# What do they mean when they say water “expands into” steam?

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I saw this video that says water “expands into” steam and that 1 cup of water can expand into as much as 1600 cups of steam.

But like… Why specifically 1600? Is steam a gas? If it’s a gas it can basically occupy any volume however large, right? The molecules will just go far apart from each other “forever”, no?

This led to a series of increasingly existential questions….

(1) Is steam not a gas? It does look a little like water when it rises from my kettle.

(2) Do different liquids “expand into” different volumes of their gaseous substances?

And more exestentially…

(3) What does the “volume” of a gas even mean if it can expand basically infinitely to “fill any container” as I was taught in school?

This is the YouTube video here: https://youtu.be/-8lXXg8dWHk

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>1 cup of water can expand into as much as 1600 cups of steam.

*At standard pressure

That’s always the bit that’s missing here

If you boil water in near vacuum then the steam will take up significantly more than 1600 cups of volume but the pressure will be significantly lower

Similarly, if you boil water in a power plant and don’t let the steam expand then you can get steam as dense as liquid water but it’s under much higher pressure than normal

Steam is a gas. When it is pure steam, it is called saturated or superheated and is invisible. You do not see steam from a kettle or boiling pot.

The “steam” you see when the kettle boils is actually liquid water droplets that are condensing out of the gaseous steam when it comes into contact with the colder air.

1600 is the approximate ratio of liquid to gas volume at the boiling point at standard temperature and pressure. So right at the moment the steam boils – or changes phase from liquid to gas – it expands 1600 times in volume.

If you continue to heat the steam , it can continue to expand – like any gas, following the ideal gas law- as long as the gas is free to expand. If it’s not free to expand it will increase in pressure.

Steam is water in gas form. Like other gases, you can compress it in a container (the number they are giving you is “at standard pressure”, meaning at the average air pressure at sea-level — 14.696 psi).

You can imagine that if you turned 1 cup of water into 1600 cups of steam crammed into a 1 cup container, the pressure would be 1600x surrounding air pressure (and the container would need to be super strong, otherwise it would just explode). For reference, the propane tank for a gas grill will explode at 42x. The strongest gas cylinders you can buy go up to 680x.

Most liquids can be heated into gases (and the gas takes up more space at standard pressure). Conversely, you can cool and compress gasses into liquids (liquid nitrogen, oxygen, etc).

1. Steam is a gas

2. Depending on the substance, a certain amount
of liquid would produce a certain amount of vapor based on how much the different molecules like to stick to each other and depending on what pressures you’re working with.

3. The volume of a gas, it’s pressure, and it’s temperature are all interconnected. If you have seen the formula PV = nRT, that’s how they relate. P is the pressure, V is the volume, T is temperature (in Kelvin), n is the number of molecules, and R is the ideal gas constant (related to the boltzmann constant). Basically, if you have something at a certain temperature, and you want to reach a desired pressure, it tells you what volume you need, or you can rearrange it to find any of the variables as long as you fix everything else in place.

Long story short, the steam expands because it’s hot, hot things expand, especially during phase changes. The only common thing that breaks that rule is that ice expands when it freezes.

Steam engineer here! 1600 times is just the measured expansion of water, it’s how much it increases in volume.

1. Yes steam is a gas. But, the steam you see rising from your kettle is not 100% steam, it is some percentage water. Once you have steam that is 100% a gas with no entrained liquid water, it is called saturated steam, and when you increase the temperature of that steam it is called superheated steam.

2. Different liquids do expand into different volumes and do so at different temperatures and pressures, this is the basis of refrigeration systems and why you can’t put just any refrigerant into every system, but that is a topic for another time.

2. A gas will expand to fill any container it is in, that is correct, but so will the other gases in the room. You would have regular air mixed in with your steam, like a steam room, you can still breath because there is still air in their.

3. The big part of this expansion is when you are making steam in a sealed system, like a boiler, or steam piping system. If the steam has no where to expand to it increases the pressure on the container. Steam for heating use doesn’t really require much if any pressure at all, but using steam for propulsion or power generation like a steam turbine does require pressure.

4. Going back to volume, the vice versa is true, if you have 1600cu.ft of steam, then turning it back into water only gives you 1 cubic foot. If you do that inside of a sealed container you create a vacuum. If you watch the videos of someone boiling water in a soda can and then placing the soda can in ice water and it rapidly crumples the can, the rapid change in volume of the steam to water caused a vacuum and sucked the can inwards, crumpling it.

Does this make sense? Would you like me to clarify anything?