Everything around us is made up of matter and everything can be either in a solid, liquid, or gas state. We know that anything that occupies space and has mass is matter, but the matter that has a specific composition and chemical characteristics is called a substance.
In this lesson, we will learn about SUBSTANCES. We will discuss:
A substance is a matter which has a specific composition and specific properties. Substances are made up of atoms and molecules.
Let us understand this.
Some references add, that chemical substances cannot be separated into their constituent elements by physical separation methods, or without breaking their chemical bonds.
All substances can be either pure or impure.
Chemists define the term 'pure' as a chemical that is natural. This is a substance that is made up of one type of atom. The purity of a substance means the substance cannot be broken down any further without losing its unique physical and chemical properties. Pure substances are further classified as elements and compounds.
The best examples of pure substances are pure elements and compounds:
Characteristics of pure substances
A substance is impure if it consists of different kinds of elements combined physically and not chemically.
Or, if in particular substances are also present in smaller or larger amounts other substances, the new substances are called impure substances. Impure substances are mixtures. In mixtures, the substances present are not chemically bonded together.
Mixtures can be classified into two main categories: homogeneous and heterogeneous. A homogeneous mixture is one in which the composition of its constituents is uniformly mixed throughout. Examples are air, saline solution, most alloys.
A heterogeneous mixture is a nonuniform mixture in which the components separate and the composition varies. Examples are a mixture of sand and water or sand and iron filings, a conglomerate rock, water and oil, a salad, etc.
Characteristics of impure substances
Let's understand what is the difference between substances and mixtures.
To understand this, we can compare baking soda (as a pure substance) and a mixture of salt and water (as an impure substance).
Baking soda has the chemical name sodium hydrogen carbonate. It is a chemical compound having structural formula ( NaHCO3 ) that contains the same composition of constituent particles which are consistent. It is composed of Carbon, Sodium, Hydrogen, and Oxygen molecules. Hence, it is a pure substance.
We can get salt water when we add salt into the water. Ordinary table salt is called sodium chloride and consists of two elements, Sodium (Na) and Chloride (Cl), so the chemical formula is NaCl. Salt is considered a pure substance because it has a uniform and definite composition. Water, H2O, is a pure substance, a compound made of Hydrogen and Oxygen.
Salt easily dissolves in water, but this mixture cannot be classified as a substance because its composition can vary. You may dissolve a small amount of salt or a large amount into a given amount of water. Hence, it is an impure substance or mixture.
