Chemical reactions involve energy transfers. In a suitably insulated experiment, a rise or fall in temperature shows whether energy has been transferred to or from the surroundings.
Melting and boiling need energy to be put in, to overcome attractive forces between the particles and increase their separation. On melting the particles stay close together; on boiling they move much further apart. Freezing and condensing transfer energy to the surroundings as the particles become more strongly attracted to one another.
During the change of state of a pure substance the temperature stays constant even though energy is still being transferred, because the energy is going into separating the particles rather than speeding them up.
An exothermic reaction gives energy out to the surroundings, so the temperature of the surroundings rises. Combustion, neutralisation and most oxidation reactions are exothermic.
An endothermic reaction takes energy in from the surroundings, so the temperature falls. Thermal decomposition is endothermic, and so is the reaction in a sports injury cold pack.
In a suitably insulated container, a temperature rise is evidence that the reaction is exothermic and a fall that it is endothermic.
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