What a solution is: dissolving, surface tension and saturation
By the end of this lesson you will be able to
- Describe dissolving at the particle level, and define solute, solvent, solution and homogeneous mixture.
- Identify solutions in all three states — brass, air, seawater — and distinguish a solution from a mixture that merely looks mixed.
- Explain surface tension using the attraction between water molecules, and outline how a detergent breaks it.
- Define solubility and saturation, interpret a solubility curve, and calculate the mass that crystallises when a saturated solution cools.
- Explain why the solubility of most solids rises with temperature while the solubility of gases falls.
The puzzle
Stir a spoonful of sugar into half a glass of water — 100 g — at 20 °C. It disappears. Add another, and another; sugar takes a remarkable amount of beating, but somewhere past the fortieth heaped teaspoon, with about 200 g of sugar in, the next spoonful sits on the bottom and will not go, however long you stir. Now warm the glass and the pile shrinks. Meanwhile a steel needle laid gently on the surface of the same water floats, although steel is eight times denser than water; one drop of dishwashing liquid and it sinks at once. Where did the sugar go, why did the water stop taking it, and what was holding the needle up?
Predict what happens to the mass when 20 g of salt is dissolved in 100 g of water: is the solution 120 g, less, or more? Then predict whether the salt is still there as salt, and how you could get it back.
Compare your answer
Exactly 120 g: dissolving rearranges particles but destroys none, so mass is conserved. The salt is still there — as separated sodium and chloride ions spread among the water molecules, too small to see and too well mixed to settle. Evaporate the water (the sun does it at Sua Pan) and the ions rebuild their crystals, all 20 g. The lesson explains what “dissolve” means in particles, and why every solvent has a limit.
The science — structured notes
1.1 Dissolving is a particle event
A solution is a mixture in which one substance, the solute, is spread through another, the solvent, as individual particles — ions or molecules — so that the mixture is the same all the way through: homogeneous. Water is the commonest solvent, but not the only one: nail-varnish remover dissolves varnish that water cannot touch; petrol dissolves grease.
1.2 Solutions come in all three states
Solutions are not only liquids. Any state can be the solvent, and any state can be dissolved in it — the rule is only that the particles are mixed one by one, evenly, throughout.
| Solvent | Solute | Example | What the particles are doing |
|---|---|---|---|
| Liquid | Solid | Seawater; sugar in tea; copper sulfate solution | Ions or molecules spread among water molecules |
| Liquid | Liquid | Vinegar (ethanoic acid in water); antifreeze; a spirit | Two kinds of molecule mixed evenly |
| Liquid | Gas | Fizzy drinks (CO₂); oxygen in river water; ammonia solution | Gas molecules held among the liquid molecules — fewer as it warms |
| Gas | Gas | Air: nitrogen with oxygen, argon, CO₂ and water vapour dissolved in it | All gases mix completely, in any proportion |
| Solid | Solid | Alloys: brass (zinc in copper), steel (carbon in iron), bronze (tin in copper), 9-carat gold | Atoms of the solute sit among the atoms of the solvent metal in the lattice |
| Solid | Gas | Hydrogen dissolved in palladium metal | Small gas atoms fit in the gaps between metal atoms |
Decide whether each is a solution: brass; muddy river water; air; a cup of tea with the leaves in it; clear tea after the leaves are removed.
Brass: yes — zinc atoms are spread evenly among copper atoms; it is one uniform metal all through and cannot be separated by filtering. A solid solution.
Muddy water: no — clay particles are millions of atoms each, visible as cloudiness, and they settle or can be filtered. This is a suspension (Lesson 7.2).
Air: yes — a solution of gases; every litre has the same 78 % nitrogen and 21 % oxygen.
Tea with leaves: not a solution as a whole — the leaves are solid pieces. The clear liquid is a solution of the molecules that came out of the leaves (colour, caffeine, flavour) in water: transparent, uniform, and it passes through a filter unchanged.
1.3 How to recognise a solution
| Test | Solution | Not a solution (suspension) |
|---|---|---|
| Look through it | Transparent — coloured or not, light passes straight through | Cloudy; a beam of light is scattered and shows as a visible path |
| Leave it standing | Nothing settles, ever | Particles sink (or float) over minutes to days |
| Filter it | Everything passes through the paper | Solid is caught on the paper |
| Sample the top and the bottom | Identical composition | Different |
| Particle size | Individual ions or molecules, under 1 nm | Clumps of thousands to millions of particles, over 1000 nm |
“A solution must be colourless.”
Reveal answer
False. Copper sulfate solution is deep blue, potassium manganate(VII) solution is purple, tea is brown — and all three are transparent, uniform and unfilterable, which is what makes them solutions. Colour is about which wavelengths the dissolved particles absorb; clarity is about whether the particles are small enough not to scatter light. A solution can be any colour, but it is never cloudy.
1.4 Surface tension — and how a detergent breaks it
Water molecules attract each other strongly (the polar attraction from 1.1 acting between water molecules themselves). A molecule in the middle of the liquid is pulled equally in every direction; a molecule at the surface has neighbours only below and beside it, so it is pulled inwards and sideways. The surface behaves like a stretched elastic skin that resists being broken or enlarged: surface tension. It is why drops are round, why a pond skater stands on a stream, why water climbs a narrow glass tube and why a needle laid flat can float — the skin dips under it but does not tear.
A detergent molecule has two ends: a long hydrocarbon tail that water molecules do not attract, and a charged or polar head that they do. At the surface the tails poke out of the water and the heads sit in it, wedging between the water molecules and weakening the pull between them. The skin loses most of its strength — the surface tension of water falls by more than half — and the needle sinks, the drop spreads, the water soaks into cloth instead of sitting on it. That is the beginning of how soap cleans, and Lesson 7.5 finishes the story.
| Liquid at 20 °C | Surface tension / mN per m | Why |
|---|---|---|
| Mercury | 485 | Metal atoms bound by very strong attractions; drops are almost spherical |
| Water | 73 | Strong polar attraction between molecules |
| Water with detergent | about 30 | Detergent heads wedge between water molecules at the surface |
| Ethanol | 22 | Weaker attraction between molecules; spreads and wets easily |
| Cooking oil | about 32 | Non-polar molecules, weak attractions |
A pond skater stands on water without wetting its feet. A student drops a little detergent into the pond upstream, and the insect sinks. Explain both observations using the particles.
Reveal answer
Water molecules attract one another strongly; at the surface a molecule has no neighbours above it, so the net pull is inwards and the surface behaves like a stretched skin. The insect is light and its feet are spread wide and coated in waxy, non-polar hairs that water does not attract, so the skin dips under each foot but the pull between the water molecules is enough to support the small weight. Detergent molecules have polar heads and non-polar tails; they collect at the surface with the tails out of the water and wedge between the water molecules, weakening their attraction for each other. The skin is now too weak to hold the insect, and with the surface tension roughly halved the water also wets the hairs, so the skater breaks through.
1.5 Solubility, saturation and the solubility curve
Every solvent has a limit. Keep adding solute and a point comes when the solution holds all it can at that temperature: it is saturated, and any more solid stays undissolved at the bottom. The solubility of a substance is the mass that dissolves in 100 g of water to make a saturated solution at a stated temperature — the temperature must be stated, because for most solids the value rises as the water is heated.
| Solute | Solubility at 20 °C / g per 100 g water | At 60 °C | At 80 °C | Pattern |
|---|---|---|---|---|
| Potassium nitrate, KNO₃ | 32 | 110 | 169 | Rises steeply with temperature |
| Copper sulfate, CuSO₄ | 21 | 40 | 55 | Rises with temperature |
| Sodium chloride, NaCl | 36 | 37 | 38 | Almost unchanged |
| Sucrose (sugar) | 204 | 287 | 362 | Very soluble; rises with temperature |
| Calcium hydroxide, Ca(OH)₂ | 0.17 | 0.12 | 0.09 | Slightly soluble; falls with temperature |
| Oxygen gas, O₂ (pure O₂ at 1 atmosphere) | 0.0043 | 0.0023 | 0.0014 | Gases dissolve less in warm water |
Plot solubility (y) against temperature (x) and you have a solubility curve. Every point on the curve is a saturated solution; points below it are unsaturated (more will dissolve); points above it cannot exist for long — the excess crystallises out.
A saturated solution of potassium nitrate is made in 100 g of water at 80 °C and cooled to 20 °C. What mass of crystals forms? And if the same is done with 250 g of water?
Read the curve: 169 g dissolves at 80 °C; only 32 g can stay dissolved at 20 °C.
Mass that must leave the solution = 169 − 32 = 137 g of crystals per 100 g of water.
For 250 g of water, scale by 250100 = 2.5: crystals = 137 × 2.5 = 342.5 g — written as 343 g, since the curve is read to the nearest gram.
This is recrystallisation: dissolve hot, filter while hot to remove insoluble dirt, cool slowly, and the pure solute crystallises while the small mass of soluble impurity stays in the liquid. It only works for solutes whose curve is steep — for sodium chloride (36 g to 38 g) cooling would recover almost nothing, which is why salt is won by evaporation instead.
Use the table in 1.5.
(a) 50 g of copper sulfate is stirred into 100 g of water at 60 °C. Is the solution saturated? What mass, if any, stays undissolved?
(b) That same solution is cooled to 20 °C. What mass of crystals forms?
(c) A student says sodium chloride “is not very soluble because heating hardly helps”. Correct the reasoning.
Reveal answer
(a) Solubility at 60 °C is 40 g per 100 g of water, so 40 g dissolves and the solution is saturated; 50 − 40 = 10 g stays undissolved on the bottom.
(b) At 20 °C only 21 g can stay dissolved, so 40 − 21 = 19 g of crystals forms — plus the 10 g that never dissolved, giving 29 g of solid in the beaker.
(c) Solubility and its change with temperature are different things. Sodium chloride is quite soluble — 36 g per 100 g at 20 °C, more than copper sulfate — but its solubility barely changes with temperature. So it is soluble but not “more soluble hot”; the flat curve tells you how to separate it (evaporate), not how much dissolves.
“Potassium nitrate has a solubility of 110 g.”
Reveal answer
Two things are missing, and the statement is meaningless without them: the mass of water (solubility is per 100 g of water — 110 g in a swimming pool is a different thing) and the temperature (110 g per 100 g is true at 60 °C; at 20 °C it is 32 g). Write: “The solubility of potassium nitrate at 60 °C is 110 g per 100 g of water.”
1.6 Gases in solution: temperature and pressure
Gases dissolve too, and they follow two rules that solids mostly do not. First, warming a solvent drives gas out (the oxygen row of the table falls from 0.0043 to 0.0014 g per 100 g between 20 °C and 80 °C). Those table values are for pure oxygen at one atmosphere; a river or a pond is in contact with air, which is only 21 % oxygen, so it holds about a fifth as much — roughly 0.0009 g per 100 g at 20 °C. The pattern with temperature is the same; only the size of the number changes. Second, pressure pushes gas in: the mass of gas that dissolves is proportional to its pressure above the liquid, which is why a fizzy drink is bottled under about four times atmospheric pressure and why opening the cap — dropping the pressure to one atmosphere — releases the carbon dioxide as bubbles. Divers breathing compressed air at depth dissolve extra nitrogen in their blood, and must surface slowly so that it leaves as gently as it entered.
What if a river in Botswana warms from 18 °C in winter to 30 °C in summer, while a factory upstream discharges warm water that raises it a further 5 °C? Predict the effect on the fish, using the solubility of gases, and explain why “the water gets dirtier” is not the reason.
Reveal answer
Oxygen dissolves less in warmer water — roughly 0.0009 g per 100 g at 18 °C falling to about 0.0007 g at 30 °C and lower again at 35 °C — so the river carries less dissolved oxygen in summer, and the factory’s warm water lowers it further. Fish take oxygen from the water through their gills; below a threshold they suffocate, and the warmer water also raises their oxygen demand. Nothing has been added to the water: the harm is a physical change in solubility, which is why “thermal pollution” counts as pollution even when the discharged water is perfectly clean.
Try it — the solubility curve
Warm 100 g of water to a set temperature and add potassium nitrate a spatula at a time, stirring the same way each time, until a few crystals refuse to dissolve; weigh what went in. Repeat at 10, 20, 30, 40, 50, 60, 70 and 80 °C, three trials each, and plot the mean mass dissolved against temperature. Then swap to sodium chloride and copper sulfate: which curves rise, and which barely moves?
Suppose your mean results for potassium nitrate in 100 g of water were: 10 °C → 23 g, 20 → 36, 30 → 51, 40 → 68, 50 → 88, 60 → 110, 70 → 135, 80 → 162 g.
(a) Describe the relationship, quoting values, and say whether solubility is proportional to temperature.
(b) Use the data to estimate the mass of crystals that would form when a saturated solution at 70 °C in 100 g of water cools to 30 °C.
(c) Explain why the reading is only valid if undissolved crystals were visible when it was taken.
Reveal answer
(a) Solubility increases with temperature, from 23 g at 10 °C to 162 g at 80 °C, and the increase gets larger each step (13 g between 10 and 20 °C, 27 g between 70 and 80 °C), so the graph is a curve steepening upwards. It is not proportional: doubling the temperature from 20 to 40 °C gives 68 g, not 72 g, and the line would not pass through the origin (about 13 g dissolves even at 0 °C).
(b) 135 g at 70 °C; 51 g at 30 °C; 135 − 51 = 84 g of crystals per 100 g of water.
(c) Solubility is defined by a saturated solution — one that holds the maximum at that temperature. If every crystal dissolved, the solution might still take more, so the mass added is a lower limit, not the solubility. Visible undissolved crystals prove that the limit was reached, so the mass dissolved equals the solubility.
Common mistakes to avoid
- “The solute melts / disappears / turns into water” — it dissolves: its particles are pulled apart by solvent particles and spread through them, unchanged in mass and recoverable.
- Solubility with no temperature or no “per 100 g of water” — both are part of the definition.
- “Cloudy means concentrated” — cloudy means a suspension; a concentrated solution is still transparent.
- “Only liquids are solutions” — air and brass are solutions; the test is particle-by-particle mixing.
- “Heating always dissolves more” — true for most solids, nearly untrue for sodium chloride, and the reverse for gases and calcium hydroxide.
- Surface tension as stickiness — it is the attraction of water molecules for each other, strongest in its effect at the surface where the pull is one-sided.
How we found out — a kitchen sink and a brewery
In 1803 the Manchester chemist William Henry, working with his friend John Dalton, measured how much of each gas water would take up at different pressures and found a simple law: the mass of gas dissolved is proportional to its pressure. Dalton used Henry’s numbers to argue that a gas is made of separate particles that push into the water independently of one another — one of the first pieces of evidence for the particle model that this course has assumed since Year 1. Brewers already knew the practical version: carbon dioxide forced into beer under pressure stayed dissolved until the barrel was tapped.
Surface tension was measured properly by someone who was not allowed into a laboratory. Agnes Pockels, born in 1862 in Venice and raised in Braunschweig, was refused a university place because she was a woman, and studied physics from her brother’s textbooks while keeping house. Washing up, she noticed how films of grease altered the way water behaved, and built an instrument from a tin trough and a button hung from a balance to measure the force needed to pull the button off the surface. She found that the smallest trace of oil cut the surface tension sharply, and that the effect appeared only when the oil film was thinner than a certain thickness — a single layer of molecules, it turned out. In 1891 she wrote to Lord Rayleigh in England, who had the letter translated and published in Nature. Her trough, refined by Irving Langmuir (Nobel Prize 1932), is still the standard instrument for studying surfaces; she was finally given an honorary doctorate in 1932, aged seventy.
Pockels found that the surface tension of water stayed almost unchanged as she added oil, until the film reached a certain thinness, and then fell sharply. Explain what that observation revealed about the oil, and why it counts as evidence for molecules of a definite size.
Reveal answer
While the oil was spread thinly over a large area, the molecules lay far apart and barely disturbed the water surface. As she squeezed the film into a smaller area, the surface tension only dropped once the film reached one thickness — the point at which the oil molecules were packed side by side in a single complete layer, each with its polar head in the water and its tail up. A layer of one molecule has a definite thickness only if molecules have a definite size, so from the volume of oil and the area at which the change happened, the length of an oil molecule could be calculated — about two millionths of a millimetre. It was a measurement of a molecule made with a button and a balance, decades before anyone could see one.
Where it is used now
- Sua Pan, Botswana. Brine pumped from beneath the pan holds sodium carbonate and sodium chloride in solution; evaporation ponds concentrate it in the sun until each salt reaches saturation and crystallises in turn — an industrial solubility curve run by the weather. Lesson 7.5 follows the soda ash it produces.
- Borehole water. Groundwater across the Kalahari has dissolved calcium and magnesium salts from the rocks it passed through: “hard” water, which scales kettles and stops soap lathering. The white crust in a kettle is a solute crystallising as the solvent boils away.
- Medicine. A saline drip is 0.9 g of sodium chloride in 100 mL of water — the same concentration as blood plasma, so cells neither swell nor shrink. Every dose of a drug in solution depends on knowing its solubility, and poorly soluble drugs are the pharmacist’s hardest problem.
- Alloys. Steel is carbon dissolved in iron; the copper-nickel coins in your pocket and the aluminium alloys in an aircraft are solid solutions whose dissolved atoms make them stronger than the pure metal.
- Fisheries and rivers. Dissolved oxygen is the first number a water-quality team measures, because warm or polluted water holds less, and the Okavango’s fish depend on it.
- Cooking. Sugar syrup, brine for pickling, tea and coffee: every kitchen dissolves solutes and relies on temperature to change how much.
Science now — and the scientist you need to be
What is happening in 2026
Solubility has become a problem for computers. Nearly half of new drug molecules dissolve too poorly in water to reach the blood, and machine-learning models trained on hundreds of thousands of measured solubilities now predict, before a molecule is made, whether it will dissolve — and suggest small changes to its structure that would help. The same tools guide the design of solvents for capturing carbon dioxide from the air, where a solution that absorbs the gas quickly and releases it cheaply on warming is worth a fortune, and for pulling lithium directly out of brines like those under the Kalahari instead of evaporating whole lakes. Meanwhile satellite salinity maps track how much salt the oceans hold region by region, month by month, and the melting of freshwater ice shows up as a measurable dilution. The oldest chemistry — what dissolves in what, and how much — is now some of the most computed.
The mindshift
What you need to be able to do
Three examinable skills from this lesson: explain dissolving with particles (solvent molecules pulling solute particles from the lattice and surrounding them), read a solubility curve to find what dissolves and what crystallises out on cooling — with the arithmetic and the units — and explain a change in surface tension with the attraction between molecules. Criterion A questions will hand you a solubility table for a substance you have not met and ask for a number and a reason; the number comes from the table, the reason from the particles.
See an example
The paper clip floated with the surface dimpled around it, and sank the instant a soapy finger touched the water. The sugar solution grew crystals on a thread over three days as water evaporated and the solution became saturated. The AI’s first answer said “the gas escapes faster when warm”; asked why, it added that gas molecules dissolve less in warmer water because they have more energy to escape the liquid, which is the solubility argument the lesson gives.
Key terms
| Solution | A homogeneous mixture in which the solute is spread through the solvent as individual particles; transparent, uniform, unfilterable. |
| Solute / solvent | The substance that dissolves / the substance it dissolves in (usually the one present in the larger amount). |
| Homogeneous | The same composition and properties throughout, down to the particle level. |
| Dissolving | Solvent particles pull solute particles from their solid structure and surround them; the particles then spread by diffusion. |
| Alloy | A solid solution of one metal (or carbon) in another: brass, steel, bronze. |
| Surface tension | The inward pull on molecules at a liquid surface, from the attraction between them, which makes the surface behave like a skin. |
| Detergent (surfactant) | A molecule with a water-attracting head and a water-repelling tail that gathers at surfaces and lowers surface tension. |
| Solubility | The maximum mass of solute that dissolves in 100 g of water at a stated temperature. |
| Saturated solution | A solution holding the maximum solute at that temperature; undissolved solute remains. |
| Solubility curve | A graph of solubility against temperature; points on the curve are saturated solutions. |
| Recrystallisation | Purifying a solid by dissolving it hot, filtering, and cooling so that the pure solute crystallises. |
Think like a scientist
Criterion A A chemistry class measured the solubility of a salt they were not told the name of, “salt X”, and of sodium chloride, in 100 g of water at five temperatures. Their results:
| Temperature / °C | Solubility of salt X / g per 100 g water | Solubility of sodium chloride / g per 100 g water |
|---|---|---|
| 20 | 34 | 36 |
| 40 | 62 | 37 |
| 60 | 108 | 37 |
| 80 | 167 | 38 |
| 100 | 245 | 40 |
(a) Compare the effect of temperature on the two solubilities, quoting values. [2]
(b) A saturated solution of salt X is made in 250 g of water at 80 °C and cooled to 40 °C. Calculate the mass of crystals that forms. [3]
(c) Explain, in terms of particles and energy, why heating increases the solubility of salt X so much. [3]
(d) Salt X is contaminated with a little sodium chloride. Describe how the data show that recrystallisation from hot water would purify it, and state one limitation. [3]
(e) The class also measured the mass of oxygen dissolved in 100 g of water and found it fell from 0.0043 g at 20 °C to 0.0014 g at 80 °C. Explain the difference from the solids, and state one consequence for a river in summer. [3]
Model answer
(a) The solubility of salt X rises steeply with temperature, from 34 g at 20 °C to 245 g at 100 °C — more than seven times — with the rise getting larger each step (28 g between 20 and 40 °C, 78 g between 80 and 100 °C), whereas the solubility of sodium chloride is almost unchanged, rising only from 36 g to 40 g over the same range. At 20 °C sodium chloride is slightly the more soluble; above 40 °C salt X is far more soluble.
(b) Per 100 g of water: 167 g dissolves at 80 °C and 62 g at 40 °C, so 167 − 62 = 105 g crystallises. For 250 g of water: 105 × 2.5 = 262.5 g, which is 263 g to the nearest gram, matching the precision of the table.
(c) To dissolve, ions must be pulled out of the lattice of salt X, which needs energy to overcome the attraction between them; the water molecules supply it when they collide with the crystal surface and then surround the freed ions. Hotter water molecules have more kinetic energy, so more collisions each second carry enough energy to free an ion, and freed ions are less likely to be recaptured by the lattice. Dissolving salt X takes in energy overall (it is endothermic), so raising the temperature shifts the balance between dissolving and crystallising strongly towards dissolving.
(d) Dissolve the impure solid in the minimum of water at 100 °C: up to 245 g of salt X dissolves per 100 g of water, and the small mass of sodium chloride dissolves too (up to 40 g). Cool to 20 °C: only 34 g of salt X can stay dissolved, so almost all of it crystallises, while the sodium chloride — present in a small mass well below its solubility of 36 g — stays in solution and is removed with the liquid when the crystals are filtered off. Limitation: the 34 g of salt X that remains dissolved at 20 °C is lost, so the yield is never 100 %; and if the sodium chloride impurity were large it could reach saturation and crystallise too.
(e) When a gas dissolves its molecules are captured by attractions to water molecules and lose freedom of movement, which releases energy rather than taking it in; warming gives the dissolved molecules more energy to escape back into the air, so the balance shifts to less dissolved. Solids are the opposite because their particles must be pulled apart from a lattice. Consequence: a river warmed by summer sun (or by a factory outflow) holds less dissolved oxygen — about a fifth less between 20 and 30 °C — so fish and other animals that take oxygen through gills can suffocate even though nothing has been added to the water.
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
Describe, in terms of particles, what happens when a crystal of copper sulfate dissolves in water.
Reveal answer
Water molecules, which are polar, are attracted to the ions at the surface of the crystal [1]; they pull the ions out of the lattice, overcoming the attraction between the ions, and surround each ion [1]; the freed ions spread evenly through the water by diffusion, so the solution is uniform and transparent (blue) [1].
Explain why brass is classed as a solution but a mixture of iron filings and sand is not.
Reveal answer
In brass, zinc atoms are spread individually and evenly among the copper atoms in one lattice, so the metal is uniform throughout and cannot be separated physically — a solid solution [1]; iron filings and sand are separate grains each containing millions of particles, not mixed at the particle level, visibly different and separable with a magnet [1].
The solubility of potassium chloride is 34 g per 100 g of water at 20 °C and 51 g at 80 °C. A saturated solution is made in 40 g of water at 80 °C and cooled to 20 °C. Calculate the mass of crystals formed.
Reveal answer
Per 100 g of water: 51 − 34 = 17 g crystallises [1]; for 40 g of water scale by 40 ÷ 100 = 0.4 [1]; 17 × 0.4 = 6.8 g of crystals [1].
Explain why a drop of water on a waxed car bonnet forms a round bead, and why a drop of water containing detergent spreads out flat.
Reveal answer
Water molecules attract each other strongly; at the surface of the drop the net pull is inwards, so the surface shrinks to the smallest area — a sphere — and the non-polar wax does not attract the water enough to pull it flat [1]; detergent molecules gather at the surface with their non-polar tails outwards and wedge between the water molecules, weakening the attraction between them [1]; with the surface tension lowered, the pull towards a bead is smaller than the attraction to the surface and the weight of the drop, so it spreads and wets the bonnet [1].
True or false, with a reason: “A saturated solution is always a concentrated solution.”
Reveal answer
False [1]: saturated means the solvent holds the maximum of that solute at that temperature, which for a slightly soluble substance like calcium hydroxide (0.17 g per 100 g) is a very dilute solution; concentrated describes how much is dissolved compared with the volume, not whether the limit has been reached [1].
A bottle of fizzy drink is opened warm and one is opened cold. Explain which fizzes over more and why, using the solubility of gases.
Reveal answer
Carbon dioxide is dissolved under pressure in the bottle; opening it drops the pressure to atmospheric, so the gas is less soluble and leaves as bubbles [1]; gases are less soluble in warm water because the dissolved molecules have more energy to escape the attractions holding them among the water molecules [1]; so the warm drink held less gas comfortably to begin with and loses far more of it at once, fizzing over, while the cold drink keeps most of its carbon dioxide dissolved [1].
Summary
- A solution is a homogeneous mixture: solute particles pulled apart and spread among solvent particles. Mass is conserved and the solute can be recovered.
- Solutions exist in every state — seawater, air, brass — because the test is particle-by-particle mixing, not the state of the result.
- Surface tension comes from the attraction between water molecules acting one-sidedly at the surface; detergents wedge in and weaken it.
- Solubility is a mass per 100 g of water at a stated temperature; a saturated solution holds the maximum. Most solids dissolve more when hot, sodium chloride barely changes, gases dissolve less.
- Read a solubility curve to find what dissolves and what crystallises on cooling — the basis of recrystallisation and of salt pans.
Next time → How much is dissolved: concentration in g per 100 mL and g/dm³, dilution, and why salt water conducts but sugar water does not.