Question

Pure substances are characterised by:
A) Variable melting points
B) Non-uniform composition
C) Variable properties
D) Fixed boiling points

Answer 1

Hint: To start the characteristics of pure substances first we need to understand the definition of pure substance. a substance is a pure substance only and only if it consists of one type of atoms or molecules throughout having constant composition and structure.

Complete answer:
Pure substances are made up of one type of atoms or molecules. Pure substances can be further divided into elements and compounds.
Elements are simplest substances which cannot be broken down further. Elements do not undergo transformation, from one element into another. They can further be classified as metals, nonmetals and metalloids. Examples: Sodium, Magnesium, Iron, Oxygen gas, Nitrogen gas, Silicon etc.
Compounds are pure substances made up of more than one type of element, combined in a fixed ratio by mass. They can be divided into its constituent elements by chemical means. Examples can be salts like NaCl, gases like Carbon dioxide, Methane, molecules with more than one type of atoms like glucose.
Let us start with characteristics of pure substances.
> They are made up of one kind of atoms or molecules.
> Composition is uniform throughout the substance.
> Have the same chemical and physical properties in the whole composition.
> Physical properties like melting point, boiling point, density are fixed.
> Chemical properties include reaction with oxygen, water and other substances. even they are fixed for every pure substance.
From the above characteristics of pure Substances, we can say that option (A), option (B) and option (C) are incorrect as pure substances have fixed melting points, composition is uniform throughout, and properties are the same for the same pure substance.

Thus, the correct option is (D) Fixed boiling points.

Note: Compounds might be sometimes confused as mixture, as it contains more than one type of atoms or elements. But we should know that they combine in fixed ratio by mass and are uniform in composition, physical and chemical properties throughout the substance. So they are pure substances.