called ‘quicksilver’). Mercury does not stick to glass, so it is used in thermometers to indicate the temperature.
Mercury compounds are often brightly coloured and were once widely used in coloured paints. However, mercury is an extremely poisonous substance and its use is now strictly controlled. Some other uses of mercury are in the manufacture of batteries and in medicine. Dentists still use a mixture of mercury and silver, in the form of amalgam, to fill cavities in teeth.
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Matter exists in three states: solid, liquid and gas. The molecules in all matter are in constant motion, and in a gas they are held together so loosely that they can move freely. Gas molecules move about rapidly and at random. This means that a gas will expand to fill any space it occupies. The molecules in a gas press against anything that restrains them, causing pressure. As a gas is heated, the molecules move further apart and move about more rapidly. If it is restricted in a container, the pressure will increase. If any gas is cooled sufficiently, it will condense into a liquid. This is why some of the outer planets of the solar system are composed of liquid methane and hydrogen.
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Radioactivity can contain large amounts of energy, which causes damage to matter, including living cells. These forms of radioactivity can knock electrons out of the atoms of a substance to produce ions, which have different properties.
Organic compounds always contain the element carbon. Carbon compounds often have a very complicated structure, and they are responsible for most of the chemical reactions that sustain life. Carbon is found in some very large molecules, such as those in proteins and plastics. It has proved relatively easy to manufacture many of these substances artificially.
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New elements are created by bombarding other elements with radiation in an atomic reactor. Scientists have made 16 artificial elements by bombarding uranium and other radioactive elements.
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Radioactivity is a form of energy given off by some types of atom with a high atomic number, such as uranium, radium and plutonium. Some forms of radiation consist of protons and neutrons bound together, while other forms consist of electrons or other particles called positrons. As radiation is emitted, the loss of particles from the original atoms changes their properties to those of another atom with a lower atomic number.
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Hydrogen is the lightest and simplest element, and its atomic number is 1. Uranium is the heaviest and its atomic number is 92 — indicating that it has 92 protons in each atom.
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The periodic table is a list of all the elements, arranged in such a way that elements with similar properties are grouped together. Each element in the table is given a number, called an atomic number, which indicates the number of protons the atom has. (A single atom has the same number of protons as electrons.) Elements with the same number of electrons in their outer shells are grouped together in the table.
How many different elements are there?
There are 92 elements that exist naturally, but it has been possible for scientists to create many more in the laboratory. These artificial elements are radioactive and they quickly decay or lose their radioactivity. Some exist for only a few seconds or less.
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Antimatter is like normal matter, but it is made up of particles that are exactly the opposite to those making up ordinary matter. A normal electron, for example, has a negative charge, but in anti-matter, the corresponding particle has a positive charge. Antimatter was first proposed in 1930, and it is now possible to make it in tiny amounts, using a machine called a particle accelerator. When antimatter and normal matter make contact, huge amounts of energy are released.
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