Polymers can be natural or synthetic. Some examples of natural polymers include starch, cotton and rubber but in this article I'm going to talk about synthetic polymers such as nylon, polyethene or perspex. The building blocks of polymers are called monomers which are connected together into long chains. Synthetic polymers can consist of chains of thousands or even millions of monomer units. We'll look at the types of monomers and how they are bonded together to make polymers of various kinds.

Synthetic polymers can be divided into two types, addition polymers and condensation polymers. They are formed from different types of monomers and with different bonds between them.

Addition Polymers

Addition polymers are always made with a group of organic compounds called alkenes. These all contain a double bond between two carbon atoms. Double bonds are more reactive than single bonds and can be induced to react together, given the right conditions which usually include the use of a catalyst, increased temperature and sometimes an increased pressure. Alkenes bond together in an addition reaction and form addition polymers.

An alkene - ethene

All bonds contain two electrons and the ethene molecule has a double bond between two carbon atoms. One of these bonds is more reactive than the other one and this bond breaks during the polymerisation process. The two electrons in the bond split up and end up on each of the carbon atoms. Now the electrons on adjacent ethene molecules can join together to form a bond between the two carbon atoms on different molecules thus joining the two molecules together with a single covalent bond. The electrons on the other carbon atoms can do the same thing and join up with other ethene molecules to start to form a chain. This process can continue until a huge chain is formed.

Formation of Polyethene

Formation of radicals

Naming of Addition Polymers

Polymers are named for the alkene monomer from which they are formed so, for instance a polymer made from the alkene called ethene forms the polymer polyethene (usually called more simply polythene). Other examples include polyvinylchloride (PVC) made from chloroethene which used to be called vinyl chloride, polystyrene and polytetrafluoroethylene (PTFE or Teflon).

Condensation Polymers

In this type of polymerisation, monomers are bonded together with the elimination of a molecule of water, hence the name condensation polymer. The monomers used in condensation polymerisation can be di-alcohols (containing two -OH groups), di-carboxylic acids (containing two -COOH groups) or di-amines (containing two NH₂ groups). When a polymer is made from a di-alcohol and a di-carboxylic acid an ester link is formed and the resulting polymer is a polyester as illustrated below.

Formation of condensation polymers

When a polymer is formed between a di-carboxylic acid and a di-amine a polyamide is formed. An example of a di-amide is nylon. The monomers used in this type of polymerisation must have reactive groups at both ends to allow chains to be formed.

Once the polymers have formed, further treatment is needed to produce plastics. Dyes, stabilisers, fillers and pigments are added to produce finished products. Polymers have different properties depending on the monomers used in their production and are used in various ways. Polyethene for example is used for plastic bags, mixing bowls etc. PVC is used for water pipes, floor tiles, waterproof articles and insulating materials. Nylon is used for textiles, ropes and carpets.
 

The chemistry behind fireworks started over 2000 years ago when, according to legend, the firecracker was invented when a Chinese cook mixed up charcoal, sulphur and saltpeter. He discovered that it would explode if packed into a bamboo tube and set alight. In the 9th century the Chinese invented gunpowder and produced fireworks for important events such as the Moon Festival and New Year using a combination of potassium nitrate (also called saltpeter), sulphur and charcoal.

Each of these chemicals burns in a different way. Charcoal burns slowly, potassium nitrate quickly and sulfur crackles and pops as it burns. Using different proportions of these chemicals produced various kinds of displays. They also invented rockets by placing gunpowder in a roll of paper and igniting it at one end.

Chemistry of fireworks

To start off the reaction energy must be supplied by lighting the fuse. Potassium nitrate acts as an oxidiser by providing oxygen for the charcoal or fuel to burn, sulphur helps to keep the reaction stable. Without the oxidiser the reaction would be too slow, the oxygen provided by the potassium nitrate speeds up the reaction. The three ingredients produce potassium sulphide, carbon dioxide and nitrogen which expand with the heat and provide the propelling force. In addition the reactions are exothermic, that is they produce heat, which contributes to the rate at which the gases expand and increases the explosive power of the reaction.

Fireworks were originally only able to produce yellow or white light which was emitted by heating up the gunpowder mixture. The effect can be varied to produce more glitter by increasing the amount of sulphur or a quick flash by adding more potassium nitrate. When white or yellow light is emitted in this way it is called incandescence. As a substance is heated it glows first with a red light (~480C) through bright red (~730C) to bright orange (~930C) and yellow (~1100C) then white at over 1400C. Until the late 18th century these were all the colours that could be produced in fireworks. Chlorates were produced industrially in the 19th century and allowed reds and greens to be produced in firework displays. It was only in the 20th century that purples and blues could be produced.

How does the oxidiser work?

When heated potassium nitrate releases oxygen and nitrogen but not all of the oxygen is released. Some remains bound to potassium ions.

When chlorates were manufactured industrially they began to be used in fireworks as they are better oxidisers than the nitrates. They release all their oxygen on heating so they are better oxidisers and can produce higher temperatures in the firework which allows more intense colours to be seen and a faster explosion.

However chlorates are fairly unstable so need very special care and today perchlorates are used as they are more stable but, as they also release all their oxygen, are also good oxidisers.

The chemistry of fireworks colors

Today we have fireworks that emit red, blue, green, yellow and lavender light so how is this possible? The answer lies in the way metals emit light as they burn.

Some metals and the colour of light they emit

                 Sodium               yellow
                 Barium                green
                 Strontium           red
                 Copper               blue
                 Potassium          lavender
                 Caesium             violet
                 Magnesium         brilliant white

You may have done flame tests at school to discover the identity of a metal by placing it in a flame and noting the characteristic colour that is emitted.

How do metals emit coloured light?

To find out what happens when we burn a metal we need to know something about the atoms of the metal. All atoms have a nucleus containing protons and neutrons (except hydrogen which is the lightest atom and doesn't have any neutrons). Electrons are in orbitals at various distances from the nucleus. The electrons will always occupy the lowest energy level possible, which are the ones closest to the nucleus.

When the electrons absorb energy, e.g. if they are heated, the energy allows them to jump to a higher energy level further away from the nucleus. Once in the higher energy levels they are unstable and will fall back to a lower energy level. When they do so they emit radiation in the form of light. The wavelength of the radiation emitted depends on the energy difference between the energy levels and is different for different metal atoms and for different energy levels within the same metal, so the colour of the light you see emitted will vary with the metal.

Some metals that burn brightly such as magnesium and titanium are used for both the bright light they emit and to increase the temperature of the burning compounds.

So next time you watch a firework display not only will you marvel at the wonderful colours and sounds but you will know more about how they are produced and the fascinating chemistry of fireworks!

 

We all like to eat meat that is tender and succulent rather than tough and stringy so what is the best way to tenderize meat? How tender meat is naturally depends on a number of factors including how the meat is treated after the animal is slaughtered, the type of meat and the age of the animal.

Meat has a high proportion of protein in the form of connective tissue, called collagen, which needs to be broken down before it is tender enough to eat. Collagen makes up around 30% of the protein found in animal tissues and is a major component of skin, cartilage, organs, bones and tendons.

What is collagen?

Collagen is a protein that is made from three intertwined poly-peptide chains. A poly peptide chain is a chain of amino acids bonded together to make a natural polymer. It is a stiff, strong structure that is hard to break down. Muscles that are weight bearing or used often contain larger amounts of collagen than other parts of the animal so legs and rump will be high in collagen. The age of an animal also has a bearing on the amount of collagen present which is why meat from older animals is tougher than that from younger ones.

Collagen

What ways are there to tenderize meat?

Hanging - meat can be hung after the animal is slaughtered. This loosens the muscle fibres.
Grinding and pounding - hitting the meat with a mallet is a popular way to tenderize meat, especially steak. The action of pounding on the meat loosens the muscle fibres by breaking up the connective tissue. Mincing or chopping up meat also has the same effect.

Cooking - cooking meat slowly with moist heat breaks down the collagen. However cooking also hardens the muscle fibres so a balance needs to made between gelatinising the collagen and preventing the muscles fibres from hardening. Moist cooking for around three hours is usually enough to break down the collagen but not long enough to harden muscle fibres. The exception to this is cooking some meats, like steak, that do not have a high collagen content. These types of meats are best cooked quickly with a dry heat as they will become tough if cooked slowly. Some meats can also be tenderized more easily in a pressure cooker. Gelatin is the product when collagen is broken down by heat.

Marinating - meat can be marinated in alcohol and acidic fruits or vinegar to tenderize it. Marinating is also used to add flavour to the meat. Marinating takes time for the ingredients to break down the connective tissue in the meat. Alcohol is effective but acids from vinegar or fruits works even better.

Using enzymes to tenderize meat - some foods contain enzymes that can be used to tenderize meat. Papaya (Paw-paw) contains the enzyme papain and pineapple contains bromelin both of which break down the collagen in meat. As we said earlier collagen is made up of three protein chains and these enzymes can break the bonds between the amino acids in the protein chains.

Individual amino acids in the protein are joined together with a peptide bond (coloured blue in the protein fragment pictured below). It is this bond that these enzymes break, thus fragmenting the protein chains and destroying the collagen structure.

Protein fragment showing peptide bond (in blue)

Conclusion

Chemistry in everyday life is a fascinating subject! Look around you and you can see examples of how chemistry comes into almost everything you do every day. I hope this has sparked an interest in this subject and that you will be eager to learn more.

Image Credits

Collagen by Nevit

papaya by Olegivvitby

Cast Iron Cooking by LarimdaMEMeat

We all enjoy cooked food and many of us cook every day but have you ever thought about the chemistry of cooking? Cooking makes food easier to digest and safer to eat as it kills micro organisms in the food. However it can also destroy some nutrients in food, such as vitamin C, so it's necessary to balance making food easier to digest and taste better without destroying the valuable nutrients that we need.

Food contains a variety of nutrients including vitamins, protein, carbohydrates, fats and minerals and we'll look at some of these and see how they are affected by the cooking processes. We can cook food in different ways using microwave radiation, dry heat (baking and grilling), moist heat (boiling) and heated fats (frying). Let's look at how the different nutrients in food are affected by these processes.
 

Proteins

Part of a protein showing amino acids bonded together

Proteins are found in eggs, meat, milk, cheese and other foods and are long chains of amino acids bonded together. They have a three dimensional structure that is held together by weak bonds between the chains.  When you boil an egg the white and the yolk contain proteins that are liquids. When these protein are heated the weak bonds are broken and the protein unravels. This is called denaturing and can also be brought about by the action of enzymes. The chains of amino acids now link together to form a network and the protein changes from a liquid state to a semi-solid state.

This is called coagulation and is what is happening as the egg white and yolk solidify. If you only boil the egg lightly the network of proteins traps water to form a soft gel that is easy to digest. If you continue boiling the egg it becomes rubbery and tough and not easy to digest at all! Coagulation is used to when you make souffles, custards and cakes. Coagulating milk protein using an enzyme called rennin results in cheese and coagulating soymilk using magnesium sulphate makes tofu.

The main protein in meat is collagen which is a tough building block found in connective tissue and needs to be softened before it is digestible. Long, moist cooking is best for this type of protein. Some meats however, such as steak, are low in collagen and are best cooked quickly with a dry heat.
 

Tenderising Meat

Meat can be tenderised in a variety of ways including:

1. Hanging - this stretches the muscle fibres so that they become loose.
2. Cooking - long moist cooking makes some meats more tender. using a pressure cooker can also help to make meats more tender.
3. Beating and grinding - hitting tough meats such as steak with a mallet or mincing can loosen connective and make the meat tenderer.
4. Use of enzymes - Some foods contain enzymes (called proteolytic enzymes) that will break down the protein chains. Pawpaw for instance contains papain which is often used to tenderise meat.
5. Salt - salt can be used to break down the connective tissues.
6. Alcohol - alcohol is often used in marinades and can denature the proteins in meat.

Starch

Part of a Starch Molecule

Starch is present in foods such as rice, potatoes and pasta. Starch is a natural polymer made from glucose molecules and is insoluble in water. When starch is boiled the grains begin to absorb water and swell. As the temperature rises above 70C the membranes of the starch grains weaken and the starch granules leak out. This is called gelatinisation and the mixture becomes jelly like and viscous.
 

Vitamin C

Vitamin C

This vitamin is present in fruits and vegetables and is essential for our health. It is water soluble so is not stored in the body so we need to eat foods containing vitamin C every day. A deficiency of this vitamin results in a disease called scurvy which was a big problem in the past, before people knew it was caused by the lack of Vitamin C. Vitamin C, or ascorbic acid, is easily oxidised to an inert form, the extent of oxidation being increased by heating, cutting or crushing the food. So if you chop up fruits or vegetables a long time before you eat them you'll be destroying the vitamin C they contain.

Oranges, lemons and limes are rich in Vitamin C

Cooking will also decrease the vitamin C content of the food so vegetables should always be cooked for the minimum time in a small quantity of water. Some cooks add a small quantity of sodium hydrogen carbonate (baking powder) to vegetables to improve the flavour but what will this do to the Vitamin C. Well as it is an acid and acids react with carbonates this will destroy the vitamin C in the vegetables! Not a good idea.
 

Raising Agents

Yeast

We've all used raising agents in cooking and baking but do you know the chemistry involved? There are two raising agents used in most recipes, yeast and baking powder. Yeast (Saccharomyces cerevisiae) is a micro-organism that contains the enzyme zymase that converts the sugars in dough into carbon dioxide and ethanol. The carbon dioxide is the raising agent.

Gluten in the dough is a fibrous compound that stretches as the bread rises and traps the carbon dioxide in an elastic framework. Yeast grows in a warm environment so the bread dough is kept warm until it rises. When it is placed in a hot oven the yeast increases production of carbon dioxide initially then dies as the temperature rises. The carbon dioxide trapped in the dough expands and the dough rises even more. Starch grains in the dough now absorb moisture and burst, the gluten becomes more rigid and sets forming a hard crust on the top of the bread. The flavour comes partly from the ethanol produced by the yeast.

Baking Powder

Baking powder is another raising agent used in baking and is sodium hydrogen carbonate. When this compound is heated carbon dioxide is produced which is the raising agent. In addition some recipes uses tartaric acid mixed with the sodium hydrogen carbonate. Acids react with carbonates to produce carbon dioxide so mixing tartaric acid with the baking powder increases the production of carbon dioxide and makes the food rise faster.

There's a lot more interesting chemistry of cooking to find out about! This is only a short introduction but I hope it has served to interest you in this fascinating aspect of chemistry in everyday life.