Science Define The Following.

The changes that are not useful or changes that harms living things or the environment are called harmful changes.

Many changes such as baking bread, cooking food are useful to us, they are called useful changes.

Man-made changes are result of human- activity. E.g. Burning of fossil fuel.

Changes that occur naturally like ripening of a fruit, spoiling of milk are called natural changes.

Copper and brass articles are coated with tin. This is called tinning.

Iron articles are given a thin coat of zinc to prevent corrosion this is called galvanization.

When an iron article rusts, a reddish brown layer (of iron oxide) forms on it. A greenish layer is seen to form on a copper article. This is called corrosion of metals.

The process by which matter changes from its vapour or gaseous state into its liquid state by cooling is called condensation.

The changes which we cannot reverse back to same thing are called Irreversible changes e.g. Burning of wood, paper.

The changes that can occur in a forward and reverse direction again and again are called reversible changes.

Changes that do not repeat themselves after a fixed interval of time are called non-periodic changes.

Some changes occur again and again after . a definite interval of time, such changes are called periodic changes.

A chemical change produces a new substance having new and different properties and which cannot be reversed by reversing the conditions.

In a physical change the material may change shapes or forms but no new compounds are formed and change can generally be reversed by reversing the conditions.