Exploring Mixtures and Their Separation Class 9 PPT Free

• A mixture forms when two or more substances are simply mixed together
• They are not joined chemically, so each keeps its own properties
• The components can be present in any ratio. Examples: sugar in water, salt in water and sand in water
• A mixture can be separated back into its parts by physical methods

• Most natural materials are mixtures, not pure substances
• We separate them to obtain one pure and useful component: sugar is separated from sugarcane, dirty water is separated to give safe drinking water, and blood is separated to run life-saving medical tests

Group A mixes salt, Group B mixes chalk powder and Group C mixes milk, each in water. Stir each mixture and note whether the particles are visible. Shine a laser beam through each and observe from the side (never look directly into the laser beam, as it harms the eyes). Leave them undisturbed and predict which mixtures will settle.

Think it over: why do mud particles settle in water over time, but milk stays mixed throughout? Because mud particles are large and heavy enough to settle, whereas the fat droplets in milk are so small that they stay spread out.

A homogeneous mixture is also called a solution.

• A solution is made of a solute and a solvent
• The solute is the substance that dissolves, present in the smaller amount
• The solvent does the dissolving and is present in the larger amount
• In sugar-water, sugar is the solute and water is the solvent. Solute plus solvent makes a solution

Gas particles move freely, so gases usually mix evenly and completely, and most mixtures of gases are homogeneous (for example, hydrogen and oxygen used as rocket fuel). But smoke, fog and dust in air are heterogeneous mixtures, because solid or liquid particles are spread unevenly in the gas.

• Concentration is the amount of solute in a given amount of solvent or solution. It tells us how strong or weak a solution is
• The correct proportion is always essential when preparing a solution. ORS (Oral Rehydration Solution) is an exact mixture of salt and sugar dissolved in clean water, and it replaces the water and salts the body loses during diarrhoea. If the concentration is wrong, the ORS will not work properly
• Farmers must mix the right amount of pesticide, or crops may be harmed

Mass of solution = mass of solute + mass of solvent.

• Solubility is the maximum amount of solute that dissolves in 100 g (or 100 mL) of solvent, and it is always stated at a particular temperature
• The solubility of most solids increases as the temperature rises, while the solubility of gases in a liquid decreases as the temperature rises
• Solubility helps us separate substances from their mixtures

• A saturated solution cannot dissolve any more solute at that temperature, and any extra solute stays undissolved at the bottom
• Solubility depends on temperature, so the temperature is always mentioned
• Heating lets a saturated solution dissolve even more solute, while cooling a saturated solution makes the extra solute separate out

A graph of solubility against temperature is called a solubility curve. The x-axis shows temperature and the y-axis shows the solubility per 100 g of water. A steep curve means the solubility rises sharply with temperature. The curve helps predict how much solute separates on cooling.

Potassium nitrate dissolves far more at higher temperatures (from 20 g at 10 °C to 110 g at 60 °C), so it crystallizes the most on cooling. Sodium chloride changes very little with temperature (35.8 g to 37.3 g).

The method depends on the properties of the components.

• Homogeneous mixtures: evaporation, crystallization, distillation and chromatography
• Immiscible liquids and solids: separating funnel, magnet and filtration
• Subliming solids: sublimation
• Fine suspended solids: centrifugation, coagulation and sedimentation

The solution is heated so that the liquid solvent escapes as vapour, and the dissolved solid is left behind (for example salt from salt water). It differs from boiling: evaporation is slow and happens only at the surface, whereas boiling is fast and takes place throughout the liquid. Unlike distillation, the solvent is lost and is not recovered.

• It forms pure crystals from a saturated solution on cooling. A crystal has its particles arranged in a regular geometric pattern
• It separates a soluble solid present in a small quantity, and is also used to purify a solid and remove impurities
• It works because solubility changes with temperature

1. Dissolve copper sulfate in warm water with a drop of dilute acid
2. Add more solute until the solution becomes saturated
3. Filter the hot solution to remove insoluble impurities
4. Cool the filtrate slowly and undisturbed to grow large blue crystals

Safety first: copper sulfate is toxic, so use adult supervision.

• Salt from seawater: seawater is evaporated to a saturated solution, and crystals then form
• The ancient Indian Karkatch salt was made by evaporating sea water in the sun; concentrated sea brine produced salt crystals of different sizes
• Large crystals also form naturally inside mines and caves, growing very slowly over extremely long periods. Quartz is one of the most beautiful natural crystals, and the Mawsmai Cave in Sohra (Cherrapunji) has fascinating formations

• It separates two miscible liquids using their different boiling points
• The liquid with the lower boiling point vaporises first (liquid → vapour → liquid)
• The vapour is cooled in a condenser and collected as a pure liquid, called the distillate
• It works when the boiling points differ by at least about 25 °C, and it can also recover a liquid from a solution of dissolved solids

A large difference in boiling points allows simple distillation (for example, separating acetone and water). A small difference (under 25 °C) needs fractional distillation, which separates the fractions of crude oil: petroleum gas (compressed into LPG), petrol, kerosene, diesel, lubricating oil and bitumen.

India’s contribution: Mitti ka ittar. Distillation captures fragrances from flowers, and the traditional deg-bhapka distillation method of Uttar Pradesh turns the earthy smell after the first rain into a natural perfume.

• It separates the coloured components of a mixture, such as ink dyes
• A solvent rises through the paper and carries the components with it; components that move faster travel higher up the paper
• This separates them into distinct coloured bands. The word means “writing with colour” (from chroma and graphein)

Activity 5.5: draw a pencil line 2 cm from the bottom of a paper strip, place a spot of black sketch-pen ink at the centre of the line, and dip the lower end in water, keeping the ink spot above the water. As the water rises, it carries the ink up the paper, and the ink separates into several different colour spots.

Immiscible liquids do not mix to form one layer; the lighter liquid floats on top of the heavier. A separating funnel separates them: oil forms the upper layer and water the lower, and the lower layer is run off through the stopcock.

It is used when one solid in the mixture is magnetic. A magnet attracts and pulls out the magnetic component, for example iron filings from sulfur powder, or iron pieces from a heap of sawdust.

• Some solids change directly into vapour without becoming a liquid. On cooling, the vapour changes back into a solid, which is called deposition
• Camphor sublimes on heating, but sand does not. So camphor separates and deposits on the cool funnel walls. Naphthalene and dry ice also sublime
• Sublimation versus evaporation: in sublimation a solid changes directly into vapour (camphor, naphthalene), whereas in evaporation a liquid changes into vapour at its surface (water, spirit)

An alloy is a homogeneous mixture of metals, or a metal and a non-metal, made by melting, mixing and then cooling to solidify. Physical methods cannot separate the components of an alloy. Brass is mainly copper and zinc, bronze is mainly copper and tin, and steel is iron with other elements.

• In a suspension, the solid particles do not dissolve but stay spread in the liquid; they are large enough to be seen with the naked eye, and settle down when the mixture is left undisturbed. It is a heterogeneous mixture. Examples: sand in water, chalk in water and tea leaves in water
• Filtration separates an insoluble solid from a liquid. The solid that stays on the paper is the residue, and the clear liquid that passes through is the filtrate. It works only when the particles are larger than the paper pores

• Centrifugation separates very fine particles by spinning the mixture at high speed. The spinning creates an outward force: the heavier particles move outward and settle at the bottom of the tube, and the lighter liquid stays on top. It is used to separate the components of blood and milk
• The Paperfuge is a simple, hand-powered, low-cost device inspired by a spinning toy. It works on the same idea as a laboratory centrifuge and spins blood samples at very high speed without any electricity, which helps detect malaria and anaemia in remote areas
• Centrifuged blood separates into layers by density: the top layer is plasma (the lightest), the middle layer holds platelets and white blood cells, and the bottom layer is the red blood cells (the heaviest)

A coagulant is added to make fine suspended particles clump together. The clumps grow heavy and settle down by gravity (sedimentation), and the clear water is then removed by decantation or filtration. Powdered alum (fitkari) is added to muddy water as a coagulant, and making paneer from milk uses lemon juice as a coagulant.

• A colloid is neither a true solution nor a true suspension. Its particles are very small and do not settle on standing, and they stay evenly spread throughout the mixture. Examples: milk, blood, tomato sauce and ice cream
• A colloid has two parts: the dispersed phase (the tiny scattered particles) and the dispersion medium (the substance they are spread through). In milk, tiny fat droplets are the dispersed phase in a water medium

The Tyndall effect is the scattering of a beam of light by particles in a colloid or suspension, which makes the path of the light beam visible. It is seen when sunlight enters a dusty room through a small gap. It was first explained by the scientist John Tyndall. A true solution does not show it, because its particles are too small to scatter light.

• An emulsion is a colloid in which both parts are liquids
• Oil-in-water emulsions include milk and vanishing cream; water-in-oil emulsions include butter, body lotion and cold cream
• An emulsifying agent keeps the two liquids mixed and stable. Proteins in milk act as natural emulsifying agents

Our kidneys separate and remove waste from the blood. Sewage is treated by sedimentation, coagulation and filtration. Dry waste is recycled and wet waste is composted. Better separation makes the world cleaner and more sustainable.

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