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Because the heat from the water could in theory build up enough air pressure to cause the bottle to burst - covering the person with scalding water. Expelling all the air from the bottle before tightening the stopper reduces the risk dramatically.

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Q: Why air from hot water bottle is expelled after filling?
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Related questions

Why do you need water in a bottle rocket?

water is needed to keep the air in until the water is expelled out.

How can you make a bottle float then sink then float?

A bottle, though heavier than water, can be made to float by trapping air inside of it. This same bottle can be made to sink by filling it with water, or a substance heavier than water. It can again float by emptying it and trapping air in it.

Why does filling a plastic bottle with cold water cause moisture on the outside of the bottleg?

The dew point is the temperature at which vapor in the air will condense and turn into liquid. The dew point is determined by the air temperature, pressure and relative humidity. Consequently, when vapor in the air comes into contact with the edge of the bottle (which, if the water is cold, is below dew point) it condenses and forms liquid on the outside of the bottle.

What will happen if a dented water bottle is put in a basin of hot water?

The air in water bottle expands

Why does the water vapour surround the cold water bottle?

The water in the bottle is much colder than the air surrounding it, so the bottle cools the air immediately surrounding it through conduction. This causes the air to cool to its dew point, whereupon some of the moisture in the air will condense onto the bottle.

Why does filling a plastic bottle with cold water cause moisture on the outside of the bottle?

It's a process called condensation caused by humid air striking a much colder surface. Generally the colder air is the less moisture it can carry. So when the air around you meets the side of the bottle it begins to cool, and can't care as much moisture as it had been. The moisture it can't carry is deposited on the side of the bottle as condensation.

Does a water bottle float on top of water?

An unopened water bottle contains a small pocket of air, allowing it to float. If the water bottle were to have this air removed, its buoyancy would be determined by the purity of the water inside and outside of the plastic.

Does freezing a water bottle make the outside of it wet?

Not exactly.The water that appears on the outside of the icy bottle is condensation of the water vapor in the air around the bottle. The cold temperature of the ice in the bottle causes the condensation. There are lots of water molecules in air -- there is more water in the air on a humid day then on a hot dry day, but there is always some water in the air. When air is cooled by coming in contact with the icy bottle, it condenses, and goes from being a gas to being a liquid (just like how steam turns back into water when it cools). It is the condensed water from the air that makes the outside of the bottle wet.If a cold bottle was in air that had no water vapor in it (unlikely except in a laboratory), then it would not get wet.

Why does an empty plastic bottle floated while the bottle filled with water sank?

Since air is less dense than water, the air pushes the bottle upward. The water filled bottle would sink because the density is greater than water.

Is an empty water bottle matter?

Yes. The bottle itself is matter, as well as the air inside it.

How do pop bottle rockets work?

There is water in the bottle and the air gets compressed to launch the bottle into the air, and when it reaches apegy, the parachute comes out and lands the bottle safely.

What happens when you put a balloon on a water bottle and put it in hot water?

Air inside the bottle expands when the bottle is heated. Some of it leaves the bottle until the pressure of the heated air remaining in the bottle equals the pressure of the air in the room. The balloon is then placed over the neck of the bottle and prevents any more air from entering or leaving the bottle. The air inside the bottle cools to the temperature of the ice water. The cooler air inside the bottle takes less space (volume) than it did when hot, so it sucks the balloon inside the neck of the bottle. Air pressure inside the bottle causes the balloon to stretch and enlarge until the air pressure inside the bottle, including the air in the balloon, has the same pressure as the air in the room.