Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. Water vapor accounts for roughly 0.25% of the atmosphere by mass.
Physical property, such as real estate, does not qualify due to the unpredictable length of the sales process. As retirement accounts, 401(k)s and IRAs operate both as liquid assets and non-liquid assets depending on the circumstance.
A non-liquid asset is any asset that requires a significant amount of time and effort to convert into cash. Real estate is an example of a non-liquid asset, as are almost all types of business inventory.