What is a physical and chemical change? Give some examples of physical and chemical changes.
Answers
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A physical change is a temporary change which can be reversed.
Some examples of physical changes are,
Water cooling-heating ice
Water heating-cooling water vapour
solid wax heating-cooling liquid wax
In a chemical change, heating or cooling results in a permanent change in the state of matter. A new substance is formed and we cannot get the old substance.
For example, when a piece of paper is burnt it changes into ash. The molecules of paper are different from the molecules of ash.
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A change can't be both physical and chemical, but physical and chemical changes can occur simultaneously. This is what's happening with the burning candle: the wax is melting, which is a physical change, and it's combusting, which is a chemical change. A physical change is a change in state, such as melting, freezing, evaporation or condensation, or a change in form, such as grinding a substance into a powder. There's no change in the chemical formula of the substance. A chemical change is one in which a new substance is formed.
The cycle of an internal combustion engine involves both a physical and chemical change. Gasoline is vaporized and the air in the cylinders is compressed, a physical change, before being ignited and combusting, a chemical change.
Digestion is another example of both. Food is physically broken down by chewing and chemically broken down by digestive enzymes in the saliva and acid in the stomach.
A third example is weathering in nature. Rocks are physically changed by wind, running water, expansion of ice as it freezes, abrasive wind and water-borne sediments. They are chemically changed by oxygen, acid rain and acidic compounds produced by decomposers.
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