Classify the given substance as elements, compunds and mixtures.

Mercury, sulphur, gold, coal, and oxygen are [[1]].
Sugar, water, and alcohol are [[2]].
Coal and sand are [[3]].

Oxygen (O), mercury (Hg), sulphur (S), and gold (Au) are Elements and Sugar $\left({\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\right)$, water $\left({\mathrm{H}}_2\mathrm{O}\right)$, and alcohol $(\mathrm{for} \mathrm{ex}. {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH})$ are Compounds where as sand and coal are examples of mixture. 

A pure substance with only one type of particle is called an element. Pure substances with multiple types of atoms or elements are called compounds. A mixture is a substance that comprises two or more elements or compounds in any quantity.

An element was previously described as the purest version of a substance with a distinct physical or chemical feature that cannot be divided into or constructed from simpler substances by any physical or chemical process. A pure substance with only one type of particle is an element, according to the updated definition. Atoms are these subatomic units.

A compound is a pure material that has two or more elements mixed in a fixed mass proportion and that can be broken down into its component parts using the proper physical or chemical processes. A compound's characteristics are entirely distinct from those of the constituent parts.

A mixture is a substance that comprises two or more elements or compounds in any quantity. A mixture's characteristics mirror those of its separate parts.

Sugar is composed of the elements carbon, hydrogen, and oxygen. Hydrogen and oxygen are the two elements that makeup water, these two are hence compounds.  There are mixtures because materials like sand and coal are composed of several compounds.

Hence, the correct answers are elements, compounds and mixtures respectively.