notes about reversible and irriversable substancces
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All changes in the world are one of the two types – Reversible changes and irreversible changes. A change which can happen backward, that is, can be reversed is called a reversible change. If you keep water in the freezer for some time, it transforms into ice. But as soon as you take it out of the freezer, it turns into water again. This is a reversible change. Similarly, if you boil water, it evaporates and becomes water vapor. When you cool this vapor down, it turns back to water.
A change which cannot happen backward, that is, it cannot be reversed is called an irreversible change. When you burn a piece of paper, it turns to ash. It cannot become paper again. Your height cannot decrease. These are irreversible changes. They cannot be reversed at all.
To understand reversible changes and irreversible changes better, we need to know what physical changes and chemical changes are.
Physical changes are changes that occur in the physical properties of the substance, like shape, size, and nature. Physical changes may or may not be reversible changes. Like, turning of water into ice is an example of reversible changes but growth in height is an irreversible change.
Chemical changes are changes that occur in the chemical properties of the substance like its flammability, radioactivity, etc. All chemical changes are irreversible changes. Once the chemical properties of a substance are changed, it becomes another substance. Thus, it cannot return to its original state.
Examples of reversible changes
Melting: Melting is when solid converts into a liquid after heating. Example of melting is turning of ice into water.
Freezing: Freezing is when a liquid converts into a solid. Example of freezing is turning of water into ice.
Boiling: Boiling is when a liquid converts into a gas. Example of boiling is turning water into water vapour.
Examples of irreversible changes
Burning: When we burn a piece of paper or wood, it turns to ash and smoke. We cannot obtain paper and wood from the ash again.
Cooking: We use heat for cooking, right? Once we cook our eggs, we cannot uncook them. Therefore, cooking is an irreversible change.
Rusting of iron: You must have seen rusting of iron. When water vapour (humidity) comes in contact with the iron, they react. Therefore, iron rusts and once this happens, we cannot reverse it.