Why is H2O a liquid but CO2 a gas ?

Question

Since two atoms of Hydrogen and one atom of Oxygen are lighter than One atom of Carbon and two atoms of Oxygen, if a molecule of CO2 is a gas, one would expect a molecule of H2O to be a gas also.

Answer 1

It's due to hydrogen bonding, put that in a search engine and read away, it's very interesting actually.

Answer 2

You'd think so, wouldn't you. But it's not just the molecular weight that contributes to whether something is a liquiid or a gas at room temperature.

In this case, a major factor is a kind of bonding between molecules, called hydrogen bonding. Oxygen is very electronegative, meaning that it doesn't share well in forming a bond. (Bad oxygen!) Instead, it tries to grab most of the electrons in the two electron oxygen-hydrogen bond for itself most of the time. The result is that it develops a partial negative charge.

This leaves the hydrogens in water with a partial positive charge. The resulting bond is "polarized" and the water molecule is polar.

The answer to your question comes from how this affects the neighboring water molecules. Electrostatic (i.e. charge-based) interactions between the partially negative oxygen of one water molecule and the partially positive hydrogen of another water molecule form what's called a hydrogen bond between the two molecules. Those bonds are weaker than regular covalent bonds (the ones IN the water molecule), but still plenty strong.

And of course all the water molecules are interacting this way, so you really have a big, massive pseudo-molecule of high molecular weight, sort of. Those hydrogen bonds are hard to break at room temperature, which is what you'd need to do to make water a gas.

Carbon is somewhat more electronegative than hydrogen, enough so that carbon dioxide is not as polarized as water. But still enough so that carbon dioxide is an easily liquified gas.

(Water also can form nice tetrahedral H-bonded structures because of its geometry--but that's just icing on the cake.)

(Electronegaivity of H = 2.1, C = 2.5, O = 3.5).

Answer 3

Strong intermolecular forces exist between the δ- oxygen atom in one water molecule and δ+ hydrogen atom in a neighbouring water molecule. This is called hydrogen bonding and is the strongest type of intermolecular force.

Hydrogen bonding only occurs between a δ+ hydrogen atom and a δ- fluorine, oxygen and nitrogen atom hence HF, NH3 and H20 all exhibit hydrogen bonding and their bp's/mp's are far higher than expected in comparison to the other hydrides of their groups (restricted to Van der Waals, which increases with the number of electrons in a molecule).

The greater the intermolecular forces, the more energy is required to overcome the bonds for the molecules to become liquids or gases. There is no hydrogen bonding in CO2.
This has nothing to do with molecular mass.

Answer 4

H2O is a highly polar molecule and CO2 is not. Therefore H2O experiences hydrogen bonding, which makes the molecules of water harder to seperate from one another in order to go from a liquid to a gas. The molecules must absorb more energy in the form of heat to do so, which is why the boiling point of water is much higher.

Answer 5

Water is liquid is not explained on the basis of molecular mass ,but due to hydrogen bonding.H atom when attached to N,O or F atom is capable of forming H bonds.

Answer 6

you can't just base a state of matter on valancy (ability to combine) or atomic/molecular mass. nonetheless, in everyday terms, because (as you rightly pointed out) co2 has a greater mass this means that the atoms are harder to push together (they need much greater pressure to liquify). On the other hand the dainty little hydrogens in h20 go together under atmopheric pressure.

here's a strange one for your chemistry teacher...measure out exactly 50ml of water and 50ml of ethanol then pour them into a 100ml measuring cylinder....see what it comes to????

Answer 7

In its most simplest form, the bonds between the carbon, oxygen and hydrogen atoms differ in strength, and it takes energy from heat, say, to alter that attachment and to produce water vapour, or a reduction of heat to produce liquid carbon dioxide.

Answer 8

They are only in this state at certain temperatures. It is quite easy to change their states by freezing / boiling them.

Answer 9

Dissociation. Water will split into H30+ and O- and bond to adjacent molecules. CO2 doesn't do this.

Answer 10

Because the boiling point and freezing point of the two are different. Nothing so special about it.