熱湯涼了(有蓋),為什麼很難開??

呢個係 一個 about pressure 的 problem

when 湯 is hot, the air near (above) it will be hot also. Thus the air density above the湯 will be less then the ambient air.
In 有蓋 case, 蓋 proivide the air above the 湯 to be in a close system ( no particle transfer between the air inside the bowl of soap with 蓋, and the air outside).
according to "PV/nRT" is conserve for a close system, when the air inside the 蓋 is cool down (i.e. T decrease),volume of the air unchange (V unchange), the resulted effect is P must be decrease. (P stant for pressure)

Before the 蓋 is used to cover the soap, the ambient air and the air above the soap is in same pressure. After using 蓋 to cover the soap and wait for sometime, the air pressure inside the 蓋 will decrease. The ambient pressure will be larger then the pressure inside the 蓋. Thus extra force is needed to over the bowl of soap.