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PHYSICAL AND CHEMICAL CHANGES
Changes can be of two types: Changes in which the physical properties of a substance change are called physical changes. The general physical properties of substances are shape, size, color, state etc. Physical changes are reversible changes. That is, change in physical properties doesnot alter its chemical composition or characterisrtics. Therefore, the change can be undone, which means that the substance can be brought back to its original state.
 * 1) Physical changes
 * 2) Chemical changes

Some examples of physical changes are given below: Now let us try to understand each case. Thus, characteristics of physical changes are: Changes in which the chemical properties of a substance are changed, resulting in the formation of a new substance are known as chemical changes. Chemical combination of a substance is its chemical property. Chemical changes are generally irreversible. That is, the change which is brought cannot be undone due to the formation of a new substance.
 * 1) Melting of wax
 * 2) Melting of ice
 * 3) Crushing a chalk piece
 * 4) Tearing a paper
 * 5) Squeezing a sponge
 * 1) Melting of wax: When wax is given temperature, it starts melting. As the temperature increases, the state of wax changes from solid to liquid. But we observe that as the temperature falls, it again regains its solid state. That is, its physical property which was changed(solid to liquid) is regained. Hence we see that it is a reversible change.
 * 2) Melting of ice: The same process as mentioned above, follows here. When temperature is increased, the state changes from solid to liquid and water is formed. When temperature falls, the solid state is regained. Hence the change is reversible.
 * 3) Crushing a chalk piece: A chalk piece exits in solid state. When it is crushed, the state remains the same, whereas its size and shape is changed. Size is reduced as the chalk is powdered. But we know that by moulding the powder, we regain the shape and size of the chalk piece. Hence the change is reversible.
 * 4) Tearing a paper: When a paper is torn, the shape and size of the paper is altered, which are physical properties. By joining the torn pieces together, we get the original shape and size of the paper. Thus we see that the change is reversible.
 * 5) Squeezing a sponge: When a piece of sponge is squeezed, its shape is changed. But once the force which we have given is taken back, the shape is regained. Thus the change is reversible.
 * 1) Physical properties are changed
 * 2) No new substance is formed
 * 3) The change is reversible

Some examples of chemical changes are given below: Let us try try to understand each case. Thus, characteristics of chemical changes are:
 * 1) Burning of wood
 * 2) Curding of milk
 * 3) Rusting of iron
 * 1) 1. Burning of wood/ burning of paper: When a wooden block or a paper is burnt, ashes are formed, which is a new substance. Thus, as a result of formation of new substance, which cannot be brought back to its initial state, it is an irreversible reaction.
 * 2) 2. Curding of milk: Milk is turned to curd due to the action of enzymes. Curd which is formed as a result of chemical reaction is a new substance, from which milk cannot be obtained back. Thus it is an irreversible reaction.
 * 3) 3. Rusting of iron: When a piece of iron is left untouched in open for a long time, a brownish layer is formed over the iron article. This coating is called rust. The water molecules in the air, also called as moisture along with oxygen molecules react with iron metal to form a new substance called iron oxide (ferrous oxide), which we call as rust. This process is called rusting of iron. Since this involves formation of a new substance (rust) due to chemical reaction, it is an irreversible reaction.
 * 1) Chemical properties of substance is changed
 * 2) New substances are formed
 * 3) The change is irreversible