Sand or silica is mainly silicon dioxide, SiO₂. It has a structure very similar to that of diamond, the structure of silica or sand is shown below. You should notice that the structure is very ordered with all the atoms arranged in a very regular way, we say it has a crystalline structure. Silica has a giant covalent structure with lots of strong bonds. It has a very high melting point due to its strong bonding and giant structure. It is an electrical insulator due to the fact that all the electrons are held tightly in covalent bonds and cannot move. It is very hard, again due to its strong bonding and giant structure.

A group of compounds called silicates are related to silica. Silicates like silica have structures based on the one shown above. However in silicates other atoms and ions maybe present as well as silicon and oxygen. Aluminosilicates are silicates where some of the silicon atoms are replaced by aluminium. Perhaps the best known aluminosilicate is clay. Clays can absorb lots of water and this allows parts of the structure to slide and move, this why it is possible to mould and shape clay when it is wet. However when the clay is heated or fired in a hot oven or kiln the water is forced out of the structure and a giant covalent structure is formed. Firing clays is used to make objects such as pottery, china and bricks. These materials made from firing clay at high temperatures are called clay ceramics.
Ceramic materials are all around us, bricks, glass and cement are ceramics that probably make up the building you are sat in, you may be having a drink from a ceramic cup or a glass tumbler while using your computer or laptop which has ceramic semi-conductors and insulators inside it. When most people think of ceramic they probably think of sinks, toilets, ceramic wall and floor tiles, roof tiles and pottery, while these are indeed ceramics, modern ceramics are very different from these rather old-fashioned idea of what a ceramic is. The image below shows some everyday ceramic materials, mostly these ceramics are made from clay and sand and are what we call clay ceramic materials.

These traditional clay ceramics have giant structures, they maybe crystalline (made up of atoms or ions arranged in a very ordered regular way - Silica or sand is crystalline because its structure is very ordered) or non-crystalline (atoms or ions in the structure are arranged in a random or disordered fashion). Glass has a non-crystalline structure, sometimes these disordered structures are said to be amorphous. In a liquid the atoms move freely and are arranged in a disordere or amorphous manner. Imagine taking a liquid and instantly freezing it so that the atoms do not have time to arrange themselves into an ordered solid structure - you would end up with a solid but the atoms in it are arranged in a random way - glasses have structures like this. Glasses are often called super-cooled liquids!. It is this random arrangement of atoms in a glass which make it transparent.

Properties of ceramics

• Heat resistant or refractory.
• Corrosion resistance.
• Low density compared to metals.
• Good thermal and electrical insulators.
• High resistance to wear and tear.
• Resistant to many acids and alkalis.
• Very strong in compression.

• Ceramics are brittle- they crack and break easily.
• Poor tensile strength - crack or break if any pulling or stretching forces are applied.

Similarly it is possible to place four tea cups under the wheels of a lorry or bus and lower the bus or lorry down onto the cups and they will not crush. However drop the cups and they will break, pottery like bricks are strong in compression but brittle. The heat shield on NASA's space shuttle is made of a ceramic material similar to a dinner plate. This "dinner plate" can withstand the heat produced when the space shuttle re-enters the Earth, but again they are very brittle and must be constantly checked to ensure they are no small cracks which could spread and lead to failure of the heat shield. When the space shuttle Columbia exploded on February 1, 2003 killing all astronauts on board, the accident was blamed on damaged ceramic tiles on its wing.

If silica or sand is heated up to around 1650⁰C it melts to form a tacky, thick viscous liquid which can be moulded and shaped. If it is cooled quickly it will form glass. Silica is a crystalline solid with a giant structure containing many strong silicon-oxygen bonds, its structure is very similar to that of diamond. Once it begins to melt the ordered crystalline structure begins to break down and some of the silicon-oxygen bonds have been broken. If this tacky liquid is allowed to cool it will slowly reform its ordered crystalline structure, however if it is cooled quickly this ordered crystalline structure will not have time to form, instead solid glass will form. Its structure will be made up of a random arrangement of atoms, it's called a non-crystalline or amorphous structur. The structure resembles that of a liquid where the atoms are arranged randomly.

To lower the melting point of the silica, calcium carbonate and sodium carbonate are added to the silica. These two substances decompose as the temperature rises to give:

calcium carbonate_(s) → calcium oxide (or lime)_(s) + carbon dioxide_(g)

CaCO_3(s) → CaO_(s) + CO_2(g)

sodium carbonate (or soda)_(s) → sodium oxide_(s) + carbon dioxide_(g)

Na₂CO_3(s) → Na₂O_(s) + CO_2(g)

Key points

• Clay ceramics include bricks and pottery and are made by shaping wet clay and then firing this in an oven or kiln.
• Glasses are made by melting sand (silica) and cooling it quickly. To lower the melting point of the sand sodium carbonate and calcium carbonate are added to it. This produces an inexpensive type of glass called soda-lime glass. Adding boron trioxide to the molten sand produces a glass which melts at a higher temperature than soda-lime glass and is also more resistant to cracking when heated.

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