Justine observed that the water level in a dish placed on the window sill decreased after a few days, and vapor formed. What bes
t explains that she observed a physical change? Vapor can be easily condensed back to form water. Water and its vapor react with substances in different ways. The molecular composition of water changed when the water level decreased. Bonds between hydrogen and oxygen broke when the water level changed.
Water molecules that only vibrate are in solid form. If heat energy is added, the molecules will speed up their vibrations until they can break loose from the organized pattern of a solid and begin to slide over each other, changing to liquid state. If more heat energy is added, the molecules will speed up more and move randomly in all directions as a gas/vapor. Removing heat energy will reverse these changes.
