Question

Is water an allotrope?

Answer 1

Allotropy is the property of any element to be able to exist in more than one form. Allotropes are modified forms of the same element with different structures. Allotropes of any element vary in their physical structural properties.

Complete answer: As the definition of allotropes suggest, allotropes are different chemical forms having structural modifications of the same element. This tells us that allotropy is the characteristic property of elements, and not compounds.
We have been asked, if water is an allotrope or not. Since water is a compound containing two hydrogen atoms, and one oxygen atom, therefore it is a chemical compound. Allotropes cannot exist in any compound form, rather they are elemental forms. So, water cannot be termed as an allotrope.
Now, the confusion arises, with different forms of ${{H}_{2}}O$, which are ice, liquid, and vapor. These forms of ${{H}_{2}}O$ are its physical modifications, rather than chemical forms and contain the same crystal structure as that of ${{H}_{2}}O$, which is $H-O-H$, so these forms of ${{H}_{2}}O$, cannot be termed as its allotropes as allotropes are chemical modifications of an element.
Hence, water cannot have allotropes as water is a compound and has different physical forms (ice, vapor, liquid) rather than chemical forms.

Additional information: Allotropes of carbon are well known, which are diamond, graphite, coke, fullerene, etc. They all have various structural modifications.

Note: The fact that ${{H}_{2}}O$ can have various physical forms, like, liquid, vapor, and ice, is the property of polymorphism that can have a compound to exist in various forms. ${{H}_{2}}O$ can exist in 3 states, solid, liquid, and gas. All these 3 forms are interrelated and can be achieved with varying the temperature of water.