A block of ice at - 10 ° C is slowly heated and converted to steam...

When the state of the substance is not changing, temperature of the substance increases when heat is supplied to it. But when the substance changes the state, the heat supplied to it used as latent heat and the temperature remains constant until all the substance converts to other state.

Therefore, temperature of ice first increases from $- 10 ° C$ to $0 ° C$. Then, remains constant at $0 ° C$ till whole ice is melted. Then, it increases from $0 ° C$ to $100 ° C$. Again it remains constant at $100 ° C$ till whole water converts into steam.