16.
OXYGEN
On Earth, oxygen is more common than any other element. It is an invisible, odourless gas that makes up 21 per cent of air. Oxygen is found in water, minerals, and almost all living things. It is essential to life. Ordinary oxygen molecules contain two oxygen atoms. Ozone, a three-atom form, is found high up in the atmosphere. Oxygen moves through the environment via the OXYGEN CYCLE.
OXYGEN FOR LIFE
Divers wear a SCUBA (self-contained underwater breathing apparatus), so they can breathe under water. The SCUBA contains a cylinder of compressed air, which divers carry on their backs. The air is compressed (or squeezed) into the cylinder to increase the amount of air the divers can carry. Divers breathe through a regulator, which decompresses the air as it comes out of the cylinder.
BIOGRAPHY: JOSEPH PRIESTLEY British, 1733-1804
In 1774, this chemist announced his discovery of oxygen. He didn’t realize that Swedish chemist Carl Scheele (1742–1786) had found it first, a year or two previously. They both showed that air is not one element. Priestley also discovered how to combine carbon dioxide with water to make fizzy water.
OXYGEN CYCLE
Almost all living things, including humans, need oxygen to survive. Both plants and animals take in oxygen from their surroundings to release energy. Underwater plants and animals cannot use the oxygen in air – instead they use oxygen dissolved in water. The oxygen cycle continuously circulates oxygen through the environment, so it is always available to all living things.
CHANGING OXYGEN
Plants are able to use the energy of sunlight to convert carbon dioxide (CO2) and water (H2O) into carbohydrates and oxygen (O2) in a process called photosynthesis. This oxygen is taken in by plants and animals to provide energy, releasing carbon dioxide and water. This process is called respiration.
17.
WATER
The simple combination of two hydrogen atoms and one oxygen atom creates a water molecule (H2O). Water is the most common compound on Earth, making up over half the weight of living things. It is vital to life, bringing nutrients to and taking away waste from every living cell. Water molecules are attracted to one another through HYDROGEN BONDS, and this gives water some unusual but useful properties.
ABUNDANCE OF WATER
Water covers around 70 per cent of the Earth’s surface. This is why Earth looks blue from space, and why it is often called the Blue Planet. Water is liquid in the oceans and forms solid ice caps at the ice caps. Water vapour is a gas in air. Humid places, such as rainforests, have a lot of water vapour. The human body is about 60 per cent water and a ripe tomato contains over 95 per cent water.
DRINKING WATER
At room temperature, pure water is a colourless liquid with a neutral pH – it is not an acid or a base. But most water is not pure. Hard water contains calcium and magnesium minerals, which have dissolved in the water as it flows over rocks. Soap does not lather well in hard water – the minerals react with the soap to form a scum. Hard water is softened by boiling or by passing it through a water softener.
HYDROGEN BOND
Water molecules have an attraction to other water molecules. This attraction is called the hydrogen bond. It is a fairly weak bond compared to the bonds within a water molecule, but it is still strong enough to give water some unusual properties. For example, water is a liquid at room temperature; other molecules of a similar size are gases. It is also less dense as a solid than as a liquid.
MOLECULAR STRUCTURE
In a water molecule, electrons are pulled closer to the oxygen atom than the hydrogen atoms. So the oxygen atom has a small negative charge, and the hydrogen atoms have a small positive charge. The slightly positively charged hydrogen atoms of one water molecule are attracted to the slightly negatively charged oxygen atoms of another water molecule. This attraction is the hydrogen bond.
Water boils at 100ºC (212ºF). This is almost 200ºC (424ºF) higher than the boiling points of other similar-sized molecules, such as hydrogen sulphide. Water’s high boiling point can be explained by its hydrogen bonds. Extra heat is needed to break the hydrogen bonds, so a water molecule can break free of other water molecules and leave the liquid’s surface as steam, which is a gas.
18.
NITROGEN
Nitrogen is needed to make proteins, which are vital to life. Plants and animals recycle nitrogen through the air and soil in a process called the NITROGEN CYCLE. As a gas, nitrogen makes up 78 per cent of air. At everyday temperatures it is very unreactive. It is used in place of air in crisp packets, for example, so the contents do not go stale. Nitrogen is also used to make industrial chemicals such as fertilizers and explosives.
LIQUID NITROGEN
When nitrogen gas is cooled to -196ºC (-320ºF), it turns to a liquid. Liquid nitrogen is so cold that it can freeze a substance in seconds. In hospitals, it is used to preserve blood and body parts for transplant. The material to be preserved is placed in a special, sealed container that is filled with liquid nitrogen. Because nitrogen is so unreactive, it does not alter the preserved materials in any way.
LIGHTNING
The heat produced by lightning forces nitrogen molecules in the air to split. Nitrogen atoms bond with oxygen to form nitrogen oxides, which dissolve in water to create nitric acid. Weak nitric acid falls to the soil, where it splits apart to form the compounds nitrates and nitrites. These compounds are essential to life for plants and micro-organisms.
FERTILIZING SOIL
Farmers often use fertilizers to help their crops grow well. Many fertilizers contain nitrogen in the form of nitrates, because this is the form that plants can use. Natural fertilizers are made from compost and manure. Synthetic fertilizers are made by combining nitrogen from the air with hydrogen from natural gas.
EXPLOSIVES
Nitrogen compounds are used to make explosives. These compounds contain chemicals that break apart easily to release huge volumes of gases extremely quickly. They can be used in a controlled way to demolish a building without harming other buildings nearby. The explosive TNT (trinitrotoluene) releases hydrogen, carbon monoxide, and nitrogen, and carbon powder, which produces black smoke.
NITROGEN CYCLE
All living things need nitrogen, but most cannot use nitrogen gas directly from the air. The nitrogen has to be fixed (combined) with other elements to form nitrites and nitrates. This is done by lightning and by nitrogen-fixing bacteria. The nitrates are taken up by plants, which are eaten by animals. This starts the continual cycle of nitrogen called the nitrogen cycle.
MOVEMENT OF ATOMS
Nitrogen from the air is fixed to make nitrates in the soil by nitrifying bacteria. The nitrates are taken up by plants to build plant protein. When an animal eats a plant, it turns the plant protein into animal protein. Denitrifying bacteria convert the nitrogen contained in animal waste and in decaying plant and animal material back into nitrogen gas again.
NITRIFYING BACTERIA IN ROOT NODULES
Nitrifying bacteria are a key part of the nitrogen cycle. Some live in the root nodules of legumes (peas and beans), like this nodule from the root of a pea plant. Others live free in the soil. Bacteria in the soil make nitrates from nitrites and other nitrogen molecules. Bacteria in legume root nodules take up nitrates from the soil.
19.
CARBON
Carbon is the sixth most common element in the universe and is the main element in every living thing on Earth. Carbon atoms are passed between living things through the CARBON CYCLE. Carbon is present as carbon dioxide in the air, and makes up a large part of coal, crude oil, and natural gas. Pure carbon is very rare in nature, although it can be found in one of several different forms, or ALLOTROPES.
CARBON AS FUEL
Anything that burns well usually contains carbon. Coal, charcoal, wood, and paper are packed full of carbon. Carbon atoms joined together store a lot of energy. When carbon burns, each carbon atom breaks away from its surrounding atoms and reacts with oxygen in the air to form carbon dioxide. The stored energy is released as heat.
ALLOTROPE
The atoms of some elements can link up in different ways to create different forms called allotropes. Carbon is found in three allotropes: diamond, graphite, and fullerene. Each allotrope has very different physical properties. Graphite, diamond, and fullerene contain only carbon atoms, but the atoms are arranged differently in each allotrope.
In a fullerene, the carbon atoms link together to form a ball-shaped cage. Fullerenes may contain 100, 80, or 60 carbon atoms. This fullerene contains 80 atoms. The first fullerene discovered was buckminsterfullerene in the 1980s, which has 60 carbon atoms. It is named after Buckminster Fuller, an American architect who designed buildings similar in shape to the fullerene molecule.
GRAPHITE
Some lubricating engine oils and all pencil leads contain graphite. Graphite has layers of carbon atoms that can slide across one another. There are strong bonds between the carbon atoms of each layer, but weak bonds between the different layers. Because the layers can move over one another, graphite is quite a soft material.
GEODESIC DOME
The stable structure of fullerenes works well on large-scale buildings. In the 1940s, architect Buckminster Fuller designed a type of building called a geodesic dome. It is made of a network of triangles that together form a sphere. This shape is very stable and encloses a lot of space with little building material, making it strong but light.
CARBON CYCLE
Carbon atoms continually circulate through the air, animals, plants, and the soil. This recycling of carbon atoms in nature is called the carbon cycle. The bodies of all living things contain carbon. The carbon comes originally from carbon dioxide gas in the air. Green plants and some bacteria take in the carbon dioxide and use it to make food. When animals eat plants, they take in some of the carbon. Carbon dioxide goes back into the air when living things breathe out, and when they produce waste, die, and decay.
MOVEMENT OF ATOMS
Green plants use carbon dioxide from the air to make food. When an animal eats a plant, it uses the carbon to build body tissue. When the animal breathes out, it returns carbon into the air as carbon dioxide. When the animal dies and decays, the carbon in its body returns to the soil. Decomposers such as worms, bacteria, and fungi, feed on the decaying remains of animals. As they feed, the decomposers breathe out carbon dioxide into the air. Green plants then take in carbon dioxide from the air, and the cycle is repeated.
20.
BIOCHEMISTRY
The study of the chemical processes of all living things is called biochemistry. These processes include respiration (breathing) and the digestion of food. Carbon atoms can combine in so many ways that living things are mainly made up of molecules containing carbon. The molecule DNA carries the chemical instructions that allow living things to create and make copies of their molecules and reproduce.
FOOD FOR ENERGY
All living things need food for the energy to make all the other body processes happen, such as growth, movement, and repair. These complicated chemical reactions are called metabolism in animals. Plants make their food, through a process called photosynthesis.
BREAKING DOWN CARBOHYDRATES
Many foods, including apples, contain molecules called carbohydrates. When broken down through the process of digestion, carbohydrates release a lot of energy. Food contains two other kinds of molecule: fats and proteins. Fats are another good source of energy, and proteins are important for growth.
DIGESTION
The carbohydrate sugar contains 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms. Once eaten, the molecules are broken down to form simpler glucose molecules through the chemical process of digestion. Digestive enzymes, such as amylase, speed up this process.
RESPIRATION
Glucose molecules pass into the bloodstream and are carried to cells around the body. Every cell uses glucose molecules in a chemical process called respiration. In respiration, the bonds within the glucose molecule break, releasing the energy in the molecule in a form our bodies can use.
ENERGY RELEASED
Glucose molecules react with oxygen in the air we breathe to release energy and create carbon dioxide (CO2) and water (H2O) molecules. Processes that release energy, such as respiration, are called catabolic reactions. Other processes that take in energy, such as building proteins, are called anabolic reactions.
FOOD FOR BUILDING MOLECULES
Living things do not only break down molecules, but they also build up complex molecules, such as muscle proteins. Protein molecules are needed for growth. They are made from amino acids, which come from protein-rich food, such as beans, pulses, and meat.
AMINO ACID BUILDING BLOCKS
Amino acid molecules are made of carbon, hydrogen, oxygen, and nitrogen atoms. They join up to form protein molecules, the building blocks of our bodies. Our 20 different amino acids make thousands of proteins.
PROTEIN CELLS
The cells of our skin, blood, hair, and muscles are all made up of proteins. There are thousands of different proteins in our bodies, including the enzymes that drive every reaction and the antibodies that fight disease.
DNA
Probably the most amazing molecule in our bodies is one called deoxyribonucleic acid, or DNA for short. This molecule contains the genes (instructions) for making every different type of protein in our bodies. Almost every cell in our bodies contains DNA, divided up into 46 parts called chromosomes. Each cell uses just the part of the instructions it needs. For example, only a muscle cell makes muscle proteins.
MUSCLES FOR MOVEMENT
Almost anything we do, such as gymnastics, talking, or reading, relies on our protein-built muscles. Inside every muscle cell, a chemical reaction converts the energy contained within the chemical bonds of ATP (adenosine triphosphate) molecules into movement. This reaction also creates heat, which is why you get hot when you exercise. When ATP molecules are made to store energy, anabolic reactions occur. When energy is released, catabolic reactions occur.
21.
ORGANIC CHEMISTRY
The study of all compounds that contain carbon is called organic chemistry. Carbon atoms are unique. They can combine with each other to make molecules that contain hundreds, even thousands, of carbon atoms. There are more CARBON COMPOUNDS than compounds of all the other elements put together. CARBON TECHNOLOGY uses carbon compounds to make many modern materials, from the interiors of aircraft to medicines.
CARBON IN ALL LIVING THINGS
From butterfly wings to the petals of a flower, all living things are made of carbon compounds. All the processes that happen in living things – such as digestion, movement, and growth – are chemical reactions involving carbon compounds. It is the ability of carbon to make so many different compounds that results in the rich diversity of life on Earth.
CARBON COMPOUNDS
Many carbon compounds contain the same few elements, but in different quantities and arranged in different ways. The most important elements to join with carbon are hydrogen, oxygen, and nitrogen. Carbon atoms can form chains of just carbon and hydrogen, which are called hydrocarbons. They can also form rings of carbon, called aromatics.
Methane is a hydrocarbon. It contains one carbon atom bonded to four hydrogen atoms. The prefix “meth-” always refers to compounds whose molecules contain only one carbon atom. Methane is a natural gas. It is used in domestic central heating.
ALCOHOLS AND ESTERS
A carbon compound called an ester gives an apple its distinctive smell. Esters are liquids with a sweet, fruity smell, and evaporate quickly. They are made when alcohol reacts with an acid. Alcohols and esters contain carbon, hydrogen, and oxygen atoms.
CARBON TECHNOLOGY
The carbon industry is one of the largest and most important industries because so many products contain organic (carbon) compounds. Carbon technology is vital to the production of medicines, paints, synthetic fabrics, food flavourings, plastics, cosmetics, and glues. The raw materials that are the basis for these products come from coal, crude oil, and natural gas.
Racing bikes are often made from carbon fibre because it is strong and light, and can be moulded into complex shapes. The carbon fibres are woven into a cloth which is then cut and layered in a mould. The moulded part is filled with a chemical called a resin and then baked in an oven to form the hard, tough carbon fibre material.
CARBON FIBRES
Polyacrylonitrile (PAN) is heated to 3,000°C (5,432°F) to create thin filaments of carbon fibre. This material is fireproof and five times lighter than steel, yet twice as strong. Carbon fibre has many uses, such as in lightweight sports equipment, car body panels, construction pipes, and on the wings and nose of space shuttles.
22.
PLASTICS
Plastics are used to make a wide range of materials, including furniture, computers, and toys. Plastics are not found in nature, but are created from the products of coal, oil, and natural gas. They are made up of carbon, hydrogen, and other atoms linked in long-chained molecules called POLYMERS. Plastics are so useful because they are strong, light and can withstand heat and chemicals better than many materials. They can also be moulded into practically any shape or size.
INSULATING PLASTIC
Modern plastics can be created with precise properties that suit a particular use. Mylar® is a plastic used to insulate space shuttles. It is a shiny, strong, and light polyester film made in very thin sheets. It reflects the intense heat generated when a space shuttle re-enters Earth’s atmosphere, protecting the craft and its crew.
INJECTION MOULDING
Plastic bowls are often shaped by injection moulding. Plastic pellets are heated until they have melted. The liquid plastic is then pushed, or injected, into a mould, held in place by a clamp. After the plastic in the mould has cooled and hardened, the finished product is pushed out. This process is used to mass-produce items such as bowls, butter tubs, and yoghurt pots.
COMMON PLASTICS
PVC
Polyvinylchloride, or PVC for short, is the plastic used to make credit cards and waterproof clothes. It is tough, flexible, cheap to produce, and easy to print on.
POLYTHENE
There are two types of polythene. Low-density polythene is used to make lightweight plastic bags. High-density polythene is stronger and is used to make plastic milk bottles.
PS
Polystyrene can be either rigid or foamed. Rigid polystyrene is used to make toys and containers. Foamed polystyrene is used for fast food packaging.
PET
Polyethylene terephthalate, more commonly called PET, is a strong plastic used to make fizzy drinks bottles. It can be recycled into carpets and ribbons for video cassettes.
PP
Polypropylene, or PP, is a plastic with a relatively high melting point of 160°C (320°F). It is used to make camera film and dishwasher-proof plastic objects.
PA
Polyamide, or PA for short, is the plastic used to package oily food such as cheese and meats. It is also known as nylon, and is used in clothing, ropes, carpets, and bristles for brushes.
POLYMERS
All plastics are synthetic polymers. Polymers are substances whose molecules are made of simpler molecules called monomers joined together in long, winding chains. The monomers contain carbon and hydrogen, and sometimes other elements such as oxygen and nitrogen. Synthetic polymers can be divided into two groups, thermoplastics and thermosets.