Most organic compounds can be composted and the resulting material used to great benefit in your garden. Organic compounds refers to anything that comes from animals and plants and usually has a high percentage of carbon and hydrogen, plus trace amounts of other elements such as sulfur, oxygen etc.
The aim of a compost pile is to make an environment ideal for the micro-organisms that are the main decomposers of the organic material. In addition ants, worms and snails also play a part. All organic waste will decompose over time but in a compost pile you are providing the ideal conditions for these organisms to thrive, thus accelerating the process. Organic waste will contain compounds such as proteins, sugars, carbohydrates, starches, cellulose etc which are broken down at different rates. Carbohydrates are easily broken down to their constituent sugars while plant remains containing cellulose take longer.
Getting the carbon to nitrogen ratio right
For optimal conditions that encourage microbial growth it is important that the ratio of carbon to nitrogen in the compost pile is correct. The ideal ratio is around thirty parts carbon to one part nitrogen. Carbon is the energy source for the growth of the microbes, nitrogen is needed in smaller amounts for growth as it is a crucial element in enzymes, proteins and DNA. However, if you supply too much nitrogen it will be turned into ammonia (NH3) which will make your compost pile smell! Too little nitrogen however will prevent the microbes growing at the fastest rate and slows down the decomposition.
So how do you ensure the ratio is correct? The ratio of 'green' to 'brown' compost materials is the best way to do this. Green materials are the fruit and vegetables scraps from the kitchen, coffee grounds, grass clippings etc which will be high in nitrogen. They also contain the bacteria that you need to begin the decomposition and to supply energy in the form of heat. Brown materials are high in carbon and are the woody materials such as paper, cardboard, dried autumn leaves and sawdust. You need to make the ratio of these materials 30:1 for the best conditions inside your compost pile. If you have too many autumn leaves make a seperate pile for composting into leaf mold. Don't make the mistake of only adding green waste to your bin or you'll end up with a rotting mess full of fruit flies with a horrible smell!
Composting is an aerobic process
Aerobic means 'with oxygen'. The microbes in the compost pile use oxygen when they process the material, using the carbon for both energy and as building material for their cells. They also use the nitrogen to make proteins and DNA but need much more carbon which is why you need only a small proportion of nitrogen in your bin. Other trace materials need by the organisms to grow include phosphorus, sulphur and a variety of metals such as iron, copper, and calcium in trace amounts. In order to encourage aerobic decomposition, oxygen needs to be supplied. This can be accomplished by agitating the pile with a fork or spade regularly. In aerobic respiration a lot of energy in the form of heat is produced.
The chemical equation for aerobic respiration follows:
If the supply of oxygen is inadequate anaerobic respiration (without oxygen) will result in the production of methane and volatile organic acids, ammonia and other compounds. Sulfur containing compounds such as hydrogen sulfide are also released. Contrary to popular belief methane has no smell, it's the other compounds that are also produced by the methanogens (the bacteria active in anaerobic decompostion) that give a badly aerated compost heap it's smell.
The chemical equation for anaerobic respiration follows:
Why does a compost pile become hot?
The microbes present in the green, moist materials added to the compost pile respire aerobically using up oxygen and releasing carbon dioxide. They also release a great deal of enegy in the form of heat. A compost pile can reach temperatures as high as 150F in a few days given the right conditions. Oxygen is needed to sustain this temperature which is why it is important to agitate the pile regularly or the aerobic microbes will die off and anearobic microbes take their place.
The pH of the compost pile
The pH level in the compost needs to be between about 5 and 8. In a new compost pile the digestion of the organic matter often produces some organic acids which lower the pH. This is not a bad thing however as this encourages fungi to grow which then digest the cellulose in plant materials. In a properly aerated pile the organic acids are themselves broken down. However if you don't aerate the pile adequately the pH can drop below about 5 because the organic acids begin to accumulate and this starts to restrict the activity of the aerobic microbes.
Conclusion
Chemistry in everyday life is fun to find out about! Now you know what is going on in your compost pile, why it gets hot and why it smells if you don't get it right! If you're eager to get started on your own compost pile see How To Make A Compost Heap In Your Garden
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!
