Chemistry only
Silicon dioxide (SiO2), often called silica is made from one silicon atom bonded to two oxygen atoms. It is the main substance in sand and can make up to 60% of some clays. In clay, the silica forms part of the mineral structure rather than existing as pure silicon dioxide. The structure of silicon dioxide is very similar to diamond as shown in the image below. Silicon dioxide is used to make glass, while different types of clays are used to make traditional clay ceramics such as porcelain, china and bricks.
You may notice that the structure of silicon dioxide or silica is very ordered; with atoms arranged in a regular three-dimensional pattern. Solids with this type of arrangement are called crystalline structures. Silica has a giant covalent structure containing many strong covalent bonds because of this it has a very high melting point. Silica is also an electrical insulator because its electrons are held tightly in covalent bonds and cannot move freely.
A group of naturally occurring minerals called silicates are closely related to silica. Silicates contain the same basic silicon–oxygen structural units as shown in the image above, but other atoms and ions are also present. Aluminosilicates for example are silicates where some of the silicon atoms are replaced by aluminium ions (Al3+).
One of the best-known aluminosilicates is clay. Clays can absorb large amounts of water allowing layers in its structure to slide over each another. This is why wet clay is soft and easy to mould. When clay is heated strongly in a kiln the water is driven out and the structure changes to form a rigid giant covalent structure. This process, called firing is used to make clay ceramics such as porcelain, stoneware and bricks.
If silica or sand is heated up to around 1650°C it melts to form a tacky, thick viscous semi-solid which can be moulded and shaped and if this semi-solid is cooled quickly it will form glass. Now silica or silicon dioxide is a crystalline solid, that is it has a very ordered giant covalent structure containing many strong silicon-oxygen bonds as shown in the image above. However if the silica is heated and it begins to melt this ordered crystalline structure begins to break down and some of the silicon-oxygen bonds break.
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 reform, instead the atoms are arranged in a very disordered random fashion similar to that found in a liquid, but the atoms are unable to move freely as you might expect in a liquid, this disordered random arrangement of atoms is called a solid glass, indeed the structure of glasses are often referred to as super-cooled liquids or amorphous solids.
To make glass silica or silicon dioxide is heated, now to lower the melting point of the silica sodium carbonate (which is often called soda ash) is added, this makes it more energy efficient to make the glass as well as making it much less expensive. Calcium carbonate is also added to the molten silica to make the glass more workable. These two substances change as the temperature inside the oven or kiln begins to rise. Calcium carbonate decomposes to give:
And for sodium carbonate, it reacts with the silica to form sodium silicate and carbon dioxide:
The glass produced here is called soda-lime glass and it is the most widely used form of glass; it is used to make bottles and windows. Unfortunately soda-lime glass expands a lot when heated and this often causes it to crack and break. However if boron trioxide is added to the silica this produces a glass that expands very little when heated and is much more resistant to changes in temperature. This type of glass is called borosilicate glass and it is used for ovenware, tea and coffee makers and for lab apparatus. You probably have some of this glass in your kitchen - it's sold under the name of Pyrex glass.
Traditional clay ceramics have giant covalent structures while glass has a non-crystalline structure or amorphous structure. In a liquid the atoms move freely and are arranged in a disordered 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 or amorphous solids. It is this random arrangement of atoms in a glass which make it transparent.
Before 1959, windows were often wavy, uneven and blurry, and certainly not fully transparent. Then a British engineer, Sir Alastair Pilkington found a way to float molten glass on a bath of molten tin, creating perfectly flat glass. Want to find out more about this revolutionary method for making glass? Then click the button below:
Most window glass is made using the float-glass process. In this method the molten silica mixture is poured onto a bath of molten tin. Because the glass floats on the tin, the upper and lower surfaces become perfectly flat and smooth.
As the glass floats across the molten tin, it slowly cools and solidifies while remaining extremely flat. The continuous ribbon then enters an annealing oven which cools it very slowly to remove internal stresses.
At the end of the process the glass ribbon is cut into sheets. Today, almost all window glass is made this way.
The raw materials include silica sand, sodium carbonate and calcium carbonate. These react together in the furnace to form molten soda-lime glass.
The thickness of the glass is controlled by the speed at which it travels across the tin bath and by gentle stretching rollers.
Before the float-glass process was invented in 1959, it was impossible to make perfectly flat transparent window glass. Earlier methods, such as blowing a cylinder and flattening it, always produced sheets with ripples, waves and uneven thickness.
Because the surface was not completely flat, light was scattered. This is why medieval church windows and older house windows appear translucent rather than fully clear.
The float process was invented by Sir Alastair Pilkington of Pilkington Brothers Ltd, a British glass company in St Helens. He discovered that molten glass would float on molten tin, producing a sheet with perfectly flat surfaces on both sides.
Pilkington’s invention completely transformed glassmaking, and nearly all flat window glass in the world today is produced using the Pilkington float-glass process.
Quickly review the main points on glass covered above by ddeciding which statements below are correct or not.
Decide if each statement is correct or incorrect, then click a button to check your answer.
1. Glass has a crystalline structure like diamond.
2. Silicon dioxide (silica) has a giant covalent structure and a very high melting point.
3. Soda-lime glass expands quite a lot when heated, so it can crack if the temperature changes suddenly.
4. Borosilicate glass is used for ovenware and lab apparatus because it expands a lot when heated.
5. Before 1959, window glass was already perfectly flat and fully transparent.