[Viv (OP)]

I need help explaining how this is not so simple to my non science boyfriend!

He thinks that you should be able to add 2 hydrogen atoms to 1 oxygen atom plus energy (usually in the form of heat) and whaaalaaa you have water ( in a liquid phase).

I told him that water exists in all three phases- i.e. solid, liquid and gas but it takes energy to convert from one phase to the other and to convert gas H2O into liquid H2O is too expensive in terms of energy required and thats why its not done on a large scale industrialized level.

Am I wrong in my explanation?
Please help  []

[eric l]

Actually, the reaction of hydrogen with oxygen is exothermic, meaning that you set free a lot of energy.  That is why you can use hydrogen as a fuel, in a car engine or a blow torch or whatever.  You set free so much energy that you will have water vapour (water in the gas phase).
In order to separate water into hydrogen and oxygen, you need to add lot of energy, e.g. by an electric current in the case of electrolysis.

[daveshorts]

Yes you can make water this way, and you do manufacture it like this in a space rocket and all you have to do to turn it into water is cool down the products, or in a fuel cell where the water is manufatured as a liquid - on the space shuttle this is a major source of their water. However on earth hydrogen is immensely more expensive than the amount of water it would manufacture  (you would have to make it from water + lots of energy or natural gas). If you lived on a planet where hydrogen and oxygen were plentiful but water was not (a circumstance unlikely to last very long) then it would be a way of making water.

[lightarrow]

I told him that water exists in all three phases- i.e. solid, liquid and gas but it takes energy to convert from one phase to the other and to convert gas H2O into liquid H2O is too expensive in terms of energy required and thats why its not done on a large scale industrialized level.

So he wants to make liquid water, e.g., condensing the water vapour which is in air? Look at the price of a dehumidifier and then, in base of the relative humidity and air temperature, evaluate how much it'll cost to have 1 L of liquid water! (A lot!)

[Viv (OP)]

thank you all for your response...