Impure Substances - Mixtures
Impure substances are commonly called mixtures. A mixture is a material containing two or more elements or compounds that are in close contact and are mixed in any proportion. The constituents of mixtures exhibit their individual properties. These constituents can be separated by physical means. Example: air, gunpowder, etc.
Mixtures can be homogeneous or heterogeneous. A homogeneous mixture has a uniform composition through out its mass. For example, sugar or salt dissolved in water, alcohol in water, etc. While in a heterogeneous mixture the composition is not uniform throughout its mass. Different portions of a heterogeneous mixture show different properties. There are visible sharp boundaries. Example: Oil and water, salt and sand, etc.
Experiment
Take a beaker of water. Add a pinch of salt (sodium chloride) and stir. The water dissolves the salt and the salt spreads out throughout the water. Two samples of this solution taken from two different parts of the beaker will have the same composition. So, the solution is said to be homogeneous. Generally, all solutions tend to be homogeneous. In a solution the solute particles divide and subdivide and finally achieve the form of molecules or ions and spread uniformly throughout the solution. These particles (ions and molecules) become so small that they cannot be seen even through a microscope. Similarly, when sugar is dissolved in water, the sugar molecules distribute over the entire bulk of water uniformly. A sugar solution has same sweetness weather we taste from the top or bottom layers.In the case of other mixtures like that of salt and sand, no matter how well it is mixed, separate particles of salt and sand can always be seen. Their composition is also not the same throughout. These are called heterogeneous mixtures.
We can now summarize the properties of mixtures as follows:- A mixture may be homogenous or heterogeneous.
- The constituents of a mixture can be separated by physical means like filtration, evaporation, sublimation and magnetic separation.
- In the preparation of a mixture, energy is neither evolved nor absorbed.
- A mixture has no definite melting and boiling point.
- The constituents of a mixture retain their original set of properties. For example, magnet attracts iron filings in a mixture of sand and iron powder.
Types of Mixtures
| Matter | MixtureType | Example |
|---|---|---|
| Solid | Solid mixture | Iron filings and sulphur |
| Solid | Liquid mixture | Common salt and water |
| Solid | Gas mixture | Air entrapped in soil |
| Liquid | Gas mixture | Oxygen dissolved in water |
| Gas | Gas mixture | Air containing hydrogen, oxygen, nitrogen, carbon dioxide etc. |
| Liquid | Liquid mixture | Water and alcohol |
Differences Between Mixtures and Compounds
| Mixtures | Compounds |
| A mixture c b spdrated into its constituents by physical processes (filtration, evaporation, sublimation, distillation) | A compound cannot be separated into its constituents by physical processes. It can be separated by chemical means. |
| A mixture shows the properties of its constituents | A compound has a new set of properties different from its constituents |
| Composition of a mixture varies and the constituents are present in any proportion by weight. ft does not have a definite formula. | The composition of a compound is fixed and the constituents are present in fixed proportions by weight. It has a definite formula. |
| The constituents do not react chemically, thus no energy changes takes place | Chemical reactions take place and energy changes in the form of heat and light are involved |
| A mixture does not have a fixed melting point and boiling point Examples: air, sand and salt | A compound has a fixed melting point and boiling point Examples: 1.120 (water), FeS (iron sulphide) |
Remember :
- The components hydrogen and oxygen cannot be separated by physical methods such as filtration or evaporation.
- Hydrogen and oxygen are present in a fixed proportion of 1: 8 by weight
- Energy changes accompany the formation of this compound i.e., heat and light are given out.
- Properties of water are entirely different from the constituents, hydrogen and oxygen.
- The boiling point of water is 100oC at 76 cm of Hg i.e., one atmospheric pressure.
Types of Mixtures
Mixtures can be classified based on the composition and can be broadly divided into three groups, depending on whether the constituents are elements or compounds or both.- Element with an Element (forming alloys)a) Oxygen and nitrogenb) Sodium and mercury (amalgam)c) Copper and zinc alloy.
- Compound with a Compounda) Water and saltb) Water and alcoholc) Salt and sugar.
- Element with a Compounda) Oxygen and water (air dissolved in water)b) Oxygen, nitrogen, carbon dioxide and water vapour (air).
Mixtures of Solids, Liquids and Gases
| Solid | Liquid | Gas | |
|---|---|---|---|
| Solid | salt and sugar | salt and water | Dust in air |
| Liquid | Mercury and copper | Alchol and water | clouds |
| Gas | Hydrogen and palladium | Oxygen and water | Air |
Characteristics of Compounds and Mixtures
| Characteristics | Compounds | Mixtures |
|---|---|---|
| Composition | Made up of atoms of elements in a fixed proportion | Made up of elements, or compounds, or both in any proportion |
| Nature | Particles are of the same kind | Particles are of different kinds |
| Structure | Always homogeneous | May or may not be homogeneous |
| Separation | Components can be separated only by chemical means | Components can be separated by physical means |
| Energy changes | Energy is always evolved or absorbed | Generally no energy is evolved or absorbed |
| Appearance | Components cannot be seen separately | Components may or may not be seen separately |
| Preparation | Always involves a chemical change | Involves only physical change |
| Properties | Entirely different from those of the constituents | No property of their own. Show the average properties of all the constituents |
