Is Matter Around Us Pure?

Class 09 Science

Mixtures are constituted by more than one kind of pure form of matter. Dissolved sodium chloride can be separated from water by the physical process of evaporation. However, sodium chloride is itself a pure substance and cannot be separated by physical process into its chemical constituents.

Types of Mixtures

Depending upon the nature of the components that form a mixture, we can have different types of mixtures.

Mixture which has a uniform composition throughout are called homogeneous mixtures or solutions. For example, (i) salt dissolved in water and (ii) sugar dissolved in water.

Mixtures which contain physically distinct parts and have non-uniform compositions are called heterogeneous mixtures. Mixtures of sodium chloride and iron filings, salt and sulphur, and oil and water are examples of heterogeneous mixtures.

Solution

A solution is a homogeneous mixture of two or more substances. A solution has a solvent and a solute as its components. The component of the solution that dissolves the other component in it (usually the component present in larger amount) is called the solvent. The component of the solution that is dissolved in the solvent (usually present in lesser quantity) is called the solute.

Examples

A solution of sugar in water is a solid in liquid solution. In this solution, sugar is the solute and water is the solvent.
A solution of iodine in alcohol known as ‘tincture of iodine’, has iodine (solid) as the solute and alcohol (liquid) as the solvent.
Aerated drinks like soda water are gas in liquid solutions. These contain carbon dioxide (gas) as solute and water (liquid) as solvent.
Air is a mixture of gas in gas. Air is a homogeneous mixture of a number of gases. Its two main constituents are: oxygen (21%) and nitrogen (78%). The other gases are present in very small quantities.

Properties of a Solution

A solution is a homogeneous mixture.
The particles of a solution are smaller than 1 nm (10^-9 metre) in diameter. So, they cannot be seen by naked eyes. Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution.
The solute particles cannot be separated from the mixture by the process of filtration. The solute particles do not settle down when left undisturbed, that is, a solution is stable.

Concentration of Solution

In a solution the relative proportion of the solute and solvent can be varied. Depending upon the amount of solute present in a solution, it can be called dilute, concentrated or saturated solution.

At any particular temperature, a solution that has dissolved as much solute as it is capable of dissolving, is said to be a saturated solution. When no more solute can be dissolved in a solution at a given temperature, it is called a saturated solution. The amount of the solute present in the saturated solution at this temperature is called its solubility.

If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution. Different substances in a given solvent have different solubilities at the same temperature.

The concentration of a solution is the amount (mass or volume) of solute present in a given amount (mass or volume) of solution.

(i) Mass by mass percentage of a solution

$$ = \left( \frac{\text{Mass of Solute}}{\text{Mass of Solution}} \right) \times 100 $$

(ii) Mass by volume percentage of a solution

$$ = \left( \frac{\text{Mass of Solute}}{\text{Volume of Solution}} \right) \times 100 $$

(iii) Volume by volume percentage of a solution

$$ = \left( \frac{\text{Volume of Solute}}{\text{Volume of Solution}} \right) \times 100 $$

Example: A solution contains 40 g of common salt in 320 g of water. Calculate the concentration in terms of mass by mass percentage of the solution.

Mass of solute (salt) = 40 g

Mass of solvent (water) = 320 g

Mass of solution = Mass of solute + Mass of solvent

= 40 g + 320 g = 360 g

$$ \text{Mass percent of solute} = \left( \frac{40}{360} \right) \times 100 = 11.11\% $$

Suspension

A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of a suspension are visible to the naked eye.

Properties of a Suspension

Suspension is a heterogeneous mixture.
The particles of a suspension can be seen by the naked eye.
The particles of a suspension scatter a beam of light passing through it and make its path visible.
The solute particles settle down when a suspension is left undisturbed, that is, a suspension is unstable. They can be separated from the mixture by the process of filtration. When the particles settle down, the suspension breaks and it does not scatter light any more.

Colloidal Solution

The particles of a colloid are uniformly spread throughout the solution. Due to the relatively smaller size of particles, as compared to that of a suspension, the mixture appears to be homogeneous. But actually, a colloidal solution is a heterogeneous mixture, for example, milk.

Because of the small size of colloidal particles, we cannot see them with naked eyes. But, these particles can easily scatter a beam of visible light. This scattering of a beam of light is called the Tyndall effect. Tyndall effect can also be observed when a fine beam of light enters a room through a small hole. This happens due to the scattering of light by the particles of dust and smoke in the air. Tyndall effect can be observed when sunlight passes through the canopy of a dense forest. In the forest, mist contains tiny droplets of water, which act as particles of colloid dispersed in air.

Properties of a Colloid

A colloid is a heterogeneous mixture.
The size of particles of a colloid is too small to be individually seen with naked eyes.
Colloids are big enough to scatter a beam of light passing through it and make its path visible.
They do not settle down when left undisturbed, that is, a colloid is quite stable.
They cannot be separated from the mixture by the process of filtration. But, a special technique of separation known as centrifugation can be used to separate the colloidal particles.

The components of a colloidal solution are the dispersed phase and the dispersion medium. The solute-like component or the dispersed particles in a colloid form the dispersed phase, and the component in which the dispersed phase is suspended is known as the dispersing medium.

Colloids are classified according to the state (solid, liquid or gas) of the dispersing medium and the dispersed phase.

Common Examples of Colloids

Liquid in Gas: Aerosol (Fog, clouds, mist)
Solid in Gas: Aerosol (Smoke, automobile exhaust)
Gas in Liquid: Foam (Shaving cream)
Liquid in Liquid: Emulsion (Milk, face cream)
Solid in Liquid: Sol (Milk of magnesia, mud)
Gas in Solid: Foam (Foam, rubber, sponge, pumice)
Liquid in Solid: Gel (Jelly, cheese, butter)
Solid in Solid: Solid Sol (Coloured gemstone, milky glass)

Physical and Chemical Changes

The properties that can be observed and specified like colour, hardness, rigidity, fluidity, density, melting point, boiling point etc. are the physical properties.

The inter-conversion of states is a physical change because these changes occur without a change in composition and no change in the chemical nature of the substance. Although ice, water and water vapour all look different and display different physical properties, they are chemically the same.

Chemical change brings change in the chemical properties of matter and we get new substances. A chemical change is also called a chemical reaction.

Elements

On the basis of their chemical composition, substances can be classified either as elements or compounds.

An element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions. Elements can be normally divided into metals, non-metals and metalloids.

Metals

They have a lustre (shine).
They have silvery-grey or golden-yellow colour.
They conduct heat and electricity.
They are ductile (can be drawn into wires).
They are malleable (can be hammered into thin sheets).
They are sonorous (make a ringing sound when hit).

Examples of metals are gold, silver, copper, iron, sodium, potassium etc. Mercury is the only metal that is liquid at room temperature.

Non-metals

They display a variety of colours.
They are poor conductors of heat and electricity.
They are not lustrous, sonorous or malleable.

Examples of non-metals are hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine etc.

Some elements have intermediate properties between those of metals and non-metals, they are called metalloids. For example: boron, silicon, germanium, etc.

Compounds

A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.

A compound, which has totally different properties compared to the combining elements. The composition of a compound is the same throughout.