Question

Why does a blacksmith heat the metal rim to fix it on a cart wheel?

Answer 1

We can say physical changes are changes in the state of matter without an accompanying change in composition and a new substance is formed that have different properties and composition from the original material in a chemical change. Physical changes are reversible and chemical changes are not reversible.

Complete step by step answer:
Let us define reversible and irreversible change.
Reversible change: Any changes that could be reversed or a temporary conversion is called reversible changes. The reactions that are reversible are known as reversible reactions. In this reaction, one substance is changed into another form but a new compound is not formed. Processes like melting, boiling, evaporation, freezing, condensation, dissolution are reversible changes.
Irreversible change: Irreversible changes are permanent changes. Reactants react to produce a new compound and cannot be obtained again. Heating, burning, mixing, powdering are some of the processes that cause irreversible changes. Cooking of raw egg which can’t be converted back to its original form is an example of irreversible change.
We know that metals expand on heating and contract on cooling. The expansion and contraction of a metal is a reversible change. The diameter of metallic rim used is slightly smaller than the wheels .The rim expands on heating and could easily lie on the wheels. On cooling the rim, it contracts and presses tightly on to the wheels.

Note:
We can say chemical changes are of two types. They are reversible and irreversible change. In reversible change, there is a change in physical properties, shape, and size of the material. Folding of paper, spring elongation are some examples of reversible changes. When a piece of wood is burned to ash, it cannot be made to wood again. This is an example of irreversible change.