If you take a clean PET bottle with screw stopper and pour in some clean fresh water from the tap, about 25- 40% full. Tighten stopper, shake violently, the sides of the bottle start to cave in. There is a vacuum formed – HOW? There is the same amount of water and air in the bottle. Unscrewing the stopper relieves the vacuum.
David Royal from Warwickshire (Age 55+)
Filed under: Answered Big Questions, Neil Shirtcliffe's Big Answers, Physics Big Questions, age 55+
The temperature of the water will affect this one. Cold tap water is about 10 C, room air about 30 C. You trap them in a bottle and then cool the air to the temperature of the water by shaking it, gasses shrink a lot more than liquids when cooled so the contents shrink, reducing the pressure. If you use hot water the opposite happens, but do not do this as it is easy to burn yourself when you open the top and hot water squirts out. If you have small children and heat bottles you this is a common problem.