Mixtures and Their Separation Class 9: Complete Chapter Notes

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About This Chapter

Mixtures and Their Separation is the first chemistry chapter in the Class 9 NCERT syllabus, though within the overall NCERT Science sequence for Class 9, it comes fifth. This chapter is important for both the half-yearly and annual exams.

Before the changes brought in around the COVID period, this chapter used to be much shorter, and the separation techniques section was removed from it entirely. With the newer "Exploration" NCERT book, that separation content has been added back in with new material and a fresh approach — so students studying this year need to build a solid conceptual understanding rather than relying on older notes.

What Is a Mixture? Everyday Examples

A mixture is formed when two or more pure substances combine to make one entity. Mixtures are everywhere in daily life:

Applications of Mixtures in Daily Life

Mixtures have wide applications across industries:

Food Industry:

• Cooking any vegetable dish (adding chilli, salt, and other spices separately)
• Baking — flour, baking soda, and eggs combined
• Curry powder — a blend of spices including black pepper, cloves, cinnamon sticks, and cumin
• Vinaigrette and sauces
• Mayonnaise — an emulsified mixture

Chemical Industry:

• Paints
• Fertilisers — chemicals sprayed on soil to increase fertility for crops
• Gasoline — a mixture of methane, butane, and propane, which burns to give energy
• Polymers

Medical Industry:

• Blood
• Saline (IV fluid) — a mixture of salts and fluids given during dehydration
• Blood plasma
• Tablets, pills, and syrups — blends of different chemicals

Homogeneous vs Heterogeneous Mixtures

A mixture is a combination of two or more substances that are physically mixed — not chemically mixed. When substances mix chemically, a new compound forms instead of a mixture. In a physical mixture, like sugar dissolved in water, the properties of both original substances remain present: the sweetness of sugar and the fluid properties of water are both there in the resulting sugar solution.

Mixtures are classified into two categories based on composition uniformity:

Homogeneous Mixtures

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

Examples: air, sugar solution, salt solution, lemonade, and bronze (an alloy — a mixture of two metals).

💡 Special Case — Blood: Blood is homogeneous inside the body, where it remains stable. But once drawn outside the body, it begins to coagulate after some time and loses that stability, making it behave as a heterogeneous mixture outside the body.

Heterogeneous Mixtures

A heterogeneous mixture has a non-uniform composition with visible, separate phases. For example, mixing chalk powder into water and letting it sit shows the chalk powder settling at the bottom while water stays on top — the composition is different at different points in the mixture, and you can see separate phases.

Examples: vinaigrette, trail mix, soil, smoke, and blood (outside the body).

Solution, Colloid and Suspension: Classification by Optical Properties

Beyond homogeneous and heterogeneous classification, mixtures can also be classified based on optical properties, using NCERT Activity 5.1.

The activity: Take 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:

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

Based on four parameters — particle visibility, laser light visibility, settling, and filtration — mixtures are classified into three categories:

Solution

A solution is generally a homogeneous mixture of atoms, ions, and substances, made up of a solute and a solvent. Its particle size is less than 1 nanometre, it doesn't show the Tyndall effect (no visible light path), and its particles neither settle nor are visible to the naked eye.

Colloid

A colloid has an intermediate particle size — greater than 1 nanometre and less than 1,000 nanometres (1 micrometre). It shows the Tyndall effect (the light path becomes visible), and it is made up of a dispersed phase and a dispersing medium. Milk is a classic example of a colloid. Colloids are generally heterogeneous, though a few, like milk, can behave as homogeneous under certain conditions.

Suspension

A suspension is generally a heterogeneous mixture with large, visible particles — average particle size greater than 1,000 nanometres (1 micrometre). In a suspension, the solute particles do not dissolve but remain suspended throughout the bulk of the medium, which is why it's called a suspension. These particles are visible, settle over time, and can be separated using filtration.

How to Identify Solute and Solvent

Identifying which substance is the solute and which is the solvent depends on the physical states of the substances being mixed:

Concentration of a Solution and How to Measure It

The amount of solute present in a given amount of solution (not just solvent) is called the concentration of the solution. Understanding concentration matters not just in science laboratories but in everyday life too — in medicine, agriculture, food, cosmetics, and even making a simple cup of tea. An ORS packet, for instance, lists the amount of each solute present on its label.

There are three common methods for measuring concentration:

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

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

Formula:

% m/m=Mass of soluteMass of solution×100\%\ m/m = \frac{\text{Mass of solute}}{\text{Mass of solution}} \times 100% m/m=Mass of solutionMass of solute​×100

A milk powder label showing "per 100 grams: fat 26g, sugar 49g, fibre 0g, protein 15.5g, sodium 0.8g" is expressing mass concentration this way.

Worked Example 1: If 10 grams of salt is dissolved in 90 grams of water, calculate the mass by mass percentage of the solution formed.

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

Worked Example 2 — Cake Recipe: A cake recipe uses 75 g of sugar, 420 g of all-purpose flour, and 5 g of sodium hydrogen carbonate (NaHCO₃).

Since all three ingredients are solids, quantity decides solute vs solvent: all-purpose flour (present in the largest amount) is the solvent, and sugar and NaHCO₃ are the two solutes.

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

Worked Example 3 — Brass Alloy: Brass is 70% copper by mass. Calculate the quantities of copper and zinc present in 120 g of brass.

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

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

This method is used when a solid is dissolved in a liquid.

Formula:

% m/v=Mass of soluteVolume of solution×100\%\ m/v = \frac{\text{Mass of solute}}{\text{Volume of solution}} \times 100% m/v=Volume of solutionMass of solute​×100

A glucose intravenous infusion labelled "5% w/v" contains 5% glucose by mass in its total volume.

Worked Example: 5 g of glucose is dissolved in water to make 100 mL of solution. Calculate the mass by volume percentage.

• Mass of solute (glucose) = 5 g
• Volume of solution = 100 mL
• % m/v = (5 / 100) × 100 = 5% w/v

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

This method is used for liquid-liquid solutions.

Formula:

% v/v=Volume of soluteVolume of solution×100\%\ v/v = \frac{\text{Volume of solute}}{\text{Volume of solution}} \times 100% v/v=Volume of solutionVolume of solute​×100

Worked Example: 1 mL of liquid pesticide is mixed with a sufficient amount of water to form 100 mL of pesticide spray. Calculate the volume by volume percentage.

• Volume of solute (pesticide) = 1 mL
• Volume of solution = 100 mL
• % v/v = (1 / 100) × 100 = 1% v/v

Solubility, Saturated and Unsaturated Solutions

Solubility is the maximum amount of solute that can be dissolved in a fixed quantity of solvent (typically 100 g or 100 mL) at a given temperature. Solubility depends on temperature — generally, as temperature increases, more solute can dissolve.

If you keep adding salt to a glass of water, spoon by spoon, it dissolves without a trace at first. But beyond a certain point, it stops dissolving and starts depositing at the bottom instead. This behaviour defines three types of solutions:

According to NCERT Activity 5.2, a graph plotting solubility (grams per 100 g of water) against temperature for two compounds, A and B, shows:

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

The key observation from this graph is that the solubility of Compound B increases faster than that of Compound A as temperature rises.

Common Alloys as Solid Solutions

An alloy is a type of solid solution, formed when solids are melted, mixed, and cooled back into a new solid.

💡 Memory Tip: Students commonly mix up Brass and Bronze. Notice that Brass (spelled with an "S") does not contain Tin — whose symbol is Sn (an "S" element) — it contains Zinc instead. Bronze (spelled with a "Z") does not contain Zinc — whose symbol is Zn (a "Z" element) — it contains Tin instead. The letters swap, which makes them easy to confuse unless you remember the trick.

Separation Techniques for Homogeneous Mixtures

Crystallisation

A crystal is a solid made up of particles arranged in a regular geometric pattern. Crystallisation is the method of forming crystals from a saturated solution, and it's specifically used to separate a solid solute from a liquid solvent.

NCERT Activity 5.3 — Copper Sulfate Crystallisation: A blue-coloured copper sulfate solution (copper sulfate is also called "blue vitriol" because of its colour) is heated in a glass container. Heating increases its solubility, and continued heating causes the water (solvent) to evaporate, gradually reducing the amount of water while the amount of solute effectively increases in proportion — until the solution becomes saturated.

The steps are:

1. Heat the solution until it becomes saturated (solvent evaporates).
2. Filter the saturated solution to remove any unwanted particles.
3. Cool the filtered saturated solution — as it cools, solubility decreases, pushing the solution into a supersaturated state, and crystals begin to form and deposit.

Applications: separating substances like rock salt, common salt, and alum.

Distillation

Distillation is used to separate two miscible liquids and works on the principle of difference in boiling points. For simple distillation to work efficiently, the boiling point difference between the two liquids should be greater than 25°C.

How it works: The mixture is heated. The liquid with the lower boiling point vaporises first; its vapour rises and passes through a condenser, where chilled water cools the vapour back into liquid, which is then collected separately. The liquid with the higher boiling point stays behind.

Example: Acetone (boiling point 56°C) and water (boiling point 100°C) — a difference of about 44°C, which is more than 25°C, so simple distillation works well here.

💡 Real-World Application — Kannauj's "Deg Bhapka" Method: Kannauj, a town in Uttar Pradesh, is known as the "Perfume Capital of India." Perfumes there are made using a distillation process called the Deg Bhapka method. Fragrant flowers and leaves are made into a solution and heated; the fragrance components, having a lower boiling point, vaporise, rise, and are condensed with water cooling to form the final perfume ("ittar"). One especially famous product is "Mitti ka Ittar" — a perfume based on the earthy fragrance of the season's first rain.

Fractional Distillation

When the boiling point difference between two liquids is less than 25°C, simple distillation isn't reliable, because evaporation (a surface phenomenon) can occur below the boiling point alongside vaporisation (a bulk phenomenon), causing both liquids' vapours to rise together. In this case, fractional distillation is used instead, with an additional device called a fractional column.

Example: Ethanol (boiling point 78.4°C) and water (boiling point 100°C) — a difference of less than 25°C.

How the fractional column helps: As vapour rises through the column, the less volatile component (with a higher boiling point) partially cools and condenses back down into the flask, while the more volatile, "superheated" component continues rising to the condenser, where it's cooled and collected separately.

Major applications:

• Separating the components of air, such as nitrogen, oxygen, and argon, which have very close boiling points.
• Petroleum refineries — crude oil is heated in a furnace and sent into a fractional column, where different components condense at different heights based on their boiling points and densities. From top to bottom: petroleum gas, petrol, aviation fuel/kerosene, diesel, lubricating oil, and bitumen (coal tar) at the very bottom — with density increasing as you move downward.

💡 Current Affairs Note: Rajasthan's first Greenfield Refinery-cum-Petrochemical Complex — also India's first such Greenfield refinery — has opened at Pachpadra in the Balotra district of Rajasthan, and uses fractional distillation as its core process.

Paper Chromatography

Paper chromatography separates the colour components of a mixture, based on the principle that different substances move at different rates. It's used to separate colours from ink, plant pigments, and food colour pigments.

Method: Take a plain tissue paper and draw a line on it. Place dots of the colour mixture on that line. Dip the paper into water, but only up to just below the line — not the line itself. As water rises through the tissue paper, it carries different colour components at different rates based on their properties, causing them to separate visibly along the paper.

Separation Techniques for Heterogeneous Mixtures

Separation of Immiscible Liquids (Decantation)

"Immiscible" means substances that cannot be mixed — they don't combine to form a uniform mixture. When two immiscible liquids are combined, they form separate layers based on density.

Examples of immiscible/heterogeneous combinations: oil and water, sand and water, iron filings and sulfur.

NCERT Activity: Mixing 5 mL of mustard oil with 20 mL of water forms two distinct layers. This mixture is separated using a separating funnel, which has a stopcock at the bottom. The denser liquid (water) is released first through the stopcock into one container; once the boundary between the layers reaches the stopcock, it's closed, the container is switched, and the remaining oil is collected separately. This overall method is called decantation.

Sublimation

Sublimation separates two solid substances where one of them is a sublimate — a substance that converts directly from the solid state to the gas state, without passing through the liquid state, when heated below its melting point.

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

Activity: Camphor is mixed with sand in a china dish. On heating, the camphor sublimates directly into gas and rises, eventually depositing back as a solid on a cotton plug placed above (or on the flask walls) as it cools. The sand, which doesn't sublimate, remains behind in the dish. The deposited camphor is then collected.

💡 Note on Dry Ice: Dry ice is solid carbon dioxide. It never converts directly into a liquid under normal conditions — heating turns it straight into gas, and pressurising the gas converts it back to solid, not liquid.

Centrifugation

Centrifugation is used to separate suspensions and colloids with relatively larger particles, using high-speed spinning. The spinning generates different radial (outward) forces on substances of different densities — heavier substances are pushed further outward and settle toward the bottom, while lighter substances remain closer to the top, forming layers based on density.

During centrifugation, the tubes holding the mixture become horizontal due to the spinning motion.

Example — Blood Test: When a blood sample is centrifuged, it separates into distinct layers — plasma, white blood cells, and red blood cells — which can then be examined or separated individually.

Coagulation

Coagulation involves fine, small particles clumping together to form a larger mass called a sludge, with the help of a substance called a coagulant.

Example — Purifying Muddy Water: Powdered alum (called "phitkari," specifically potash alum) is added to muddy water. The alum causes the suspended mud particles to clump together into a sludge, which settles at the bottom due to gravity (a process called sedimentation). The clear water above can then be separated by decantation or filtration. This method is especially common during the monsoon season, when water from taps, wells, and rivers often turns muddy.

Example — Making Paneer: Warm (not boiling, not cold) milk has vinegar or an acidic juice added to it. The acid causes coagulation — the milk proteins clump together while water separates out. The clumped milk solids are strained to remove moisture and water, then pressed into shape to form paneer.

The Tyndall Effect

The Tyndall effect is the scattering of light by particles in a mixture, which makes the path of the light beam visible. It is shown by colloids and suspensions, but not by true solutions, since a solution's particles are too small to scatter light this way.

A common everyday example is seeing beams of light streaming through a window into a dusty room or stadium — the dust particles scatter the light, making its path visible.