Solute, solvent and solution
A spoon of sugar stirred into tea seems to vanish, yet the last sip is as sweet as the first. A glass of nimbu pani tastes the same from top to bottom.
A solution is a uniform mixture. The part that dissolves is the solute and the part that dissolves it is the solvent. Solute + solvent = solution.
Key idea 1: Solute and solvent — When a solid dissolves in a liquid, the solid is the solute and the liquid is the solvent. When two liquids mix, the one present in the smaller amount is called the solute and the other the solvent. In a thick sugar syrup there is far more sugar than water, yet water is still called the solvent.
- Easy: In salt water, salt is the solute and water is the solvent.
- Medium: When 5 mL of ink is mixed into 200 mL of water, the ink is the solute.
- Exam-type: A syrup has 300 g of sugar in 100 mL of water; water is still the solvent and sugar the solute.
In the figure: The solute disappears into the solvent and the result looks the same everywhere.
Key idea 2: Solution or not? — Every drop of a solution looks and tastes the same. Sugar or salt in water gives a solution. Chalk powder, sand or sawdust in water does not, because the bits are not spread evenly and they settle. A mixture of gases like air is also uniform, so it is a gaseous solution. Water is a good solvent because it dissolves many things, such as sugar and salt, though not oil.
- Easy: Salt in water is a solution; sand in water is not.
- Medium: Air is a solution of gases because it is a uniform mixture.
- Exam-type: Chalk powder in water gives a non-uniform mixture, so it is not a solution.
In the figure: Mixtures that look the same everywhere are solutions; the ones with settling bits are not.
A solute dissolves in a solvent to make a solution, and a solution is always uniform.
Calling the liquid the solute. When a solid dissolves in a liquid, the solid is the solute.
Solid that dissolves is the solute. Memory clue: Solute Slips in, Solvent Swallows it.
Spot the difference
| Feature | Solute | Solvent |
|---|---|---|
| job | gets dissolved | does the dissolving |
| amount (two liquids) | smaller | larger |
| example | sugar | water |
Practice questions
- 1What is a solution?
- 2In salt water, name the solute and the solvent.
- 3Is chalk powder mixed in water a solution?
- 4Why does every sip of lemonade taste the same?
- 5Ink is mixed with a large amount of water. Which one is the solute?
- 6A thick syrup has more sugar than water. Which is still called the solvent?
- 7Say whether each is a solution: tea with sugar, sawdust in water, air, sand in water.
- 8A student says that the substance present in the larger amount is always the solvent. Is this always true?
Show answers (try them yourself first)
- 1A uniform mixture of a solute dissolved in a solvent.
- 2Salt is the solute and water is the solvent.
- 3No. The chalk is not spread evenly and settles, so it is a non-uniform mixture.
- 4Because lemonade is a solution, in which the parts are spread evenly.
- 5The ink, because it is present in the smaller amount.
- 6Water, because a solid dissolving in a liquid is the solute and the liquid is the solvent.
- 7Tea with sugar: yes; sawdust in water: no; air: yes; sand in water: no.
- 8No. For two liquids it is true, but when a solid dissolves in a liquid, the liquid is the solvent even if the solid is more, as in syrup.
Saturation, concentration and solubility
After a few spoons of sugar in a small cup of tea, the next spoon just sits at the bottom. A hot cup of tea can hold more sugar than a cold glass of water.
Solubility is the largest amount of a solute that dissolves in a fixed amount of solvent at a given temperature.
Key idea 1: Unsaturated and saturated — In an unsaturated solution more solute can still dissolve. In a saturated solution no more solute can dissolve at that temperature, so any extra settles at the bottom.
- Easy: If the added salt settles at the bottom, the solution is saturated.
- Medium: If one more spoon of sugar dissolves, the solution was unsaturated.
- Exam-type: A solution with undissolved solid at the bottom is saturated at that temperature because it cannot dissolve any more.
In the figure: The first beaker can take more solute, the second is full, and in the third the extra solute settles at the bottom.
Key idea 2: Concentration and temperature — Concentration is the amount of solute in a fixed amount of solution. A solution with less solute is dilute and one with more is concentrated; these words compare two solutions. For most solids, solubility rises when the temperature rises, so warm water dissolves more.
- Easy: More solute in the same amount of water means a more concentrated solution.
- Medium: 3 spoons of sugar in 100 mL of water is more concentrated than 3 spoons in 200 mL.
- Exam-type: Warming a saturated solution with solid at the bottom usually dissolves that solid, because solubility rises with temperature.
In the figure: The warmer the water, the more spoons of sugar it can hold.
Solubility depends on the solute, the solvent and the temperature.
Calling a solution saturated just because it is very sweet or salty. Saturated means no more solute can dissolve.
Compare the solute in the same amount of solution. Memory clue: Saturated = Stuffed, nothing more fits.
Spot the difference
| Feature | Unsaturated | Saturated |
|---|---|---|
| more solute dissolves | yes | no |
| solid at the bottom | no | yes, if extra is added |
| limit | not reached | reached |
Practice questions
- 1Define solubility.
- 2What is a saturated solution?
- 3Which is more concentrated: 1 spoon or 3 spoons of salt in the same water?
- 4Why does extra salt settle at the bottom of the glass?
- 5Which is more concentrated: 3 spoons of sugar in 100 mL or 3 spoons in 200 mL of water?
- 6How does warming the water usually change how much solid dissolves?
- 7A glass has undissolved sugar at the bottom. How can you dissolve it without adding water?
- 8Is a dilute solution the same as an unsaturated solution? Explain.
Show answers (try them yourself first)
- 1The largest amount of solute that dissolves in a fixed amount of solvent at a given temperature.
- 2A solution that cannot dissolve any more solute at that temperature.
- 33 spoons of salt.
- 4The water has reached its limit and cannot dissolve any more salt.
- 53 spoons in 100 mL, because the same solute is in less water.
- 6It usually dissolves more, because solubility of most solids rises with temperature.
- 7Warm the water and stir, because the solubility rises.
- 8No. Dilute and concentrated compare the amount of solute; unsaturated and saturated tell whether more can dissolve.
Density and relative density
A small iron nail sinks, yet a big wooden plank floats. A bag of cotton and a bag of rice may be the same size, but the rice bag feels much heavier.
Density is the mass present in a unit volume of a substance. Density = mass ÷ volume. It is measured in kg/m³ or g/cm³. Relative density is the density of a substance divided by the density of water; it has no unit.
Key idea 1: Finding density — Divide the mass by the volume. A block of 120 g with a volume of 40 cm³ has a density of 3 g/cm³. Density does not depend on size, so each half of the block also has 3 g/cm³. Water has a density of about 1 g/cm³.
- Easy: A mass of 120 g in 40 cm³ gives a density of 3 g/cm³.
- Medium: A material of density 2.5 g/cm³ with a mass of 500 g has a volume of 200 cm³.
- Exam-type: Block A is 150 g in 50 cm³ and block B is 90 g in 45 cm³; their densities are 3 and 2 g/cm³, so A is denser.
In the figure: Two measurements and one division give the density.
Key idea 2: Float or sink, and relative density — An object floats in a liquid if it is less dense than the liquid, and sinks if it is denser. Relative density tells how many times denser than water a substance is. If it is more than 1 the object sinks in water; if less than 1 it floats.
- Easy: Wood floats on water because it is less dense than water.
- Medium: A block of density 3 g/cm³ has a relative density of 3, so it sinks in water.
- Exam-type: A pack of oil is 910 g in 1 litre, so its density is 0.91 g/mL, less than water, and it floats on water.
In the figure: The wood is less dense than water so it floats; the iron is denser so it sinks.
Density is mass per unit volume, and relative density has no unit.
Thinking heavy objects always sink. A big log floats while a small coin sinks; it is density that decides.
Less dense floats, more dense sinks. Memory clue: Denser Dives, Lighter Lifts.
Spot the difference
| Feature | Mass | Density |
|---|---|---|
| tells | amount of matter | mass in each unit volume |
| depends on size | yes | no |
| unit | g, kg | g/cm³, kg/m³ |
Practice questions
- 1Write the formula for density.
- 2State the SI unit of density.
- 3Does relative density have a unit?
- 4Find the density of a block of 240 g and volume 80 cm³.
- 5A wooden block has a density of 0.6 g/cm³. Does it float or sink in water?
- 6Why does half of a block have the same density as the whole block?
- 7A metal of density 5 g/cm³ has a mass of 350 g. Find its volume.
- 8Object X is 200 g in 50 cm³ and object Y is 180 g in 30 cm³. Which is denser? Give both relative densities.
Show answers (try them yourself first)
- 1Density = mass ÷ volume.
- 2kg/m³.
- 3No, it has no unit.
- 4240 ÷ 80 = 3 g/cm³.
- 5It floats, because 0.6 is less than 1.
- 6Both the mass and the volume become half, so their ratio stays the same.
- 7350 ÷ 5 = 70 cm³.
- 8X is 4 g/cm³ and Y is 6 g/cm³, so Y is denser; their relative densities are 4 and 6.
Density and relative density
In the app you can move this figure with your finger.
Open in the appMeasuring mass and volume
A shopkeeper weighs rice on a scale and a milkman measures milk in a litre measure. Both are measuring, but one finds mass and the other finds volume.
Mass is the amount of matter in an object, measured with a balance in g or kg. Volume is the space an object takes up; for liquids it is measured with a measuring cylinder in mL, where 1 mL = 1 cm³ and 1 L = 1000 mL. The SI unit of volume is m³.
Key idea 1: Using a balance — Make sure a digital balance reads zero. Put the empty container on it and press the tare button so the reading goes back to zero. Then add the substance, and the reading is its mass alone. Mass is the amount of matter in kg, while weight is the pull of the Earth in newtons; they are different.
- Easy: A balance measures mass in g or kg.
- Medium: Pressing tare removes the mass of the container so only the substance is measured.
- Exam-type: Mass is measured in kg and weight in newtons, so the two are different quantities.
In the figure: Taring comes before adding the substance.
Key idea 2: Reading volume — Water curves in a measuring cylinder; this curve is the meniscus. Keep the eye level with it and read the bottom of the curve for water and other colourless liquids, and the top for coloured liquids. The value of one small division is the gap between two big marks divided by the number of small parts. The volume of an irregular solid like a stone is the rise in water level when it is lowered in. A box has a volume of length × width × height.
- Easy: Keep your eye level with the meniscus while reading.
- Medium: The water rises from 30 mL to 38 mL, so the stone has a volume of 8 cm³.
- Exam-type: Two big marks are 10 mL apart with 5 small parts between them, so each small part is 2 mL.
In the figure: The rise in the water level is the volume of the stone.
Mass is measured with a balance in g or kg; volume with a measuring cylinder in mL; 1 mL = 1 cm³.
Reading the meniscus from above or below. Keep the eye level with it.
For clear liquids read the bottom of the curve with eyes straight. Memory clue: Eyes straight, Bottom of the Bend.
Spot the difference
| Feature | Mass | Volume |
|---|---|---|
| tool | balance | measuring cylinder |
| common unit | g, kg | mL, cm³, L |
| SI unit | kg | m³ |
Practice questions
- 1Which instrument measures mass?
- 21 mL is equal to how many cm³?
- 3What is the curved surface of a liquid in a cylinder called?
- 4Why do we press tare after placing the container on the balance?
- 5A stone raises the water level from 25 mL to 31 mL. Find its volume.
- 6Find the volume of a box 8 cm long, 5 cm wide and 3 cm high.
- 7Two big marks are 20 mL apart with 4 small parts between them. Find one small part, and the reading 3 small parts above 40 mL.
- 8A stone of mass 60 g raises the water level from 20 mL to 40 mL. Find its density.
Show answers (try them yourself first)
- 1A balance.
- 21 cm³.
- 3The meniscus.
- 4So that the reading shows only the mass of the substance.
- 531 − 25 = 6 cm³.
- 68 × 5 × 3 = 120 cm³.
- 7One small part is 20 ÷ 4 = 5 mL; the reading is 40 + 15 = 55 mL.
- 8Volume = 40 − 20 = 20 cm³; density = 60 ÷ 20 = 3 g/cm³.
Effect of temperature and pressure on density
The air just above a heater rises. A sealed balloon left in hot sunshine swells a little. A hot air balloon floats up into the sky.
When a substance is heated, its volume usually increases while its mass stays the same, so its density decreases. Cooling does the opposite. Pressure changes the density of gases a lot, but of liquids and solids very little.
Key idea 1: Temperature and density — On heating, the particles move apart, the volume rises and the density falls. Hot air is less dense than the cool air around it, so it rises; this is why a hot air balloon floats. On cooling, the particles come closer and the density rises.
- Easy: Heating a substance lowers its density, and its mass does not change.
- Medium: Hot air above a candle rises because it is less dense than the cool air around it.
- Exam-type: A sealed balloon swells in the sun; its mass is the same but its volume is larger, so its density is lower.
In the figure: More space for the same mass means lower density.
Key idea 2: Pressure and water — Squeezing a gas pushes its particles closer, so its density rises; liquids are almost incompressible, so pressure hardly changes their density. Water is densest at 4 °C, and ice is less dense than water, so ice floats. A layer of floating ice on a lake keeps the water below liquid, so fish can survive. Inside the Earth, pressure and temperature rise and the layers get denser towards the centre.
- Easy: Squeezing a gas to half its volume doubles its density.
- Medium: Ice floats because it is less dense than liquid water.
- Exam-type: Water is densest at 4 °C, and the floating ice cover keeps the lake water below from freezing, so fish survive.
In the figure: Ice is less dense than water, and water is less dense than seawater.
Heating lowers density and cooling raises it, while the mass stays the same.
Thinking heating changes the mass. Only the volume changes.
Heat spreads particles, so density goes down. Memory clue: Hot makes room, density Low; Cold squeezes close, density Climbs.
Spot the difference
| Change | Volume | Density |
|---|---|---|
| heating | increases | decreases |
| cooling | decreases | increases |
| squeezing a gas | decreases | increases |
Practice questions
- 1What happens to the density of a substance when it is heated?
- 2Why does hot air rise?
- 3At what temperature is water the densest?
- 4A gas of fixed mass is squeezed to half its volume. What happens to its density?
- 5Why does ice float on water?
- 6Why does pressure change the density of a liquid very little?
- 7A sealed balloon is carried from a cool room into hot sunshine. What happens to its mass, volume and density?
- 8How does a layer of floating ice help the fish in a freezing lake?
Show answers (try them yourself first)
- 1It decreases.
- 2It is less dense than the cool air around it.
- 34 °C.
- 4It doubles.
- 5Ice is less dense than liquid water.
- 6Liquids are almost incompressible, so their volume hardly changes.
- 7The mass stays the same, the volume increases and the density decreases.
- 8The ice layer keeps the water below from freezing, so the fish have liquid water to live in.