Why do liquids increase in volume when they change into gases?  

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Volume, in the context of matter, is the amount of space that is occupied by an object (matter). In some cases, the boundary of this space is evident. For instance, for a solid, the shape of the object dictates the space it occupies. It is a different case for fluids...

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Volume, in the context of matter, is the amount of space that is occupied by an object (matter). In some cases, the boundary of this space is evident. For instance, for a solid, the shape of the object dictates the space it occupies. It is a different case for fluids - or matter that flows - like gases and liquids. 

For fluids, the shape of the particular gas or the liquid is the same shape as the container. If you pour water from a water bottle to a bowl, the shape of the water changes from that of the water bottle to that of the bowl. The same is true for a gas. A gas inside a star-shaped balloon is star-shaped, and it is round in a round balloon. Liquids and gases take up the shape of the container because of the nature of the interactions in the atoms or molecules that make up the object. For solids the atoms/molecules are very tightly packed. This is not the case for liquids and gases where the molecules have more freedom.

A main difference between molecules in liquids and gases is that molecules in gases have a higher kinetic energy and hence more freedom. Liquids still have substantial interaction between its molecules. Imagine water being boiled. As the water heats up some of the molecules increase in kinetic energy due to increasing temperature and escape the liquid phase as water vapor. Molecules in a liquid (without boiling) are generally confined to a certain volume. Meanwhile, due to the kinetic energy in gases, the molecules occupy every available space. If I have 1L of a liquid and pour it into a 5L container, while it will take the shape of the new container, it will still be 1L. However, in the case of a gas, moving it from a 1L container to a 5L container would mean changing its volume from 1L to 5L - again, this is because the molecules of a gas are in a continuous random motion, and the lack of interaction between them gives them the ability to occupy and fill up the rest of the container.

This difference in the property of liquids and gases leads to the answer to your question. Gases occupy more volume than liquid because of the inherent properties of the atoms and molecules of a gas (according to the Kinetic Molecular Theory of Gases). The molecules do not interact with each other (hence no attraction to keep them in place as in solids and liquids) and they are in a continuous random motion allowing them to occupy the available space.

A slightly related concept is thermal expansion. As you heat up an object, it expands. This is because heating increases the temperature and gives the molecules more kinetic energy, and hence more motion. In solids, they vibrate more and move a little bit away from each other. The same applies for liquids and gases but with a more dramatic effect.

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