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.
Liquid assets are accounts or securities that can be easily converted to cash at little or no loss of value. These include cash, money in bank accounts, money market mutual funds, and US Treasury bills. Actively traded stocks, bonds, and mutual funds are liquid in the sense that they are easy to sell, but the price is not guaranteed and could be less than the amount you paid to buy the asset. In contrast, selling fixed assets, such as real estate, usually requires time and negotiation.