What is a Mixture?

Mixtures are constituted by more than one kind of pure form of matter, known as a substance. A substance cannot be separated into other kinds of matter by any physical process. We know that dissolved sodium chloride can be separated from water by the physical process of evaporation. However, sodium chloride is itself a substance and cannot be separated by physical process into its chemical constituents. Similarly, sugar is a substance because it contains only one kind of pure matter and its composition is the same throughout. Soft drink and soil are not single substances. Whatever the source of a substance may be, it will always have the same characteristic properties.Therefore, we can say that a mixture contains more than one substance.

 

Types of Mixtures

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

Activity : 1

 Let us divide the class into groups A, B, C and D.

• Group A takes a beaker containing 50 mL of water and one spatula full of copper sulphate powder.

• Group B takes 50 mL of water and two spatula full of copper sulphate powder in a beaker.

• Groups C and D can take different amounts of copper sulphate and potassium permanganate or common salt (sodiumchloride) and mix the given components to form a mixture.

• Report the observations on the uniformity in colour and texture.

Observations:

• Groups A and B have obtained a mixture which has a uniform composition throughout. Such mixtures are called homogeneous mixtures or solutions. Some other examples of such mixtures are: (i) salt in water and (ii) sugar in water. Compare the colour of the solutions of the two groups. Though both the groups have obtained copper sulphate solution , the intensity of colour of the solutions is different. This shows that a homogeneous mixture can have a variable composition.

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

 Activity: 2

Let us again divide the class into four groups – A, B, C and D.
Distribute the following samples to each group:

•  Few crystals of copper sulphate to group A.
•  One spatula full of copper sulphate to group B.
•  Chalk powder or wheat flour to group C.
•  Few drops of milk or ink to group D.

Each group should add the given sample in water and stir properly using a glass rod. Are the particles in the mixture visible?
Direct a beam of light from a torch through the beaker containing the mixture and observe from the front. Was the path of the beam of light visible?

Leave the mixtures undisturbed for a few minutes (and set up the filtration apparatus in the meantime). Is the mixture stable or do the particles begin to settle after some time? Filter the mixture. Is there any residue on the filter paper?

Inference:

• Groups A and B have got a solution.
• Group C has got a suspension.
• Group D has got a colloidal solution.

 

Now, we shall learn about solutions, suspensions and colloidal solutions....

What is a Solution?

A solution is a homogeneous mixture of two or more substances. You come across various types of solutions in your daily life. Lemonade, soda water etc. are all examples of solutions. Usually we think of a solution as a liquid that contains either a solid, liquid or a gas dissolved in it. But, we can also have solid solutions (alloys) and gaseous solutions (air). In a solution there is homogeneity at the particle level. For example, lemonade tastes the same throughout. This shows that particles of sugar or salt are evenly distributed in the solution.

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 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 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 a Solution

In activity 2, we observed that groups A and B obtained different shades of solutions. So, we understand that 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 a dilute, concentrated or a saturated solution. Dilute and concentrated are comparative terms. In activity 2, the solution obtained by group A is dilute as compared to that obtained by group B.

Activity: 3

 

• Take approximately 50 mL of water each in two separate beakers.
• Add salt in one beaker and sugar or barium chloride in the second beaker with continuous stirring.
• When no more solute can be dissolved, heat the contents of the beaker.
• Start adding the solute again.

Is the amount of salt and sugar or barium chloride, that can be dissolved in water at a given temperature, the same?
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. In other words, 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.

What would happen if you were to take a saturated solution at a certain temperature and cool it slowly?

We can infer from the above activity that different substances in a given solvent have different solubilities at the same temperature.

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

Concentration of solution =Amount of solute/Amount of solution

or

Amount of sloute/Amount of solvent

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

(i) Mass by mass percentage of a solution

= (Mass of solute/Mass of solution) x 100

(ii) Mass by volume percentage of a solution

= (Mass of solute/Mass of solution) x 100

 

 What is a Suspension?

Non-homogeneous systems, like those obtained by group C in activity 2, in which solids are dispersed in liquids, are called suspensions. 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.

 

What is a Colloidal Solution?

The mixture obtained by group D in activity 2 is called a colloid or a 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 as observed in activity 2. This scattering of a beam of light is called the Tyndall effect after the name of the scientist who discovered this 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.

Properties of a Colloid

• A colloid is a heterogeneous mixture.
• The size of particles of a colloid is too small to be individually seen by 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.

 

What you have learnt ?

 

• A mixture contains more than one substance (element and/or compound) mixed in any proportion.

• A solution is a homogeneous mixture of two or more substances.

• The major component of a solution is called the solvent, and the minor, the solute.

• The concentration of a solution is the amount of solute present per unit volume or per unit mass of the solution/solvent.

• Materials that are insoluble in a solvent and have particles that are visible to naked eyes, form a suspension. A suspension is a heterogeneous mixture.

• Colloids are heterogeneous mixtures in which the particle size is too small to be seen with the naked eye, but is big enough to scatter light. Colloids are useful in industry and daily life. The particles are called the dispersed phase and the medium in which they are distributed is called the dispersion medium.