Final answer:
Cement is capable of setting and hardening when it reacts with water, a process known as hydration. It does not dissolve in water nor does it become soft when wet. The key chemical reaction for creating cement is the highly exothermic reaction of quicklime with water to produce slaked lime.
Step-by-step explanation:
Cement is capable of a few distinct properties that make it vital for construction purposes. The characteristic most associated with cement is its ability to set and harden by a reaction with water. This process, known as hydration, involves the formation of products like calcium hydroxide from the reaction of compounds like calcium oxide with water. This transformation—which includes releasing heat—results in a hard, stone-like material that binds sand and gravel to form concrete.
When it comes to dissolving in water, cement does not perform this way. It does not become soft when exposed to water, on the contrary, water facilitates the setting and hardening process. Finally, while it reacts with carbon dioxide in the air to some extent, it doesn't form a solid mass through the reaction with air alone but rather through the chemical reaction with water. Therefore, option 1 is correct: Cement sets and hardens by reacting with water, which makes it the preferred choice in the construction of buildings and other structures.
The process of mixing quicklime, CaO, with water to produce slaked lime, Ca(OH)2, is a key reaction in the creation of cement and is highly exothermic. This exothermic reaction signifies a transfer of energy, which is one indication of chemical reactions. Also, the laws of conservation of mass apply to the cement-making process where atoms in the reactants recombine to form the products without any creation or destruction of atoms.