Ice

One of the most abundant materials in an inhabited solar system, ice is often mined and transported for a variety of reasons.

Sources

The most obvious source of ice is from a polar cap on a habitable planet which has one (either seasonal or permanent). While this is a handy source for people living on that world, it is not so convenient for space-dwellers, who must overcome what is in most cases a substantial gravity well to transport the ice into space. Instead, they will usually source their ice from comets or moons in outer systems, or in some cases deposits can be found in permanently-shadowed craters on otherwise-waterless bodies. The biggest sources of ice will exist outside of a system's frost line, the point at which solar radiation is too weak to melt it even if directly exposed to sunlight.

Use

Aside from its role in sustaining human life and Terran ecology, ice is handy for many scientific and engineering applications as well. Its high specific heat capacity makes it an easy sink for heat-producing devices such as shipboard laser weapons, and its density makes it an excellent radiation shield when structure mass is not a concern (for instance, ground-based solar flare shelters). Ice is also commonly mined to be melted into water which is then used as reaction mass in thermal rockets. While a water-using rocket will not have as high a delta-v compared to one using remass with a better specific impulse (such as hydrogen), water in the form of ice is so plentiful that refueling can be had on almost any location visited, often for free provided one brings along the right equipment.
Ice comets are also a critical feedstock for terraforming, being redirected at planets to build up oceans and water-vapor atmospheres which can be processed into oxygen.

Boiling / Condensation Point

100 Celsius

Melting / Freezing Point

0 Celsius