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.
Mortgages are sometimes considered a liquid asset, but they are much less liquid than many other types. Real estate is also more likely to sold at less than its value if it must be liquidated quickly; if the market is unstable, it may be difficult to determine the true value of real estate as well.