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.
An example of a non-liquid asset is a real estate investment because it can take months for a person or company to receive cash from the sale. For example, suppose a company owns real estate property and wants to liquidate because it has to pay off a debt obligation within a month.