Gas or liquid?

[mango juice]

If I boild 3 kg of water into steam, will the steam weigh 3 kg as well? Why are most stuff transported in the form of liquid rather than gas?

[greenday]

Sounds like homework to me, however, liquids occupy less volume than gases, so transporting liquids means you get more stuff in your container. The steam will still weigh 3 kg, assuming you don't let any escape.

[Dacetine]

Yes, 3 kg water will produce 3 kg steam as the amount of matter has not changed. Products are generally transported at ambient temperature to avoid the need for heating or cooling (which requires energy and thus costs money). Incidentally, I would have thought that out of solids, liquids and gases, that gases would be the least convenient phase of matter to transport.

[jake-the-peg]

There is a common confusion between mass and weight. Mass is how much matter is present and is measured in Kg, weight is the gravitational force and is measured in Newtons.

a 1Kg mass of lead weighs about 10 Newtons on Earth, It is still 1Kg on the moon but weighs considerably less about 1.6 Newtons

Similarly 3 Kg of water is still 3 Kg whether it is solid, liquid or gas however as a gas it takes up a hugely greater volume and because of this it is bouyed up more by the air.

Imagine a baloon filled with steam, in a vacuum it would weigh about 30 Newtons, now let the air in and it will weigh 30 Newtons less the weight of the air it displaces.

So things certainly weigh less (in air) when they are gasses but imagine trying to transport 30Kg of water as steam - just imagine how big that would be!

p.s. if this is homework try to get your head around this and put it in your own words ;c)

[mango juice]

thanks everybody! Yeap that's my homework!

but why does liquid occupy less volume? just curious...

[Loosehead]

Because is is more dense. In your example the 3kg's of water occupies 3000 cubic centimeters but the steam, assuming none escapes, has the same mass but takes up a lot more volume, ie it is less dense.

[jake-the-peg]

And it's less dense because when it turns to a gas the forces that are binding the molecules together are overcome and they start flying around banging into air molecules, if they hit another water molecule it's with too much energy to stick.

As the temperature cools water molecules that colide do so with less energy and start to stick together forming what we scientists like to call clouds and rain

[JohnPPotts]

As to why - imagine the molecules of water as tennis balls. As water they are stuck together in clumps and don't move around much. As a gas they are free of each other and move around like a serve from Tim Henman (actually, faster) so will tend to spread out more, unless contained by something.