Answer: This is because of increase in air pressure inside the rubber raft.
Explanation:
According to Gay-Lussac's Law: 'The pressure of the gas at constant volume is directly proportional to the temperature in Kelvins'.
[tex]Pressure\propto Temperature[/tex](at constant Volume)
or [tex]\frac{Pressure}{Temperature}=constant[/tex]
So, when we left rubber raft in a sun on a summer day temperature of the air inside the rubber raft increases due to which pressure of an air inside the rubber raft also increases. And by this increase in pressure rubber raft bursts.
