The given graph shows the heating curve for a pure substance. The temperature rises with time as the substance is heated :
(a) What is the physical state of the substance at the points A, B, C and D ?
What is the melting point of the substance ?
(c ) What is its boiling point ?
(d) What happens to the temperature while the substance is changing state ?
(e ) The substance is not water. How can you judge from the graph ?

Explanation

(a) At point A : The substance is in the solid state.
At point B : The substance has started melting. It exists both in the solid and liquid states.
At point C : The substance is in the liquid state.
A point D : The substance has started boiling. It exists both in the liquid and gaseous states.
(b) the melting point of the substance is `15^(@)C`.
(c ) The boiling point of the substance is `110^(@)C`.
(d) The temperature remains the same during the change of state.
(e ) Had substance been water, its melting point should have been `0^(@)C` and boiling point `100^(@)C`. It is therefore, not water.