Final answer:
Water's unique properties, like needing a large amount of energy to change temperature and ice being less dense than liquid, are crucial for it to stay in a liquid state across various climates, including not freezing at the poles or boiling in the tropics.
Step-by-step explanation:
Water is not frozen at the poles and boiling in the tropics primarily due to its unique physical properties. Water requires an unusually large amount of energy to change temperature, which means it changes temperature slowly as heat is added or removed. This is why, even though the poles are cold, oceans and seas do not instantly freeze solid, and similarly, tropical waters do not reach boiling temperatures despite the intense heat. Moreover, ice is less dense than liquid water, which causes ice to float, forming a surface layer that insulates the water below, allowing marine life to survive beneath. This layer prevents bodies of water from freezing solid from top to bottom, which would be lethal for aquatic life.
On Earth, these properties create a balanced environment in which water can exist in its liquid state over a broad range of temperatures. In contrast, Mars lacks the pressure and temperature conditions for stable liquid water on its surface; low pressures mean that water would either freeze or boil away. Salts can lower the freezing point of water, but the general conditions on Mars are not favorable for liquid water to persist on the surface, unlike on Earth where salt is used to lower the freezing point on icy roads.