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.
Roth IRAs can be invested in literally anything you want: index funds, lifecycle funds, individual stocks, or even alternative investments. In contrast, there are some limits to the types of assets a Traditional IRA can hold.