That is, in this chapter you will learn that water has stronger intermolecular forces than carbon dioxide, and that is why it is a liquid, while carbon dioxide is a gas. So why at ambient conditions is Carbon Dioxide (the heavier molecule at 44 amu) a gas, but water (the lighter molecule at 18 amu) is a liquid? Answer That is, shouldn't it be easier to vaporize the lighter molecule than the heavier one? The answer lies within the Intermolecular Forces, and if you review molecular shapes and polarity you will see that water is a polar molecule while carbon dioxide is non-polar, and this leads to different physical properties like the boiling and melting points. The problem is, there are often other factors, as indicated in the next question, on why is carbon dioxide a gas at ambient questions while water is a liquid? Note: This image is for the velocity distribution of a gas at a given temperature, and not a liquid, but the concept is valid, in that if all other things are equal, lighter molecules tend to move faster, and gas phase systems move faster than liquid.

Look at the case of carbon dioxide and water (the next question deals with this). The problem is we need to identify the intermolecular forces that attract the molecules to each other, and if the lighter molecule had stronger intermolecular forces, this would be wrong. From our study of gases and the Kinetic Molecular Theory we learned that the average kinetic energy of a molecular system is proportional to the absolute temperature, and both molecules have the same average kinetic energy, and so at first thought, we would predict that the lighter molecule would tend to have the higher velocity (review Graham's Law of Effusion) and be easier to vaporize (see image below).

Properties of Matter that Depend on IMFsĪt a given temperature, would it be easier to vaporize a light molecule or a heavy molecule? Answer.

Shortcomings of the term Intermolecular.

What are the states of matter