Exploring Mixtures and their Separation

Mixtures are classified as homogeneous or heterogeneous and also as solutions, suspensions, or colloids.

A well-stirred mixture of sugar and water is equally sweet in the first and the last sip. Such a mixture is called a homogeneous mixture or a solution. Other examples of homogeneous mixtures are vinegar (acetic acid in water), aerated drinks like soda (carbon dioxide in water), etc. A solution always remains homogeneous.

A stirred mixture of sand and water is not uniform. The sand particles are easily visible in the water and settle with time. Such a mixture is called a heterogeneous mixture.

Solutions

Solutions are prepared when a solute (the substance that gets dissolved) is mixed with a solvent (the substance that dissolves the solute). In the mixture of sugar and water, sugar is the solute, and water is the solvent.

Concentration of a Solution

The amount of solute dissolved in a given amount of solvent or solution is termed as the concentration of the solution.

How do we Express Concentration?

The concentration of a solution can be expressed in several ways.

A. Mass by mass percentage (% m/m or % w/w)

This method is commonly used to express the concentration of homogeneous mixtures. It tells us how many grams of solute are present in 100 grams of the total solution.

$$ \text{Mass by mass percentage} = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100 $$

This method of expressing concentration is also used for heterogeneous mixtures, such as milk powder and spice mixtures. It is also used to label the composition of packaged foods, showing how much salt, sugar, or protein is present in them.

B. Mass by volume percentage (% m/v or % w/v)

This method is used where measuring the volume of a liquid is easier than weighing it, for example, in medicines and laboratories. A common example is 5% glucose solution. It tells us how many grams of the solute is present in 100 millilitres of the solution.

$$ \text{Mass by volume percentage} = \frac{\text{Mass of solute (g)}}{\text{Volume of solution (mL)}} \times 100 $$

C. Volume by volume percentage (% v/v)

This method is used when two miscible liquids are mixed, for example, in perfumes, cosmetics and vinegar. The volume by volume percentage tells us how many millilitres of the solute is present in 100 millilitres of the solution.

$$ \text{Volume by volume percentage} = \frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100 $$

Solubility of Substances

The maximum amount of solute that dissolves in a fixed quantity of the solvent (100 mL or 100 g) is called its solubility at a given temperature. A solution that cannot dissolve any more solute at that temperature is called a saturated solution.

The solubility of a solid solute in a liquid solvent generally increases with temperature. On the other hand, in the case of gases dissolved in liquids, their solubility generally decreases with an increase in temperature.

Crystallization

A crystal is a solid that is made up of particles arranged in a regular geometric pattern. In laboratories, the process of forming crystals from a saturated solution is called crystallization.

This method can be used for the separation of two solids in which one of the compounds is present in small quantity and both are soluble in the same solvent. This method can also be used for purification of solids. The principle of purification by crystallization is based on the differences in the solubility of a substance at different temperatures.

Distillation

A homogeneous mixture of two miscible liquids can be separated by heating the mixture until the liquid with the lower boiling point vaporises. The vapour is then cooled, turning back into liquid. This process is known as distillation.

It allows the recovery of the solvent or the separation of liquids that differ in boiling point by at least about 25 °C. This method can also be used to separate a liquid from a solution containing dissolved solids.

During the process, vapours of the lower-boiling liquid are passed through a condenser where they are cooled, usually by circulating water or air and condensed as a pure liquid. The pure liquid is collected in a separate vessel, while the solid or other liquid from the mixture remains in the distillation flask.

A mixture of acetone and water can be separated by distillation because their boiling points differ sufficiently. Acetone boils at about 56 °C and water boils at 100 °C. A large difference in boiling points allows one liquid to vaporise before the vapours of the other liquid are formed in significant amount.

Paper Chromatography

It uses differences in the interactions of the components with the solvent and the paper to separate them. The liquid carries the substances up the paper, separating them based on how fast they move.

Sublimation

Camphor, on heating (below its melting point), changes directly from the solid state to the vapour state without passing through the liquid state, this process is called sublimation.

On cooling, vapours of camphor condense back into a solid without becoming a liquid, this is called deposition.

Similarly, solid carbon dioxide, known as dry ice which is used for ice cream storage, also undergoes sublimation.

Suspensions

Heterogeneous mixtures in which the solid particles (sand) do not dissolve but remain suspended throughout the bulk of the medium (water) are called suspensions. In the case of a suspension, the particles of the undissolved substance are larger in size than the particles present in the solution. Particles of a suspension are visible to the naked eye.

A. Centrifugation

This process involves spinning a mixture in a tube at a high speed. During centrifugation, the tubes become horizontal and the centrifugal force (outward force acting on a body moving in circular motion) causes the heavier particles to move outwards, where they settle at the bottom of the tube, while the lighter liquid remains at the top.

Centrifugation is widely used in laboratories to separate the components of blood, such as red blood cells and plasma and in many chemical industries.

B. Coagulation

Powdered alum (fitkari) can be added to a beaker containing muddy water. Alum is a white crystalline chemical substance often used in water purification. The alum causes the fine suspended particles to clump together. This process is called coagulation and the alum is said to act as a coagulant. These larger clumps thus formed, settle by gravity (sedimentation), and can be separated from water by decantation or filtration.

The formation of cheese (paneer) from milk also involves coagulation, in which acid (lemon juice or vinegar) is used as a coagulant. It causes the coagulation of milk proteins, forming cheese.

Colloids

Blood is neither a solution nor a true suspension. It is a colloid. Some other examples of colloids are milk, tomato sauce and ice cream.

Solutions, suspensions and colloids differ mainly in the size of the dissolved or the suspended particles. The particle size of a solute is the smallest (less than 1 nm diameter) in a solution. Colloids have larger particles (1 - 1000 nm), while suspensions have much larger particles (more than 1000 nm in diameter).

Unlike suspension, the particles in a colloid do not settle over time from the mixture. They are uniformly dispersed throughout the mixture as in a solution.

Tyndall Effect

The scattering of light by particles is known as the Tyndall effect. It is called so because it was first explained by the scientist John Tyndall, who studied the scattering of light by particles.

This effect can also be observed when a fine beam of light enters a dark room through a small hole. This happens because light gets scattered by the dust and smoke particles in the air.

The components of a colloid are called 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 dispersion medium.