CBSE Science Notes Class 9 | Is Matter Around Us Pure?

📝 Is Matter Around Us Pure?

Classification of Matter

Are these consumable items pure? 

Pure Substance

➤ A pure substance consists of a single type of particle, which means that all the constituent particles of that substance are the same in their chemical nature.

It cannot be separated by physical means, for example, sodium chloride is itself a pure substance and cannot be separated by physical process into its chemical constituents.

Mixture

➤ Mixtures are constituted by more than one kind of pure form of matter.

It can be separated by physical means, for example, sodium chloride can be separated from water by the physical process of evaporation.

Types of Mixtures 

1. Homogenous mixture
2. Heterogeneous mixture

1. Homogenous mixture

In a homogeneous mixture, the components completely mix with each other. This means particles of components of the mixture are uniformly distributed throughout the bulk of the mixture and its composition is uniform throughout. Sugar solution and air are examples of homogeneous mixtures.

2. Heterogeneous mixture

In a heterogeneous mixture, the composition is not uniform throughout and sometimes different components are visible. For example, mixtures of salt and sugar, grains and pulses along with some dirt (often stone pieces), are heterogeneous mixtures.

Pure substances can further be classified into 

1. Elements 
2. Compounds.

1. Elements

Particles of an element consist of only one type of atom. These particles may exist as atoms or molecules.  Some elements, such as sodium or copper, contain atoms as their constituent particles, whereas, in some others, the constituent particles are molecules that are formed by two or more atoms.

A representation of atoms and molecules

2. Compounds

When two or more atoms of different elements combine together in a definite ratio, the molecule of a compound is obtained.

A depiction of molecules of water and carbon dioxide  

Differentiate between mixtures and compounds

Solution 

A solution is a homogeneous mixture of two or more substances. Lemonade, soda water, etc. are all examples of solutions.

 Lemonade

In a solution, there is homogeneity at the particle level. For example, lemonade tastes the same throughout. This shows that particles are evenly distributed in the solution.

Alloys: Alloys are mixtures of two or more metals or a metal and a non-metal and cannot be separated into their components by physical methods. But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition. For example, brass is a mixture of approximately 30% zinc and 70% copper.

Solvent 

The component of the solution that dissolves the other component in it (usually the component present in a larger amount) is called the solvent. 

Solute 

The component of the solution that is dissolved in the solvent (usually present in lesser quantities) is called the solute.

Examples: 

◆ A solution of sugar in water is solid in a 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 etc., 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

(1) A solution is a homogeneous mixture.

(2) The particles of a solution are smaller than 1 nm (`10^-9` metre) in diameter. So, they cannot be seen with naked eyes.

(3) 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.

(4) 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.

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.

Solubility

The amount of the solute present in the saturated solution at this temperature is called its solubility.

Unsaturated solution.

If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.

The concentration of a solution

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

There are various ways of expressing the concentration of a solution, but here we will learn only three methods.

￭ Mass by mass percentage of a solution.

￭ Mass by volume percentage of a solution.

￭ Volume by volume percentage of a solution

Mass by mass percentage of a solution

 Mass by volume percentage of a solution

Volume by volume percentage of a solution

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

(1) Suspension is a heterogeneous mixture.

(2) The particles of a suspension can be seen by the naked eye.

(3) The particles of a suspension scatter a beam of light passing through it and make its path visible.

(4) 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 anymore.

Colloidal Solution 

A colloidal is a kind of solution in which the size of the solute particles is intermediate between those in true solutions and those in suspensions. Examples: soap solution, milk, ink, blood, etc. 

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 as observed in activity

(a) Solution of copper sulphate does not show Tyndall effect, (b) mixture of water and milk shows Tyndall effect.

Tyndall Effect 

The scattering of a beam of light by colloidal particles is known as Tyndall effect.

Tyndall effect can 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 

Tyndall effect can also 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 the air.

Properties of a colloid

(1) A colloid is a heterogeneous mixture.

(2) The size of particles of a colloid is too small to be individually seen by naked eyes.

(3) Colloids are big enough to scatter a beam of light passing through it and make its path visible.

(4)They do not settle down when left undisturbed, that is, a colloid is quite stable