The universe is forever changing. Changes are a constant presence in the world and in the environment around us. Melting of an ice-cream, ripening of fruits, curdling of milk, seeds germinating, a matchstick burning, rusting of an iron nail, etc. are all examples of changes that occur around us in our daily lives.
Chemistry classifies these changes into various categories as follows-
• Slow and fast changes-
Some changes are very fast and occur within a few seconds. Such changes are called fast changes. Bursting of a balloon, match-stick burning, spinning of a top, etc. are all examples of fast changes.
Some changes take days, months or years to take place. Such changes are called as slow changes. Growing of a mango tree, changing of seasons from summer to autumn, rusting of iron, etc. are examples of slow changes.
• Reversible and irreversible changes-
In some changes, the products of the change can be converted back into their original form. In other words, these changes can be reversed. Such changes are known as reversible changes. Melting of ice, lighting of an electric bulb, burning of a platinum wire are all examples of reversible changes.
In some changes, the products that are formed as a result of the change cannot be converted back into their original form. In other words, these changes cannot be reversed. Such changes are known as irreversible changes. Burning of paper, curdling of milk, burning of a matchstick, etc. are examples of irreversible changes.
• Periodic and non-periodic changes-
Certain changes occur again and again, generally at fixed intervals of time. Such changes are known as periodic changes. The changing of seasons, movement of a pendulum in a clock, high and low tides, day and night, etc. are examples of periodic changes.
Some changes do not occur at fixed intervals of time and can occur at any time. Such changes are known as non-periodic changes. Rusting of iron, lightning and thunder, earthquakes, etc. are examples of non-periodic changes.
All changes can broadly be classified into two kinds-
• Physical change
• Chemical change
A change that alters the physical properties of a substance, without bringing about a change in the chemical properties of the substance, is known as a physical change. Melting of ice, magnetisation of iron, heating of a platinum wire, glowing of a light bulb, dissolving salt in water, etc. are examples of physical changes. A physical change displays the following characteristics-
• A physical change can be easily reversed.
• A physical change is generally temporary.
• No new substance is formed.
• The change is only in the physical properties of the substance.
• The molecular composition and chemical properties of the original substance are not changed.
• There is no change in weight during physical change
• Hardly any heat is absorbed or released during a physical change.
A change which alters the chemical properties of a substance, by bringing about a change in its chemical composition and molecular structure, is known as a chemical change. Burning of sulphur to give sulphur dioxide, burning of sugar to give carbon and water vapour, rusting of iron, burning of a match-stick, etc. are examples of chemical changes. A chemical change displays the following characteristics-
• A chemical change cannot be reversed easily.
• A chemical change is generally permanent.
• An entirely new substance or substances are formed.
• The molecular composition and chemical properties of the original substance are changed.
• Chemical change results in a change in weight.
• A lot of heat energy is generally absorbed or released during a chemical change.