PHYSICAL AND CHEMICAL
CHANGES
PHYSICAL CHANGE:-
Those properties which describe the look or feel of a substance e.g., colour, hardness, density texture and phase are called physical properties. Every substance has its own characteristic physical properties that we used to identify the substance.
The physical property of a substance can change on changing conditions but the substance remains same or no new substance is created. e.g., if the water is subject to boil, the formation of bubbles show that the water is changing into a gas. The gas disappears into the air as water vapour. If this air touches the lid or plate covering the water, the water vapour will change back into droplets of water.
Therefore a physical change is one in which the substance undergoing the change is not destroyed and no new element or compound is formed. In some physical changes the properties like colour, shape, or size of the substance may change e.g. glowing of an electric bulb is a physical change, melting of ice is a physical change.
Look at the picture of water boiling and changing into steam. Steam is another form of water. Heating water did not create a new material. In changing the water from a liquid to a gas, only the state of the water is changed.
CHEMICAL CHANGE :-
Those properties which allow a chemical substance to react with other substances to transform from one substance to another are known as chemical properties for example copper has the chemical property of reacting with carbon dioxide and water to form greenish blue solid known as patena.
A chemical change is one in which the identity of the original substance is changed and a new substance or new substances are formed. In a chemical change the properties of the substance before and after the change are entirely different. e.g. souring of milk, burning of paper, burning of candle, etc.
Look at the picture of the burning candle. The wax of a candle burns into ash and smoke, The original materials are changing into something different.
Note:- A chemical change involves a change in the way the atoms in the molecules are chemically bonded to one another. A chemical bond is the force of attraction between atoms that holds them together.A methane molecule for example is made of a single carbon atom bonded to four hydrogen atoms, and an oxygen molecule is made of two oxygen atoms bonded to each. Figure given below shows the chemical change in which the atoms in a methane molecule and those in two oxygen molecules first full apart and then form new bonds with different patterns resulting in the formation of carbon dioxide and water molecules.
Therefore, we can say that any change in a substance that involves a rearrangement of the way atoms are bonded is called a chemical change.
class 8
,
Physics
,
Science
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