Mixtures and Their Separation Class 9: Complete Chapter Notes

About This Chapter

Mixtures and their separation class 9 is the first chemistry chapter in the Class 9 NCERT Science book, although it comes fifth in the overall Science sequence. It carries good weightage in both the half-yearly and annual exams, so it is worth preparing thoroughly. You can check how it fits into the full course in the Class 9 syllabus.

The newer NCERT "Exploration" book has added the separation techniques section back with fresh activities and examples. Students studying this year should therefore build real conceptual understanding instead of relying on older notes. These mixtures and their separation class 9 notes follow the current book, activity by activity.

What Is a Mixture? Basics of Mixtures and Their Separation Class 9

A mixture is formed when two or more pure substances combine to make one entity without chemically reacting. Mixtures are everywhere in daily life:

Applications of Mixtures in Daily Life

Mixtures are not just theory: mixtures and their separation class 9 concepts show up in kitchens, factories and hospitals.

Food industry

• Cooking any vegetable dish (chilli, salt and other spices are mixed with the vegetables)
• Baking: flour, baking soda and eggs combined
• Curry powder: a blend of spices such as black pepper, cloves, cinnamon and cumin
• Vinaigrette and sauces
• Mayonnaise: an emulsion of oil and water-based ingredients

Chemical industry

• Paints
• Fertilisers, which are added to soil to improve fertility for crops
• Petrol (gasoline): a mixture of many hydrocarbons that burns to release energy
• LPG: a mixture mainly of propane and butane
• Polymers

Medical industry

• Blood and blood plasma
• Saline (IV fluid): a solution of salts in water given during dehydration
• Tablets, pills and syrups: blends of different chemicals

Homogeneous vs Heterogeneous Mixtures in Mixtures and Their Separation Class 9

A mixture is a combination of two or more substances that are physically mixed, not chemically combined. When substances combine chemically, a new compound forms instead. In a physical mixture such as sugar dissolved in water, both original substances keep their properties: the sweetness of sugar and the fluid nature of water are both still there.

In mixtures and their separation class 9, mixtures are first classified into two categories based on how uniform their composition is.

Homogeneous Mixtures

A homogeneous mixture has the same composition throughout its volume. If you drink a well-stirred sugar solution, the first sip and the last sip taste equally sweet because the sugar concentration is uniform.

Examples: air, sugar solution, salt solution, lemonade (without ice), vinegar, soda water and bronze (an alloy, which is a mixture of metals).

Heterogeneous Mixtures

A heterogeneous mixture has a non-uniform composition with visible or detectable separate phases. If you mix chalk powder in water and leave it, the chalk settles at the bottom while water stays on top. The composition differs from point to point.

Examples: vinaigrette, trail mix, soil, smoke, muddy water and sand in water.

💡 Special case: blood and milk. Blood and milk look uniform to the naked eye, but under a microscope you can see cells in blood and fat globules in milk. They are therefore treated as heterogeneous mixtures (colloid-like systems), even though they appear homogeneous.

Solution, Colloid and Suspension: Classification by Optical Properties

Beyond the homogeneous and heterogeneous classification, mixtures can also be classified by how they interact with light. This is the idea behind NCERT Activity 5.1 in mixtures and their separation class 9.

The activity: Prepare three mixtures, each in a 1:2 ratio of solute to water:

• A: salt and water
• B: chalk powder and water
• C: milk and water

When a laser beam is passed through each:

• Mixture A (salt water): no beam path is visible.
• Mixture B (chalk in water): a faint, diffused, scattered path is visible.
• Mixture C (milk in water): a clear, straight beam path is visible.

Using four parameters (particle visibility, laser path, settling and filtration), mixtures fall into three categories:

Solution

A solution is a homogeneous mixture of atoms, ions or molecules, made up of a solute and a solvent. Its particle size is less than 1 nanometre, it does not show the Tyndall effect, and its particles neither settle nor can they be seen.

Colloid

A colloid has an intermediate particle size, from 1 nanometre up to 1,000 nanometres (1 micrometre). It shows the Tyndall effect and consists of a dispersed phase and a dispersing medium. Milk is the classic example. Colloids look homogeneous but are actually heterogeneous.

Suspension

A suspension is a heterogeneous mixture with large particles, greater than 1,000 nanometres. The solute particles do not dissolve; they stay spread through the medium, hence the name. The particles are visible, settle over time and can be removed by filtration.

How to Identify Solute and Solvent

Knowing the solute and the solvent is the first step in every mixtures and their separation class 9 numerical. It depends on the physical states of the substances being mixed:

Concentration of a Solution: Formulas in Mixtures and Their Separation Class 9

The amount of solute present in a given amount of solution (not just solvent) is called the concentration of the solution. It matters in laboratories and in daily life: medicine, agriculture, food, cosmetics and even a cup of tea. An ORS packet, for instance, lists the amount of each solute on its label.

There are three common ways to express concentration in mixtures and their separation class 9.

1. Mass by Mass Percentage (% m/m or % w/w)

This is the most commonly used method for homogeneous mixtures. It tells you how many grams of solute are present in 100 grams of solution.

Formula:

% m/m = (Mass of solute ÷ Mass of solution) × 100

A milk powder label saying "per 100 g: fat 26 g, sugar 49 g, protein 15.5 g" is expressing mass concentration in this way.

Worked Example 1: 10 g of salt is dissolved in 90 g of water. Find the mass by mass percentage.

• Mass of solute (salt) = 10 g
• Mass of solution = 10 g + 90 g = 100 g
• % m/m = (10 ÷ 100) × 100 = 10% w/w

Worked Example 2: Cake recipe. A recipe uses 75 g of sugar, 420 g of all-purpose flour and 5 g of sodium hydrogen carbonate (NaHCO₃). All three are solids, so the largest quantity decides the solvent: flour is the solvent; sugar and NaHCO₃ are solutes.

• Total mixture = 75 + 420 + 5 = 500 g
• Sugar = (75 ÷ 500) × 100 = 15% w/w
• NaHCO₃ = (5 ÷ 500) × 100 = 1% w/w
• Flour = 100% − (15% + 1%) = 84% w/w

Worked Example 3: Brass. Brass is 70% copper by mass. Find the masses of copper and zinc in 120 g of brass.

• Copper = 70% of 120 g = 84 g
• Zinc = 120 g − 84 g = 36 g

2. Mass by Volume Percentage (% m/v or % w/v)

Use this when a solid is dissolved in a liquid.

Formula:

% m/v = (Mass of solute ÷ Volume of solution) × 100

A glucose IV infusion labelled "5% w/v" contains 5 g of glucose in every 100 mL of solution.

Worked Example: 5 g of glucose is dissolved in water to make 100 mL of solution.

• % m/v = (5 ÷ 100) × 100 = 5% w/v

3. Volume by Volume Percentage (% v/v)

Use this for liquid-liquid solutions.

Formula:

% v/v = (Volume of solute ÷ Volume of solution) × 100

Worked Example: 1 mL of liquid pesticide is mixed with water to make 100 mL of spray.

• % v/v = (1 ÷ 100) × 100 = 1% v/v

Want more practice on mixtures and their separation class 9 numericals? Work through the NCERT Solutions for Class 9 after finishing this chapter.

Solubility, Saturated and Unsaturated Solutions

Solubility questions are common in mixtures and their separation class 9 exams. Solubility is the maximum amount of solute that dissolves in a fixed quantity of solvent (usually 100 g) at a given temperature. For most solids, solubility increases as temperature rises.

Keep adding salt to a glass of water, spoon by spoon. At first it dissolves completely, but beyond a certain point it stops dissolving and settles at the bottom. This leads to three types of solutions:

NCERT Activity 5.2 asks you to read a graph of solubility (g per 100 g of water) against temperature for two compounds, A and B:

• Compound A: rises from about 30 g at 0°C to about 70 g at 80°C.
• Compound B: rises from about 180 g at 0°C to about 360 g at 80°C.

The key observation: the solubility of Compound B increases faster with temperature than that of Compound A.

Common Alloys as Solid Solutions

Alloys are the main solid-solution examples in mixtures and their separation class 9. An alloy is formed when metals are melted, mixed and cooled back into a solid.

💡 Memory tip: Brass contains zinc (Zn); bronze contains tin (Sn). The spelling trick: "bronze" has a Z but no zinc, and "brass" has an S but no tin. The letters swap, so remember the swap.

Mixtures and Their Separation Class 9: Techniques for Homogeneous Mixtures

The homogeneous-mixture techniques in mixtures and their separation class 9 are crystallisation, distillation, fractional distillation and paper chromatography.

Crystallisation

A crystal is a solid whose particles are arranged in a regular geometric pattern. Crystallisation forms crystals from a saturated solution and is used to separate a dissolved solid from its liquid solvent.

NCERT Activity 5.3: Copper sulfate crystallisation. A blue copper sulfate solution (copper sulfate is also called "blue vitriol") is heated in a glass container. Water evaporates, so the solution gradually becomes saturated.

Steps:

1. Heat the solution until it becomes saturated.
2. Filter the hot saturated solution to remove unwanted particles.
3. Cool the filtrate slowly. Solubility falls as it cools, so crystals form and deposit.

Applications: purifying rock salt, common salt and alum.

Distillation

Distillation, a core technique in mixtures and their separation class 9, separates two miscible liquids using the difference in their boiling points. For simple distillation to work well, the boiling points should differ by more than 25°C (25 K).

How it works: The mixture is heated and the liquid with the lower boiling point vaporises first. Its vapour passes through a condenser, where cold water turns it back into liquid, which is collected separately. The higher-boiling liquid stays behind.

Example: Acetone (boiling point 56°C) and water (100°C) differ by 44°C, so simple distillation works.

💡 Real-world application: Kannauj's Deg Bhapka method. Kannauj in Uttar Pradesh is known as the "Perfume Capital of India". Its traditional attar is made by distilling fragrant flowers and leaves. The aromatic components vaporise, are condensed with cooling, and are collected as perfume. A famous product is "Mitti ka Ittar", which captures the smell of the first rain on dry soil.

Fractional Distillation

When the boiling points differ by less than 25°C, simple distillation is unreliable. Evaporation (a surface phenomenon) happens below the boiling point, so vapours of both liquids rise together. In this case we use fractional distillation, which adds a fractional column between the flask and the condenser.

Example: Ethanol (78.4°C) and water (100°C) differ by less than 25°C.

How the column helps: As vapour rises through the column, the less volatile component (higher boiling point) condenses and falls back into the flask. The more volatile component keeps rising to the condenser, where it is cooled and collected.

Major applications:

• Separating the gases of air (nitrogen, oxygen, argon) after liquefaction, because their boiling points are close.
• Petroleum refineries: crude oil is heated in a furnace and fed into a fractional column. Fractions condense at different heights according to their boiling points. From top to bottom: petroleum gas, petrol, kerosene (aviation fuel), diesel, lubricating oil and bitumen at the bottom.

💡 Current affairs note: HPCL's Rajasthan Refinery at Pachpadra in Balotra district is a 9 million tonnes per annum greenfield refinery-cum-petrochemical complex that has begun operations in 2026. Its crude distillation unit uses fractional distillation as the first step of refining.

Paper Chromatography

Paper chromatography separates the colour components of a mixture because different substances move at different speeds. It is used to separate colours in ink, plant pigments and food colours.

Method: Draw a pencil line near the bottom of a strip of chromatography or tissue paper. Place a dot of the colour mixture on the line. Dip the strip in water so that the water level stays just below the line. As water rises up the paper, it carries each colour at a different rate, and the colours separate visibly.

Mixtures and Their Separation Class 9: Techniques for Heterogeneous Mixtures

The heterogeneous-mixture techniques in mixtures and their separation class 9 are the separating funnel, sublimation, centrifugation and coagulation.

Separating Immiscible Liquids (Separating Funnel)

Immiscible liquids do not mix; they form separate layers according to density. Oil and water is the standard example. (Sand and water, or iron filings and sulfur, are also heterogeneous mixtures, but they are separated by other methods such as filtration, sedimentation or a magnet.)

NCERT activity: Mix 5 mL of mustard oil with 20 mL of water and two layers form. Pour the mixture into a separating funnel, which has a stopcock at the bottom. Let it settle, then open the stopcock and release the denser liquid (water) into one container. Close the stopcock just as the boundary reaches it, and collect the oil in a second container.

Sublimation

Sublimation separates a mixture in which one solid is a sublimate: a substance that changes directly from solid to gas on heating, without melting.

Common sublimates: camphor, naphthalene, ammonium chloride, iodine and dry ice (solid CO₂).

Activity: Mix camphor with sand in a china dish and cover it with an inverted funnel plugged with cotton. On heating, camphor vapour rises and re-solidifies on the cooler walls and cotton. The sand stays behind in the dish.

💡 Note on dry ice: Dry ice is solid carbon dioxide. At normal pressure it turns straight into gas (about −78.5°C) and never forms a liquid.

Centrifugation

Centrifugation separates suspensions and colloids by high-speed spinning. The spinning produces an outward force that pushes denser particles to the bottom of the tube while lighter ones stay nearer the top, so the mixture separates into layers by density. During spinning, the tubes swing out to a horizontal position.

Example: blood test. A centrifuged blood sample separates into plasma, white blood cells and red blood cells, which can then be examined individually.

Coagulation

Coagulation is the clumping of very fine particles into a larger mass called a sludge, with the help of a coagulant.

Example: purifying muddy water. Add powdered alum (phitkari, potash alum) to muddy water. The mud particles clump together and settle by gravity (sedimentation). The clear water above can then be separated by decantation or filtration. This is common in the monsoon, when tap, well and river water turn muddy.

Example: making paneer. Add vinegar or an acidic juice to warm (not boiling, not cold) milk. The milk proteins coagulate and separate from the watery part. Strain the solids and press them into shape to get paneer.

The Tyndall Effect

The Tyndall effect is a favourite exam topic in mixtures and their separation class 9. The Tyndall effect is the scattering of light by particles in a mixture, which makes the path of the light beam visible. Colloids and suspensions show it; true solutions do not, because their particles are too small to scatter light.

A common example is a beam of sunlight streaming through a window into a dusty room: the dust scatters the light and makes the beam visible.

Quick Revision: Mixtures and Their Separation Class 9 at a Glance

Use this table as a one-minute revision for mixtures and their separation class 9.

Important Questions on Mixtures and Their Separation Class 9

These questions on mixtures and their separation class 9 follow the NCERT pattern.

Q1. Why is brass a solid solution? Brass is made by melting copper and zinc together and cooling the mixture. The zinc is uniformly mixed in copper, so it has a single uniform phase, like a solution in solid form.

Q2. 20 g of sugar is dissolved in 80 g of water. Find the % m/m. Mass of solution = 20 + 80 = 100 g, so % m/m = (20 ÷ 100) × 100 = 20%.

Q3. Why does a colloid show the Tyndall effect but a solution does not? Colloid particles (1 to 1,000 nm) are large enough to scatter light. Solution particles (below 1 nm) are too small to scatter it.

Q4. Why is fractional distillation used for ethanol and water? Their boiling points (78.4°C and 100°C) differ by less than 25°C, so simple distillation cannot give a clean separation. The fractional column allows repeated condensation and evaporation.

Q5. Which technique would you use to separate iodine from sand? Sublimation, because iodine converts directly from solid to vapour on heating while sand does not.