What is defined as an alloy?

An alloy is defined as a combination of two or more elements that results in improved properties compared to the individual components. This means that by mixing different metals or a metal with non-metals, engineers can create materials with enhanced characteristics such as increased strength, corrosion resistance, or improved ductility. For example, steel is an alloy made primarily of iron and carbon, which gives it significantly improved strength and durability compared to pure iron.

The other choices do not accurately represent the definition of an alloy. A single element in a solid state refers to a pure substance rather than a mixture, while a metal in its liquid form is simply a phase of matter and does not imply any combination of elements. A process used to harden metals refers to techniques like heat treatment or alloying processes but does not define what an alloy is. Thus, the correct choice captures the essence of what an alloy is and its purpose in engineering and materials science.