If the melting temperature of hydrogen is -259°C and the melting temperature of oxygen is -218°C then why is the melting temperature of water (H2O) 0°C?


Shouldn’t it be around -245°C?

In: Chemistry

To extrapolate using your logic: if hydrogen is highly flammable, and oxygen is required for the flame, shouldn’t water be one of the most flammable substances known? 😉

You can’t just average the melting point like that. There are many different things that influence melting point, such as the composition of whatever you’re melting (the closer you pack atoms together, the higher the melting point will be), and how the atoms are bonded together. That’s why even a relatively small change can change the melting point drastically.

This may be explaining it a bit too simply, so I invite anyone to give a better detailed answer. But the reason is because you aren’t averaging out the properties of the two chemicals. You are adding the two to make an entirely new substance with its own properties.

Because water molecules are polar, they tend to attract each other and easily form a liquid. Therefore you have to introduce a lot more energy (heat) to get them to separate into a gas.

Hydrogen (H2) and Oxygen (O2) are very small and symmetrical molecules. Two hydrogen mopecules aren’t attracted to each other very much. The more attraction, the higher the melting point.

For attraction to happen you need different charges. The hydrogen molecule itself has no charge. The cores of the atoms are positively charged, the hull is negatively charged. The exact distribution of the charge within the hull varies slightly, so one side might be a bit more negative than the other. This little bit of variance creates a little bit of attraction hence the melting point being -259 and not e.g. -270. Oxygen molecules are a bit bigger, so there is more room for variation, so it’s melting point is a bit higher.

Water (H2O) is not symmetrical. The oxygen atom has a very positively charged core, much more so than either of the hydrogen atoms. So the negative charge in the hull (the electrons they share to form the bond) gathers closer to the oxygen atom rather than the hydrogen cores. The two hydrogen atoms don’t attach opposite of another but at an angle of ~120°. So the side of the molecule where the apex of that angle is (the side of the oxygen atom) is significantly more negative than the side with the hydrogen atoms.

This difference makes the negative oxygen side attract to the positive hydrogen side of the next molecule over. And vice versa. Big attraction means high melting point.

Well, the melting point is set by the molecules of the substance. The bigger a molecule is, the higher the melting point, and the smaller the molecule is, the lower the melting point. The gas hydrogen (H^2) is very small, so its melting point is very low. It is actually the second lowest melting point in the universe! Helium has the lowest melting point of all. Likewise, oxygen molecules (O^2) contain only two atoms, but water molecules (H^2 O) contain three atoms, meaning its melting point is a bit higher.

Melting point also is changed by polarity. All hydrogen molecules are positively charged and repel each other, just like all oxygen molecules are negatively charged and repel each other. This means they want to break apart, and so the melting point of pure hydrogen and pure oxygen is very low. Water is different, because one end of each molecule is positively charged, and the other end is negatively charged. This means that water molecules attract each other. This leads to a lot of very unique properties in water, like how it forms droplets instead of spreading out. It also means that its solid form, ice, is very stable, and so its melting point is much higher than hydrogen or oxygen.