Properties such as shape, size, colour and state of a substance are called its physical properties.
A change in which a substance undergoes a change in its physical properties is called a physical change.
A physical change is generally reversible. In such a change no new substance is formed.
A change with which you are quite familiar is the rusting of iron.
When we leave a piece of iron in the open for some time, it acquires a film of brownish substance. This
substance is called rust and the process is called rusting where magnesium hydroxide is a base.
Magnesium oxide is a new substance formed on burning of magnesium.
Magnesium hydroxide is another new substance formed by mixing magnesium oxide with water.
Change in colour of the copper sulphate solution due to reaction with iron.
A change in which one or more new substances are formed is called a chemical change.
A chemical change is also called a chemical reaction.
vinegar and baking soda together produced carbon dioxide, which turned lime water milky.
For rusting, the presence of both oxygen and water (or water vapour) is essential.
Prevent iron articles from coming in contact with oxygen, or water, or both. One simple way is to apply a
coat of paint or grease.
In fact, these coats should be applied regularly to prevent rusting. Another way is to deposit a layer of a metal
like chromium or zinc on iron. This process of depositing a layer of zinc on iron is called galvanisation.
Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does
not rust.
Large crystals of pure substances can be formed from their solutions. The process is called crystallisation. It is
an example of a physical change.
COAL AND PETROLEUM
These were formed from the dead remains of living organisms (fossils) known as fossil fuels.
Coal: It is as hard as stone and is black in colour. Under high pressure and high temperature, dead plants got
slowly converted to coal.
As coal contains mainly carbon, the slow process of conversion of dead vegetation into coal is called carbonisation.
Since it was formed from the remains of vegetation, coal is also called a fossil fuel.
Coke: a tough, porous and black substance.
It is almost pure form of carbon.
Coke is used in the manufacture of steel and in the extraction of many metals.
Coal tar is a black, thick liquid with unpleasant smell. It is a mixture of Products obtained from coal tar are used as starting materials for manufacturing various substances used in everyday life and in industry, like synthetic dyes, drugs, explosives, perfumes, plastics, paints, photographic materials, roofing materials, etc.
Interestingly, naphthalene balls used to repel moths and other insects are also obtained from coal tar.
These days, bitumen, a petroleum product, is used in place of coal-tar for metalling the roads.
Coal gas was used for street lighting for the first time in London in 1810 and in New York around 1820.
Now a days, it is used as a source of heat rather than light.
Petroleum was formed from organisms living in the sea.
As these organisms died, their bodies settled at the bottom of the sea and got covered with layers of sand and clay.
Over millions of years, absence of air, high temperature and high pressure transformed the dead organisms into petroleum and natural gas.
the layer containing petroleum oil and gas is above that of water because the layer containing petroleum oil and gas is above that of water.
Petroleum is a dark oily liquid.
In India, the Petroleum Conservation Research Association (PCRA) advises people how to save petrol/diesel while driving.
Social