Changes in matter

 Rusting of iron is a chemical change 

physical change 

Physical change When an ice cube melts, it is easy to make the water freeze again. You can get the salt back from a salt solution by drying out the solution. Melting and dissolving are examples of physical changes.

In a physical change, only the behaviour of the particles changes. The actual particles are the same throughout and the total mass of the substance remains the same.

Ice turns to water at 0°C. It is easy to change the water back to ice again by cooling it. No new chemical substance is formed in this change.

When no new chemical substance is formed, a change is called a physical change.

Physical changes are easy to reverse.

Chemicals change

Chemical change When iron rusts, or a candle burns, new chemical compounds are made. It may be impossible to reverse the process. These types of changes are called chemical changes or chemical reactions.

In a chemical reaction the following things happen:

1. A new substance is made;

2. energy is either given out or taken in;

3. the change is almost impossible to reverse;

4. although the atoms of each element combine in different ways, the same atoms that were present at the start, are there at the end of the reaction. Because of this, the total mass of the substances remains the same.

Some yellow sulphur powder and iron filings are mixed together. The mixture can easily be separated by stirring it with a magnet. The iron filings will stick to the magnet. When heated, the mixture will glow brightly. The yellow specks of sulphur disappear. A black solid is formed. The black solid is not at all like the mixture. It is not affected by a magnet.

Sulfur

Iron filings can be separated from the mixture 

The black solid is obviously a new chemical substance. The iron and sulphur have reacted together to form a compound iron sulphide.

The above chemical change can be written in a word equation:

iron + sulphur → iron sulphide

Or it can be written in symbols:

Fe + S ⇒ FeS

(reactants) (products)

Although we have to heat the iron and sulphur to start the reaction, once it starts it gives out heat.

Reversible reaction

There are some reactions which can be reversed. In these reactions reactant forms products that in turn react to give the reactant back. An example of reversible change is the heating of blue copper sulphate. It decomposes to give white copper sulphate powder and water vapour. If water is added to the white copper sulphate the reaction is reversed and blue copper sulphate is re-formed. This is called a reversible reaction. The symbol for this is: Another example of a reversible reaction is when ammonia gas and hydrogen chloride gas come into contact with each other. A white cloud of ammonium chloride vapour If solid ammonium chloride is heated, it decomposes to give ammonia gas and hydrogen chloride gas.

Changing raw materials into products

Fertilizers

Plants need water from the soil. They also need different elements. The three elements theu need most are nitrogen, potassium, and phosphorus. Plants use up these more quickly than other elements in the soil. Small amounts return to the soil in animal manure and when dead animals and plants decay. Some nitrogen is also put back by lightning and by bacteria in the soil, Soon the soil is too poor for new crops. So the farmer must replace the missing elements by adding fertilizers.

Fertilizers are chemicals like these:

1: ammonium nitrate

2: ammonium sulphate

3: ammonium phosphate

4: calcium nitrate

5: potassium sulphate

6: sodium nitrate

7: urea

Ammonium nitrate is a useful fertilizer. It is a salt which contains nitrogen, an element essential for plant growth. A good fertilizer contains the elements needed to promote healthy growth in plants. It must also be cheap to produce and soluble in water. In industry, ammonium nitrate is made by reacting ammonia with nitric acid.

Harmful effects of the improper use of chemical fertilisationm

. Long-term use of chemical fertilizers can be harmful to soils.

. They can lead to a breakdown of soil crumbs and an accumulation of organic acids in soils. When a soil's crumb structure is destroyed, it becomes a fine powder which is easily blown away by the wind and washed away by water, causing soil erosion.

. Excess fertilizers sprayed onto fields are washed into rivers and lakes where they cause algae to grow fast. The algae spread over the surface blocking out light for the plants below. The plants die and are decomposed by bacteria, which use up all the oxygen. So fish and other water life die too.

. They may enter drinking water and cause serious health hazards.

Plastics

Many objects that used to be made of metal are now made of plastic. A plastic is a synthetic material which can easily be shaped. Plastics are usually tough and versatile. They are very good electrical insulators. They can be spun into fibres to make clothes and carpets, or moulded to make objects such as cups or chairs.

Plastics are polymers. Polymers contain very large molecules that are made by adding together many small molecules called monomers. Some familiar polymers are polyethene, polystyrene, and poly vinyl chloride (PVC). Nylon, Perspex, and Terylene are the common names of some other important plastics.

Polyethene 

Polyethene is made from the monomer ethane. Long chains of about 50,000 ethene molecules added together are formed.

Nylons

Nylon is a plastic that can be made into thin fibres, which can be woven into cloth.

Plastics Uses

polyethene:

plastic bags, beakers, buckets, bowls, bottles, dustbins

polystyrene:

cups, packaging materials, ballpoint pens, thermal insulation

PVC:

car seat covers, raincoats, hosepipes, electrical insulation

PTFE:

non-stick saucepans, oven floors, plumbing tape (poly tetra- flourethene)

Nylon:

rope, brush bristles, clothing

terylene:

seat belts, yacht sails, clothing (polyester)

melamine:

unbreakable plates and mugs, work surfaces

phenolic resins:

electric plugs, saucepan handles

Perspex:

rulers, car windscreens, signboards, substitute for glass

Bakelite:

electrical fittings

Fossil fuels

Hundreds of millions of years ago, while ancient forests were starting to form coal on the land, other fossil fuels were being made under the sea. We use them now as oil and natural gas. Oil was made from the microscopic plants and animals which lived in the sea. As they died, their bodies collected at the bottom of the ocean. Here they were covered by mud and sand. Over thousands of years the layers of mud and sand became very thick. High temperature and pressure changed them into a thick, black liquid called crude oil.

Coal is a fossil fuel

When crude oil is pumped out of the ground it is a black, smelly liquid. It mixture of many chemicals which are very useful. They can be separated by a technique called fractional distillation. The crude oil is heated to about 400°C. This turns it into a mixture of gases which are pumped into a tall tower. The hot gases rise up the tower.

Refining crude oil

Crude oil is a mixture of compounds called hydrocarbons. How is this mixture separated?

Crude oil is refined by fractional distillation, which takes place in a tall tower called a fractionating tower. There the oil is heated up. The compounds with low boiling points boil off first. They rise to the top of the tower. Compounds with higher boiling points are collected lower down. The different groups of compounds are called fractions.

Fats

Fats are organic compounds that are made up of carbon, hydrogen, and oxygen. They are a source of energy in foods. Fats belong to a group of substances called lipids, and theu come in liquid and solid forms. All fats are a combination of saturated and unsaturated fatty acids. Fats serve as a storage substance of the body’s extra calories. It fills the fat cells and helps insulate the body. Fats are also an important energy source.

Making margarine

Some of the fats and plant oils can be turned into valuable food such as margarine. The oil for making margarine comes from the seeds of plants such as sunflowers. The sunflower seeds are crushed and squeezed to extract oil. The oil is refined by heating with sodium hydroxide. The impurities in the oil react to form a sort of soap. The purified oil is separated and then washed. Hydrogenation of the unsaturated acids in the oil takes place when the oil is heated with hydrogen under pressure over a nickel catalyst (the catalyst speeds up the reaction which would otherwise be slow). The fat is then heated and steam is blown over it to take away the unpleasant smell. Colouring, flavouring, and salt may be added. Vitamins are also added to make it healthier to eat.

soap

Detergents and soap

Detergents are chemicals which, when dissolved in water, can remove dirt and grease from cloth, metal, ceramics and, of course, human skin. Soap is a detergent made from animal fats or plant oils.