Chapter 5: Exploring Mixtures and their Separation

A mixture is formed when two or more substances are combined physically, and each substance retains its own properties. Mixtures are very common in daily life. For example, air, milk, muddy water, salt solution, sugar solution, blood, smoke, fog, and seawater are all mixtures.

The separation of mixtures is very important in daily life, industry, agriculture, medicine, and environmental protection. For example, sugar crystals are obtained from sugarcane juice, salt is obtained from seawater, diseases can be detected from blood samples, and waste materials can be separated for recycling.

This chapter explains different types of mixtures and the methods used to separate their components.

Mixtures can be mainly classified into two types:

1. Homogeneous Mixtures

A homogeneous mixture has a uniform composition throughout.

In such mixtures, the components are mixed so well that we cannot see them separately. Every part of the mixture has the same properties.

Examples:

Salt solution, sugar solution, vinegar, soda water, air, brass, bronze.

A homogeneous mixture is also called a solution.

For example, when sugar is dissolved in water, the sweetness is the same in every sip. This shows that sugar is uniformly distributed in water.

2. Heterogeneous Mixtures

A heterogeneous mixture does not have a uniform composition throughout.

In such mixtures, the components can often be seen separately or are not evenly distributed.

Examples:

Sand and water, oil and water, muddy water, smoke, fog, dust in air, chalk powder in water.

For example, when sand is mixed with water, the sand particles remain visible and settle down after some time.

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

A solution has two main components:

Solute

The substance that gets dissolved is called the solute.

Solvent

The substance that dissolves the solute is called the solvent.

Example:

In a sugar solution:

Sugar = solute Water = solvent

In a salt solution:

Salt = solute Water = solvent

Solutions are uniform throughout and their particles cannot be seen with the naked eye.

The concentration of a solution tells us how much solute is dissolved in a given amount of solvent or solution.

In simple words, concentration tells us whether a solution is dilute or concentrated.

A solution with a small amount of solute is called a dilute solution. A solution with a large amount of solute is called a concentrated solution.

Importance of concentration

Correct concentration is very important in many situations.

For example:

ORS must contain the correct amount of salt and sugar in water. If the amounts are changed, it may not work properly. Similarly, farmers must mix pesticides with water in the right proportion. Too little pesticide may not protect the crop, while too much pesticide may harm crops, soil, and the environment.

The concentration of a solution can be expressed in different ways. In this chapter, three percentage methods are discussed:

• Mass by mass percentage
• Mass by volume percentage
• Volume by volume percentage

A. Mass by Mass Percentage

% m/m or% w/w

Mass by mass percentage tells us how many grams of solute are present in 100 grams of solution.

Formula

Mass by mass percentage = Mass of solute / Mass of solution × 100

Here,

Mass of solution = Mass of solute + Mass of solvent

Example

If 10 g of salt is dissolved in 90 g of water:

Mass of solute = 10 g Mass of solvent = 90 g Mass of solution = 10 g + 90 g = 100 g

Mass by mass percentage = 10 / 100 × 100 = 10%

So, the concentration of the solution is 10% m/m.

Uses

This method is used for food labels, milk powder, spice mixtures, salt content, sugar content, protein content, and alloys.

B. Mass by Volume Percentage

% m/v or% w/v

Mass by volume percentage tells us how many grams of solute are present in 100 mL of solution.

Formula

Mass by volume percentage = Mass of solute / Volume of solution × 100

Example

If 5 g of glucose is dissolved in water to make 100 mL of solution:

Mass of solute = 5 g Volume of solution = 100 mL

Mass by volume percentage = 5 / 100 × 100 = 5%

So, the concentration is 5% m/v.

Uses

This method is commonly used in medicines and laboratories.

For example, saline solution used in hospitals is usually 0.9% m/v sodium chloride, which means 0.9 g of salt is present in 100 mL of solution.

C. Volume by Volume Percentage

% v/v

Volume by volume percentage tells us how many millilitres of solute are present in 100 mL of solution.

This method is used when both solute and solvent are liquids.

Formula

Volume by volume percentage = Volume of solute / Volume of solution × 100

Example

If 1 mL of liquid pesticide is mixed with water to make 100 mL solution:

Volume of solute = 1 mL Volume of solution = 100 mL

Volume by volume percentage = 1 / 100 × 100 = 1%

So, the concentration is 1% v/v.

Uses

This method is used for vinegar, perfumes, cosmetics, liquid medicines, and pesticide solutions.

The solubility of a substance is the maximum amount of solute that can dissolve in a fixed amount of solvent at a particular temperature.

Usually, solubility is expressed as grams of solute per 100 g of water or per 100 mL of water.

Saturated solution

A solution that cannot dissolve any more solute at a given temperature is called a saturated solution.

Effect of temperature on solubility

For most solids dissolved in liquids, solubility increases when temperature increases.

For gases dissolved in liquids, solubility usually decreases when temperature increases.

For example, more sugar dissolves in hot water than in cold water. But gases like carbon dioxide escape more easily from warm soda than cold soda.

Solubility is an important property used in separation methods such as crystallization.

Homogeneous mixtures are uniform mixtures. Their components cannot be seen separately. Some important methods used to separate homogeneous mixtures are:

• Crystallization
• Distillation
• Paper chromatography

Crystallization is the process of forming pure solid crystals from a saturated solution.

A crystal is a solid in which particles are arranged in a regular geometric pattern.

Principle of crystallization

Crystallization is based on the difference in solubility of a substance at different temperatures.

Usually, a hot saturated solution can dissolve more solute. When it is cooled slowly, some solute separates out as crystals.

Example

If a saturated solution of copper sulfate is prepared in hot water and then cooled slowly, blue copper sulfate crystals are formed.

Steps of crystallization

• Prepare a hot saturated solution of the substance.
• Filter the hot solution to remove insoluble impurities.
• Allow the solution to cool slowly without disturbance.
• Crystals form as the solution cools.
• Filter and dry the crystals.

Uses of crystallization

Crystallization is used to:

Obtain pure solids from solutions. Purify impure substances. Separate a solid from impurities. Prepare crystals of substances like copper sulfate, salt, and sugar.

Daily life examples

Rock salt crystals, sugar crystals, frost, snowflakes, and quartz are examples of crystals.

Salt is also obtained from seawater by evaporation and crystallization.

Distillation is a method used to separate a liquid from a solution or to separate two miscible liquids having different boiling points.

Principle of distillation

Distillation is based on the difference in boiling points of the components.

The liquid with a lower boiling point vaporises first. The vapour is then cooled in a condenser and collected as a liquid called the distillate.

Conditions for simple distillation

Simple distillation is useful when two miscible liquids have a boiling point difference of at least about 25°C.

Example

A mixture of acetone and water can be separated by distillation.

Acetone boils at about 56°C. Water boils at 100°C.

Since the difference in boiling points is large, acetone vaporises first and can be collected separately.

Steps of distillation

• Heat the mixture in a distillation flask.
• The liquid with lower boiling point changes into vapour.
• The vapour passes through a condenser.
• In the condenser, vapour cools and changes back into liquid.
• The pure liquid is collected in a separate flask.

Uses of distillation

Distillation is used for:

Separating miscible liquids. Recovering a solvent from a solution. Obtaining pure water from salt water. Preparing perfumes and fragrances. Separating useful components from crude petroleum through fractional distillation.

Fractional distillation

Fractional distillation is used when the boiling points of liquids are close to each other, usually with a difference of less than 25°C.

It is used in petroleum refineries to separate crude oil into petroleum gas, petrol, kerosene, diesel, lubricating oil, and bitumen.

Paper chromatography is a method used to separate different components of a mixture based on their different rates of movement on paper.

Principle of paper chromatography

Different components of a mixture move at different speeds on paper because they interact differently with the solvent and the paper.

The component that dissolves more in the solvent and is less strongly held by paper moves faster. The component that is less soluble or more strongly held by paper moves slower.

Example

Black ink may look like one colour, but it can contain different coloured dyes. Paper chromatography can separate these dyes.

Steps of paper chromatography

• Take a strip of chromatography paper or filter paper.
• Draw a pencil line near the bottom.
• Put a small spot of ink on the line.
• Place the paper in a container containing a little solvent.
• The solvent level should remain below the ink spot.
• The solvent rises up the paper and carries the ink components with it.
• Different colours separate at different heights.

Uses of paper chromatography

Paper chromatography is used to separate:

Dyes in ink. Pigments in leaves. Pigments in flower petals. Components of food colours. Substances in medicines and laboratory samples.

Heterogeneous mixtures are non-uniform mixtures. Their components may be visible or may exist in different phases.

Methods used for separating heterogeneous mixtures include:

• Separating funnel
• Sublimation
• Filtration
• Sedimentation and decantation
• Centrifugation
• Coagulation

Liquids that do not mix with each other are called immiscible liquids.

Examples:

Oil and water, mustard oil and water, kerosene and water.

Immiscible liquids form separate layers because they have different densities.

Separating funnel

A separating funnel is used to separate two immiscible liquids.

Example

Mustard oil and water can be separated using a separating funnel.

Mustard oil is less dense than water, so it forms the upper layer. Water is denser, so it forms the lower layer.

Steps

• Pour the mixture into the separating funnel.
• Allow it to stand undisturbed.
• Two separate layers are formed.
• Open the stopcock to drain the lower layer.
• Close the stopcock when the lower layer is removed.
• Collect the upper layer separately.

Uses

This method is used to separate oil and water and other immiscible liquid mixtures.

Sublimation is the process in which a solid changes directly into vapour without becoming a liquid.

The reverse process, in which vapour changes directly into solid, is called deposition.

Principle of sublimation

Sublimation is used to separate a sublimable solid from a non- sublimable solid.

Examples of sublimable substances

Camphor, naphthalene, ammonium chloride, iodine, dry ice.

Example

A mixture of camphor and sand can be separated by sublimation.

When heated, camphor changes directly into vapour. The vapour cools and deposits as solid camphor on the inner wall of the funnel. Sand remains in the china dish.

Steps

• Place the mixture of camphor and sand in a china dish.
• Cover it with an inverted funnel.
• Plug the stem of the funnel with cotton.
• Heat the mixture gently.
• Camphor sublimes and deposits on the cooler wall of the funnel.
• Sand remains behind.

A suspension is a heterogeneous mixture in which solid particles do not dissolve but remain suspended in the medium.

The particles of a suspension are large and visible to the naked eye.

Examples

Sand in water, muddy water, chalk powder in water, sawdust in water, tea leaves in water.

Properties of suspensions

Suspensions are heterogeneous. Particles are visible to the naked eye. Particles settle down when left undisturbed. They can usually be separated by filtration. They show the Tyndall effect.

How Can We Separate Mud from Water?

Muddy water is a suspension. If left undisturbed, heavy mud particles settle at the bottom. This process is called sedimentation.

The clear water can then be poured off carefully. This process is called decantation.

But sometimes very fine particles remain suspended and the water still looks cloudy. In such cases, methods like centrifugation and coagulation are used.

A. Centrifugation

Centrifugation is a method of separating heavier particles from lighter particles by spinning the mixture at high speed.

Principle

When a mixture is spun rapidly, heavier particles move outward and settle at the bottom of the tube, while lighter liquid remains above.

Uses of centrifugation

Centrifugation is used to:

Separate blood components. Separate cream from milk. Separate fine suspended particles from liquids. Separate mixtures in laboratories and industries.

Example

Blood can be separated into red blood cells, white blood cells, platelets, and plasma using centrifugation.

Bridging Science and Society – The Paperfuge

A paperfuge is a simple hand-powered centrifuge made using a cardboard disc, thread, and handles.

It does not need electricity. It works by spinning samples at high speed. It can separate heavier blood components from lighter ones.

The paperfuge can help in detecting diseases like malaria and anaemia in remote areas where electric centrifuges may not be available.

B. Coagulation

Coagulation is a process in which very fine suspended particles clump together to form larger particles.

A substance that causes coagulation is called a coagulant.

Example

Powdered alum, also called fitkari, is added to muddy water. Alum causes fine suspended particles to join together and form larger clumps. These larger clumps settle down by gravity.

After this, clear water can be separated by decantation or filtration.

Daily life example

Paneer formation from milk is also an example of coagulation. Lemon juice or vinegar acts as a coagulant and causes milk proteins to coagulate.

A colloid is a mixture in which very small particles are uniformly dispersed in a medium.

A colloid is neither a true solution nor a true suspension.

Examples

Milk, blood, tomato sauce, ice cream, fog, smoke, butter, cream.

Properties of colloids

Colloids appear homogeneous but are actually heterogeneous. Their particles are larger than solution particles but smaller than suspension particles. Particles cannot be seen with the naked eye. Particles do not settle down on standing. They cannot be separated by ordinary filtration. They show the Tyndall effect.

Particle size comparison

Solution: particles less than 1 nm Colloid: particles between 1 nm and 1000 nm Suspension: particles larger than 1000 nm

Components of a colloid

A colloid has two parts:

Dispersed phase

The solute-like component present as tiny particles is called the dispersed phase.

Dispersion medium

The component in which the dispersed phase is present is called the dispersion medium.

Example

In milk, fat droplets are dispersed in water.

Bridging Science and Society – Donate Blood

Blood is a colloid and an important fluid in our body. Donated blood can save lives during emergencies, surgeries, and serious illnesses.

Blood can be separated into components like:

Plasma Platelets White blood cells Red blood cells

These components are stored safely in blood banks and used when needed.

The Tyndall effect is the scattering of light by particles in a colloid or suspension.

When a beam of light passes through a true solution, its path is not visible because solution particles are too small to scatter light.

But when light passes through a colloid or suspension, the particles scatter the light and make the path of light visible.

Examples of Tyndall effect

Sunlight passing through gaps in trees. Light entering a dark room through a small hole. Headlights visible in fog. Floodlights in a sports stadium. Dust particles visible in a beam of sunlight. Light scattering in milk diluted with water.

Tyndall effect in different mixtures

Salt solution: does not show Tyndall effect. Chalk powder in water: shows Tyndall effect. Milk and water: shows Tyndall effect.

Mixture

A substance formed by physically combining two or more substances.

Homogeneous mixture

A mixture having uniform composition throughout.

Heterogeneous mixture

A mixture having non-uniform composition.

Solution

A homogeneous mixture of solute and solvent.

Solute

The substance that dissolves in a solvent.

Solvent

The substance that dissolves the solute.

Concentration

The amount of solute present in a given amount of solvent or solution.

Solubility

The maximum amount of solute that can dissolve in a fixed amount of solvent at a particular temperature.

Saturated solution

A solution that cannot dissolve any more solute at a given temperature.

Crystallization

The process of obtaining crystals from a saturated solution.

Distillation

A method of separating liquids based on difference in boiling points.

Chromatography

A method used to separate components of a mixture based on their different movement rates on paper.

Immiscible liquids

Liquids that do not mix with each other.

Separating funnel

An apparatus used to separate immiscible liquids.

Sublimation

The process in which a solid changes directly into vapour.

Deposition

The process in which vapour changes directly into solid.

Suspension

A heterogeneous mixture in which large particles remain suspended for some time and settle later.

Centrifugation

A method of separating particles by spinning the mixture at high speed.

Coagulation

A process in which small particles clump together to form larger particles.

Colloid

A mixture having particles bigger than solution particles but smaller than suspension particles.

Tyndall effect

The scattering of light by particles in a colloid or suspension.

Mixtures are formed when two or more substances are physically combined. They may be homogeneous or heterogeneous. Homogeneous mixtures have uniform composition and are called solutions. Heterogeneous mixtures do not have uniform composition.

A solution contains a solute and a solvent. The amount of solute present in a given amount of solution or solvent is called concentration. Concentration can be expressed as mass by mass percentage, mass by volume percentage, or volume by volume percentage.

Solubility is the maximum amount of solute that can dissolve in a fixed quantity of solvent at a particular temperature. A saturated solution cannot dissolve more solute at that temperature.

Homogeneous mixtures can be separated by methods such as crystallization, distillation, and paper chromatography. Crystallization is used to obtain pure crystals from saturated solutions. Distillation is used to separate liquids with different boiling points or to recover a solvent. Paper chromatography is used to separate coloured substances like ink dyes or plant pigments.

Heterogeneous mixtures can be separated by methods such as separating funnel, sublimation, filtration, sedimentation, decantation, centrifugation, and coagulation. A separating funnel is used for immiscible liquids like oil and water. Sublimation is used for sublimable substances like camphor and naphthalene. Centrifugation separates particles based on density by spinning. Coagulation helps tiny suspended particles clump together and settle.

Suspensions, colloids, and solutions differ mainly in particle size. Suspensions have large particles that settle down. Colloids have intermediate-sized particles that do not settle but scatter light. Solutions have very small particles that neither settle nor scatter light.

The scattering of light by colloidal or suspended particles is called the Tyndall effect. This effect is seen in fog, smoke, milk, dusty air, and sunlight passing through tree leaves.

Overall, separation techniques are important in daily life, laboratories, industries, medicine, agriculture, and environmental protection. They help us obtain pure substances, clean water, useful materials, and recyclable resources.