Homogeneous and heterogeneous mixtures; solutions, suspensions and colloids
Stir a spoon of sugar into tea and the tea tastes equally sweet in every sip. Stir a spoon of mud into water and you can see the grains drifting and soon sinking. Both are mixtures, but they behave very differently.
A homogeneous mixture has the same composition throughout, and a heterogeneous mixture does not. A homogeneous mixture of a solute in a solvent is a solution. In a solution the particles are smaller than 1 nm, so they cannot be seen, never settle and pass through filter paper. In a colloid the particles are 1 to 1000 nm: they stay spread out but scatter light. In a suspension the particles are larger than 1000 nm: they are visible, settle down and stay on filter paper.
In the figure : Particle size decides whether a mixture is a solution, a colloid or a suspension.
Rule 1: Judge by particle size — Below 1 nm is a solution, 1 to 1000 nm is a colloid and above 1000 nm is a suspension.
- Easy : Salt in water has tiny particles, so it is a solution.
- Medium : Milk has medium particles, so it is a colloid.
- Exam-type : Chalk powder in water has large particles, so it is a suspension.
Rule 2: Test by settling, filtering and light — Ask three questions: does it settle, does it stay on filter paper, does it scatter light?
- Easy : A mixture that settles and is caught by filter paper is a suspension.
- Medium : A mixture that does not settle but scatters light is a colloid.
- Exam-type : A clear mixture that does not settle and does not scatter light is a solution.
Size, settling, filtering and light: four checks tell the three types apart.
Thinking that a mixture that looks uniform must be a true solution. Milk looks uniform but is a colloid.
Memory clue: S-M-L = Small, Medium, Large — under 1 nm is a solution, 1 to 1000 nm a colloid, over 1000 nm a suspension.
Spot the difference
| Property | Solution | Suspension |
|---|---|---|
| Particle size | Below 1 nm | Above 1000 nm |
| Settles on standing | No | Yes |
| Stays on filter paper | No | Yes |
Practice questions
- 1What is a homogeneous mixture?
- 2Name the size range of particles in a colloid.
- 3What is the solute in tea with sugar?
- 4Why does milk not settle but muddy water does?
- 5Why is a solution called a homogeneous mixture?
- 6How can filter paper help you tell a suspension from a solution?
- 7A mixture has particles of diameter 0.5 nm. What type is it?
- 8A mixture has particles of diameter 5000 nm. What type is it and what happens to it on standing?
Show answers (try them yourself first)
- 1A mixture with uniform composition throughout.
- 21 nm to 1000 nm.
- 3Sugar.
- 4Milk particles are small and stay dispersed, while mud particles are big and sink.
- 5Because its composition is the same everywhere.
- 6A suspension leaves residue on the paper, a solution does not.
- 7A solution.
- 8A suspension; its particles settle down.
Tyndall effect and the parts of a colloid
Sunlight entering a dark room through a small hole shows a bright dusty beam, yet the same light across the clear air is invisible. The dust in the beam is making the light visible.
The Tyndall effect is the scattering of light by the particles of a mixture. It occurs in colloids and suspensions but not in true solutions. A colloid has two parts: the dispersed phase (the tiny particles) and the dispersion medium (what they are spread in). A colloid with a liquid dispersed in a liquid is an emulsion. Milk is oil-in-water, butter is water-in-oil, and an emulsifying agent such as soap or milk protein keeps an emulsion stable.
In the figure : Particles that are big enough scatter light and make its path visible.
Rule 1: Scattering needs particles that are big enough — Particles below 1 nm are too small to scatter light, so a solution shows no beam.
- Easy : A torch beam is not seen in clear sugar water.
- Medium : The beam is seen in starch and water.
- Exam-type : The beam of a stadium floodlight is seen because of particles in the air.
Rule 2: Name the two parts of a colloid — The spread-out particles are the dispersed phase, and the material around them is the dispersion medium.
- Easy : In fog, water droplets are the dispersed phase and air is the medium.
- Medium : In blood, plasma is the medium and cells are dispersed in it.
- Exam-type : In an oil-in-water emulsion such as milk, oil droplets are dispersed in water.
Beam visible means particles are scattering; name the dispersed phase and the dispersion medium.
Believing the Tyndall effect appears in every clear liquid. A true solution does not scatter light.
Memory clue: T-C-S — Tyndall shows in Colloid and Suspension, Solution says no.
Spot the difference
| Mixture | Beam visible? | Reason |
|---|---|---|
| Salt solution | No | Particles too small |
| Milk (colloid) | Yes | Particles scatter light |
| Muddy water | Yes | Large particles scatter light |
Practice questions
- 1What is the Tyndall effect?
- 2Name the two parts of a colloid.
- 3What is an emulsion?
- 4Why is the path of a laser seen in milk but not in sugar solution?
- 5Why do smoky cities look hazy?
- 6Why is soap added when oil is shaken with water to make an emulsion?
- 7In fog, name the dispersed phase and the dispersion medium.
- 8A student finds the beam visible in two beakers and not visible in a third. Which beaker is likely a true solution?
Show answers (try them yourself first)
- 1The scattering of light by particles.
- 2Dispersed phase and dispersion medium.
- 3A colloid in which a liquid is spread in another liquid.
- 4Milk has colloidal particles that scatter light; sugar solution has particles too small.
- 5Smoke and dust particles in the air scatter light.
- 6Soap acts as an emulsifying agent and keeps the layers from separating.
- 7Dispersed phase: water droplets; medium: air.
- 8The third beaker, where the beam is not visible.
Concentration of a solution (percentage methods)
Tea that is too sweet or too weak tells us that how much is in the cup matters, not only what is in it. A doctor, a farmer and a cook all have to measure out the right proportion.
Concentration is the amount of solute dissolved in a given amount of solution or solvent. We often show it as a percentage. Mass by mass percentage (% m/m) = (mass of solute ÷ mass of solution) × 100. Mass by volume percentage (% m/v) = (mass of solute in g ÷ volume of solution in mL) × 100, used for medicines. Volume by volume percentage (% v/v) = (volume of solute ÷ volume of solution) × 100, used for liquids that mix.
In the figure : Percentage concentration compares the solute with 100 parts of solution.
Rule 1: Mass of solution = solute + solvent — Always add the solute to the solvent before dividing.
- Easy : 20 g sugar in 80 g water gives 100 g of solution, so 20% m/m.
- Medium : 9 g salt in 141 g water gives 150 g of solution, so 6% m/m.
- Exam-type : 4 g of drug in 36 g of water gives 40 g of solution, so 10% m/m.
Rule 2: Choose the method that matches what you can measure — Use % m/m for solids and mixtures weighed in grams, % m/v when you measure the volume of a liquid, and % v/v for two liquids.
- Easy : A talcum powder label uses % m/m.
- Medium : A 0.9% m/v saline drip has 0.9 g of salt in every 100 mL.
- Exam-type : A 5% v/v pesticide spray has 5 mL of pesticide in every 100 mL of spray.
Write the formula, put the right quantity on top and bottom, then multiply by 100.
Dividing by the mass of the solvent instead of the mass of the whole solution.
Memory clue: Solute over Solution × 100 — 'S over S times 100'.
Spot the difference
| Method | Formula | Used for |
|---|---|---|
| % m/m | g of solute ÷ g of solution × 100 | Solids and food labels |
| % m/v | g of solute ÷ mL of solution × 100 | Medicines |
| % v/v | mL of solute ÷ mL of solution × 100 | Mixed liquids |
Practice questions
- 1What is concentration?
- 2Write the formula for % m/m.
- 3Which percentage method is used for medicines?
- 4Why must the proportion be right in a medicine solution?
- 5Why is a drink with some sugar and salt not always ORS?
- 6Why do we divide by the mass of solution and not of the solvent?
- 7A student mixes 12 g sugar in 88 g water. Find the % m/m.
- 8How much medicine is in 250 mL of a 4% m/v solution?
Show answers (try them yourself first)
- 1The amount of solute dissolved in a given amount of solution or solvent.
- 2(mass of solute ÷ mass of solution) × 100.
- 3Mass by volume percentage (% m/v).
- 4Too little may not work and too much may harm.
- 5ORS needs fixed amounts of salt and sugar in a fixed amount of water.
- 6Because the percentage is defined per 100 parts of the whole solution.
- 712%.
- 810 g.
Solubility, saturated solutions and solubility curves
A cup of hot tea can hold many spoons of sugar, but cold tea soon refuses to take any more. The amount a liquid can hold depends on its temperature.
Solubility is the maximum amount of solute that dissolves in a fixed amount of solvent (usually 100 g) at a given temperature. A solution that cannot dissolve more solute at that temperature is saturated. For most solids the solubility rises with temperature, but for gases in liquids it falls. A graph of solubility against temperature is a solubility curve; a steep curve means the solubility changes a lot, and a flat curve means it changes little.
In the figure : Solubility depends on temperature and is read from solubility curves.
Rule 1: Always state the temperature with solubility — Solubility changes with temperature, so a value is meaningless without it.
- Easy : A solid with solubility 60 g at 40 °C holds 60 g in 100 g of water at that temperature.
- Medium : In 50 g of water the same saturated solution holds 30 g.
- Exam-type : At a higher temperature the same solid may dissolve much more.
Rule 2: Read the curve to find how much will separate — The drop in solubility between two temperatures is the amount that comes out on cooling.
- Easy : From 80 g at 60 °C to 30 g at 20 °C, 50 g separates from 100 g of water.
- Medium : A flat curve gives almost no solid on cooling.
- Exam-type : A steep curve gives a large amount of solid on cooling.
Read solubility at each temperature, then subtract to find the solid that separates.
Thinking that every substance dissolves more when heated. Gases dissolve less, and some solids change very little.
Memory clue: Solids go up, gases go down as temperature goes up.
Spot the difference
| Substance | Effect of heating | Curve |
|---|---|---|
| Most solids | More soluble | Rises |
| Common salt | Almost no change | Flat |
| Gases in liquid | Less soluble | Falls |
Practice questions
- 1What is solubility?
- 2What is a saturated solution?
- 3How does heating affect the solubility of most solids?
- 4Why is a cold drink with dissolved gas flatter when warm?
- 5Why must the temperature be given with a solubility value?
- 6How can a solubility curve show how much solid will separate on cooling?
- 7A salt has solubility 40 g at 20 °C. How much of it makes a saturated solution in 250 g of water at 20 °C?
- 8A solid has solubility 90 g at 70 °C and 40 g at 30 °C. How much separates from a saturated solution made with 100 g water when cooled from 70 °C to 30 °C?
Show answers (try them yourself first)
- 1The maximum solute that dissolves in a fixed amount of solvent at a given temperature.
- 2One that cannot dissolve any more solute at that temperature.
- 3It increases the solubility.
- 4Gases are less soluble in warm liquids, so the gas escapes.
- 5Because solubility changes with temperature.
- 6The drop in solubility between the two temperatures is the amount that separates.
- 7100 g.
- 850 g.
Solubility, saturated solutions and solubility curves
In the app you can move this figure with your finger.
Open in the appCrystallization
Rock candy (mishri) grows slowly in a pot of thick sugar syrup. Left alone for days, the syrup builds glittering sugar crystals on a thread.
A crystal is a solid whose particles are arranged in a regular geometric pattern. Crystallization is the process of forming crystals from a saturated solution, and it is used to purify solids. A hot saturated solution is first filtered to remove insoluble impurities, then cooled slowly without disturbance so that large, well-shaped crystals grow. The crystals are filtered, rinsed with a little cold water and dried. The method works because solubility falls on cooling.
In the figure : Slow cooling of a saturated solution gives pure crystals.
Rule 1: Make a saturated solution and filter it hot — The solution is saturated at a high temperature and filtered to remove solids that did not dissolve.
- Easy : Copper sulfate is added to warm water until no more dissolves.
- Medium : Sand mixed with the salt remains on the filter paper.
- Exam-type : Only the dissolved substance passes into the filtrate.
Rule 2: Cool slowly and leave it undisturbed — Slow cooling gives the particles time to settle into a regular pattern and form big crystals.
- Easy : Slow cooling gives large shiny crystals.
- Medium : Quick cooling in ice gives tiny crystals.
- Exam-type : Shaking the beaker breaks the crystals and spoils their shape.
Saturate, filter hot, cool slowly, collect and dry.
Thinking crystallization means boiling the liquid until it is completely dry. Some solution is left behind so that soluble impurities stay dissolved.
Memory clue: S-F-C-D — Saturate, Filter, Cool, Dry.
Spot the difference
| Step | What happens | Reason |
|---|---|---|
| Hot filtration | Insoluble impurities removed | They do not dissolve |
| Slow cooling | Large crystals grow | Time to arrange |
| Rinse with cold water | Surface liquid removed | Crystals do not dissolve much |
Practice questions
- 1What is a crystal?
- 2Why is a hot saturated solution filtered?
- 3Why is the solution cooled slowly?
- 4Why does crystallization give purer solid than simple evaporation?
- 5Why do salt makers evaporate sea water in shallow pans?
- 6Why must the beaker not be disturbed while it cools?
- 7A saturated solution holds 150 g of solid at 70 °C and 90 g at 30 °C. How much solid crystallizes on cooling?
- 8Two hot saturated solutions of equal mass give 15 g and 40 g of crystals on cooling. Which solid's solubility depends more on temperature?
Show answers (try them yourself first)
- 1A solid whose particles are arranged in a regular geometric pattern.
- 2To remove insoluble impurities.
- 3To give the particles time to form large, well-shaped crystals.
- 4Soluble impurities stay in the leftover solution.
- 5Evaporation increases the concentration until crystals appear.
- 6Disturbance breaks the crystals and spoils their shape.
- 760 g.
- 8The one giving 40 g of crystals.
Distillation and fractional distillation
A glass of salty sea water can be turned into fresh drinking water without any filter, simply by boiling it and catching the steam on a cold surface.
Distillation separates a mixture of miscible liquids, or a liquid from a solution, by heating until the liquid with the lower boiling point vaporises, and then cooling the vapour in a condenser back to a liquid called the distillate. It works well when the boiling points differ by at least about 25 °C. When they differ by less than 25 °C, fractional distillation is used, as in a refinery that separates crude petroleum into petroleum gas, petrol, kerosene and diesel.
In the figure : The liquid with the lower boiling point vaporises first and is condensed.
Rule 1: Look at the boiling points — The liquid that boils at the lower temperature is collected first.
- Easy : Acetone (56 °C) comes over before water (100 °C).
- Medium : A liquid and a dissolved solid: the liquid distils over and the solid stays behind.
- Exam-type : Salt solution gives pure water as the distillate and salt in the flask.
Rule 2: Choose the method by the boiling point gap — A gap of at least 25 °C is suitable for simple distillation; a smaller gap needs fractional distillation.
- Easy : Alcohol (78 °C) and benzene (80 °C) are close, so they need fractional distillation.
- Medium : Crude oil is separated into fractions by fractional distillation.
- Exam-type : Mitti ka ittar is captured by the traditional distillation Deg-Bhapka method.
Heat, vaporise, condense, collect: the lower boiling point goes first.
Using distillation for liquids that do not mix. Immiscible liquids are separated with a separating funnel.
Memory clue: H-V-C-C — Heat, Vapour, Condense, Collect.
Spot the difference
| Part | Role | Example |
|---|---|---|
| Flask | Holds the mixture while heating | Salt solution |
| Condenser | Cools vapour to liquid | Water-cooled tube |
| Distillate | Pure condensed liquid | Pure water |
Practice questions
- 1What is distillation?
- 2What is the name of the liquid collected after condensing?
- 3What does the condenser do?
- 4Why can acetone be separated from water by distillation?
- 5Why is fractional distillation used for liquids with close boiling points?
- 6Why does salt stay behind when salt water is distilled?
- 7Liquid X boils at 62 °C and liquid Y boils at 98 °C. Which distils first and is simple distillation suitable?
- 8In fractional distillation of crude oil, which fraction rises higher in the column, the one with a lower or a higher boiling point?
Show answers (try them yourself first)
- 1Heating a liquid to vapour and cooling the vapour back to liquid.
- 2The distillate.
- 3It cools the vapour so that it turns back into liquid.
- 4Acetone boils at a much lower temperature than water, so it vaporises first.
- 5Because it separates liquids that boil close together.
- 6Salt does not vaporise, so it stays in the flask.
- 7X distils first; the gap is 36 °C, so simple distillation is suitable.
- 8The one with the lower boiling point.
Paper chromatography and separating immiscible liquids
A black sketch pen mark touched with a drop of water spreads into a rainbow of colours. The water is carrying different dyes at different speeds.
Paper chromatography separates the coloured substances of a mixture because they move up the paper at different rates when a solvent rises through it. The starting line is drawn with a pencil and the solvent level must be below the spot. Immiscible liquids, such as oil and water, form two layers and are separated with a separating funnel: the denser liquid forms the lower layer and is drained out first through the stopcock.
In the figure : Chromatography separates dyes by speed; a funnel separates liquids by layers.
Rule 1: In chromatography the solvent carries the dyes upwards — Dyes that move faster travel farther, so each one ends up in a different place.
- Easy : Black ink gives several coloured spots.
- Medium : Spinach extract can show green and yellow pigments.
- Exam-type : A pencil line is used because it does not run with the solvent.
Rule 2: Immiscible liquids settle by density — The denser liquid is the lower layer and is drained out first.
- Easy : Oil floats on water, so water is drained first.
- Medium : Close the stopcock before the upper layer reaches it.
- Exam-type : Liquids of equal density would not form clear layers.
Chromatography: spot above solvent, pencil line. Funnel: layers by density, drain the lower layer first.
Using a separating funnel for liquids that mix, or filtration for two liquids. Liquids pass through filter paper.
Memory clue: Pencil-Spot-Solvent for chromatography and Heavy goes Down for the funnel.
Spot the difference
| Technique | Based on | Separates |
|---|---|---|
| Paper chromatography | Different speeds of dyes | Coloured substances |
| Separating funnel | Different densities, no mixing | Oil and water |
| Filtration | Particle size | Solid from liquid |
Practice questions
- 1What does chromatography mean?
- 2Which apparatus separates two immiscible liquids?
- 3Which layer forms at the top when oil and water are mixed?
- 4Why is the starting line drawn in pencil?
- 5Why must the solvent level be below the sample spot?
- 6Why can oil and water be separated by a funnel but not by filter paper?
- 7One dye travels 8 cm and another 3 cm in the same time. Which moves faster?
- 8Kerosene (lighter than water) and water are in a funnel. Which layer is on top and which is drained first?
Show answers (try them yourself first)
- 1Writing with colour.
- 2A separating funnel.
- 3The oil layer.
- 4Pencil marks do not spread with the solvent as ink would.
- 5If the spot dips into the solvent, the dye washes away.
- 6Both liquids pass through filter paper, but a funnel separates the layers by density.
- 7The dye that travelled 8 cm.
- 8Kerosene is on top; water is drained first.
Sublimation, centrifugation, coagulation and alloys
A hand-held centrifuge toy, a pinch of camphor in a hot dish, and a pinch of alum in muddy water all separate mixtures in different ways. Each one uses a different property of the mixture.
Sublimation is the change of a solid directly into vapour on heating, as camphor and naphthalene do, so they can be separated from sand; the vapour turning back into a solid on cooling is deposition. Centrifugation spins a mixture so heavier particles move outwards and settle at the bottom, as when blood is separated into cells and plasma. Coagulation uses a coagulant such as alum to clump fine particles together so they settle. An alloy is a uniform mixture of metals, such as brass of about 80% copper and 20% zinc.
In the figure : Choose the method that uses a different property of each component.
Rule 1: Use the property that differs — Sublimation uses the ability to turn into vapour, centrifugation uses density, coagulation uses clumping, and a magnet uses magnetism.
- Easy : Naphthalene sublimes but sand does not.
- Medium : Iron filings are pulled out of sulfur by a magnet.
- Exam-type : Fine mud settles after alum makes it clump.
Rule 2: Plan the steps in order — For a mixture such as sand, salt and naphthalene, remove the easiest component first.
- Easy : Heat to remove naphthalene by sublimation.
- Medium : Add water and filter to keep the sand on the paper.
- Exam-type : Evaporate the filtrate to get the salt.
Match each component to the property that makes it different from the others.
Thinking an alloy can be separated by filtering. Its metals are mixed uniformly like a solution.
Memory clue: S-C-C-A — Sublimation, Centrifugation, Coagulation, Alloy.
Spot the difference
| Method | Principle | Example |
|---|---|---|
| Sublimation | Solid turns directly to vapour | Camphor from sand |
| Centrifugation | Spinning pushes heavy particles down | Blood components |
| Coagulation | Alum clumps fine particles | Clearing muddy water |
Practice questions
- 1What is sublimation?
- 2What is added to muddy water to coagulate the particles?
- 3What is an alloy?
- 4Why can camphor be separated from sand by heating?
- 5Why does a centrifuge push heavier particles to the bottom of the tube?
- 6Why can an alloy not be separated by filtering?
- 7A mixture contains sand, salt and naphthalene. State the order of the steps to get all three.
- 8100 g of brass contains about 80% copper. How much zinc is there?
Show answers (try them yourself first)
- 1The change of a solid directly into vapour on heating.
- 2Alum.
- 3A uniform mixture of two or more metals, or a metal and a non-metal.
- 4Camphor sublimes on heating while sand does not.
- 5Spinning produces an outward force that moves heavier particles outwards and down.
- 6Its metals are mixed uniformly like a solution.
- 7Heat to sublime the naphthalene, add water and filter to get the sand, then evaporate the filtrate to get salt.
- 820 g.