Solutes, Solvents and Solutions

Stir a spoonful of sugar into a glass of water and it seems to vanish. Nothing has been destroyed. The sugar has broken up into particles far too small for your eye to catch, and those particles have spread themselves evenly through the water. What you are holding now is a solution.

Every solution has two jobs to fill. The substance that dissolves and disappears from view is the solute. The substance that does the dissolving is the solvent, and it is usually the one present in the larger amount. In a glass of sugar water, sugar is the solute, water is the solvent, and the sweet liquid as a whole is the solution. A short way to hold this in your head is that the solute is the guest, the solvent is the host, and the solution is the gathering in which you can no longer tell one from the other.

The proper word for what a solution is like is homogeneous, which simply means the same all the way through. You can test this at home. Make a glass of nimbu pani, stir it well, and taste a sip from the top and then a sip from near the bottom. Both taste the same, because the dissolved sugar and salt are spread uniformly. Then leave the glass untouched on a shelf and come back the next morning. Nothing has settled into a layer at the bottom, and no cloudiness has appeared. A true solution stays mixed on its own, with no stirring needed to keep it that way.

Not everything dissolves. A substance that dissolves in a given solvent is said to be soluble in it, and one that refuses is insoluble in it. Salt, sugar, alum and glucose are soluble in water; chalk powder, sand and sawdust are not. Notice the words in a given solvent, because being soluble is never a property of the solute on its own. Chalk will not dissolve in water, and grease will not dissolve in water either — yet grease dissolves quite happily in kerosene. Nail polish stays exactly where it is when you wash your hands, and comes off at once with remover. It is the pair that matters, never just one member of it.

When both the solute and the solvent are liquids, we swap in a different pair of words. Two liquids that mix completely and stay mixed are miscible, like vinegar and water. Two liquids that separate into layers again once you stop shaking them are immiscible, like mustard oil and water. Water dissolves such a wide range of substances that people often describe it as the universal solvent, but as the grease and the nail polish show, even water has limits.

Once you know what a solution is, the next question is how much solute is sitting inside it. Three words do most of this work: dilute, concentrated and saturated. The first two are comparisons and the third is a limit, and mixing them up causes a great deal of confusion.

A dilute solution contains a small amount of solute compared with the amount of solvent. A concentrated solution contains a large amount. These two words only make full sense side by side. One spoon of sugar in a glass of water gives a dilute solution; five spoons in the same glass gives a concentrated one. Neither is dilute or concentrated all by itself — each is only dilute or concentrated compared with the other. You can watch this happen with potassium permanganate. A few crystals in a beaker of water give a pale pink liquid. Keep adding crystals and the pink deepens into a strong purple. Now take a little of that purple liquid and add plenty of fresh water, and it fades back to pink. Adding solvent dilutes a solution; adding solute concentrates it.

Now push the idea further. Keep stirring sugar into the same glass of water without warming it, and at some point the sugar stops disappearing. Grains begin to lie on the bottom however long you stir. The solution has reached its limit for that temperature, and we call it a saturated solution. A solution that can still take in more solute is unsaturated.

There is a simple test for saturation that needs nothing more than a glass and a spoon. Add a little more solute and stir patiently. If it dissolves, the solution was unsaturated. If it settles and stays undissolved even after long stirring, the solution is saturated. Patience really matters here, because a solution that is close to its limit dissolves those last few grains very slowly, and an impatient student often decides a solution is saturated when it is not.

Two cautions deserve a place in your notebook. First, saturated does not mean the same thing as concentrated. A saturated solution of chalk in water holds hardly any chalk at all, so it is saturated and dilute at the very same time. Second, a solution is only saturated at a stated temperature. Warm that same glass of sugar water and the leftover grains at the bottom begin to disappear, because warm water can hold more sugar than cold water can. Let it cool again and the extra sugar may come back as crystals. That is why any statement about saturation is incomplete unless it carries a temperature with it.

Saying that sugar dissolves better than salt is fine for a kitchen conversation, but science needs a number. That number is solubility. The solubility of a substance in a solvent is the largest mass of that substance that will dissolve in a fixed amount of the solvent at a stated temperature, producing a saturated solution. The fixed amount is usually taken as 100 g of water, so solubility is normally written as so many grams per 100 g of water.

Two parts of that sentence are doing quiet but important work. The first is a fixed amount of the solvent. If you dissolve salt in 200 g of water instead of 100 g, about twice as much salt will go in, but the salt has not become more soluble. By always comparing against the same 100 g of water, solubility gives a fair comparison between one substance and another. The second is at a stated temperature. A solubility figure with no temperature attached is incomplete, because the same substance behaves differently in cold water and in warm water.

The numbers themselves tell an interesting story. Common salt feels like the most soluble thing in the kitchen, yet only about 36 g of it will dissolve in 100 g of water at ordinary room temperature. Table sugar is far more generous, and roughly two hundred grams of it can go into that same 100 g of water. Chalk, sand and marble powder sit at the other end of the range, dissolving in amounts so tiny that we simply call them insoluble. Strictly speaking almost nothing is perfectly insoluble; insoluble is a practical word for a solubility so small that it makes no difference in everyday life.

Measuring solubility in a school laboratory follows straight from the definition. Take a weighed amount of water and keep it steady at one temperature. Weigh out some solute, then add it a little at a time with constant stirring until no more will dissolve. Filter off whatever is left undissolved, dry it and weigh it. Subtracting that leftover mass from the mass of solute you started with tells you how much actually went into the water. Scale that answer up or down to 100 g of water and you have the solubility at that temperature.

Because solubility describes a solute and a solvent together, one substance has many different solubility values, one for each solvent it meets. Sugar is very soluble in water and far less so in oil. This is not a fussy detail. A doctor choosing a medicine, a farmer choosing a fertiliser and a cook making sherbet are all quietly relying on solubility, because each of them needs a substance that will dissolve properly in the particular liquid they actually have.

Solubility is not a fixed number carved into stone. For a solid solute two things decide it, and when the solute is a gas, pressure joins them as a third. There is also a separate group of things that is very often confused with all of these, and we will settle that confusion at the end.

The first is the nature of the solute and the solvent. Some pairs suit each other and some do not, and nothing you do can force an unwilling pair together. Salt and sugar dissolve readily in water while chalk does not, and no amount of stirring, grinding or heating will change that. Turn it around and grease, which ignores water completely, dissolves easily in kerosene. This is why a mechanic reaches for kerosene and not for a bucket of water when cleaning greasy parts.

The second is temperature, and here solids and gases behave in opposite ways. For most solid solutes, raising the temperature of the solvent increases the solubility. To see this properly you have to compare like with like: take two glasses holding the same amount of water, one cold and one hot, and stir sugar into each until no more will dissolve. By the time both have given up, the hot glass will have taken in more sugar altogether. Note carefully that this is a statement about how much finally dissolves, not about how quickly it goes in. A hot, nearly saturated solution that is then allowed to cool can no longer hold all it was holding, so the extra solute comes out as crystals — which is exactly how crystals of alum, sugar candy and blue copper sulphate are grown. Common salt is worth remembering as a quiet exception. Its solubility rises so little between cold water and boiling water that heating the water barely helps at all.

Gases go the other way entirely. Put a pan of water on the stove and long before it boils you see small bubbles clinging to the bottom and the sides. Those are not steam. They are the air that was dissolved in the cold water being driven out as the water warms up. Dissolved gas escapes as temperature rises, and this is one reason why fish struggle in water that has become too warm. Pressure matters for gases as well. A sealed bottle of a fizzy drink holds carbon dioxide dissolved under pressure. Open the cap, the pressure falls, and the gas comes rushing out as fizz.

Now for the confusion. Stirring the mixture, grinding the solute into a powder and warming the solvent all make dissolving happen faster. Stirring keeps fresh solvent in contact with the solid instead of letting a saturated layer sit around it. Grinding a lump into powder gives the solvent far more surface to work on at once. Warming makes the particles move about more quickly. But of these three, only warming changes how much can finally dissolve. Stir a saturated solution for a whole hour and it is still saturated. How fast a solid dissolves and how much of it dissolves are two completely separate ideas, and keeping them apart is one of the real gains of this chapter.

Most of the solutions you meet in a science class are solids dissolved in liquids, and that makes it easy to believe a solution must always be a liquid with a powder stirred into it. It need not be. A solution is any homogeneous mixture, whatever the states of its parts, and once you start looking you find solutions in all three states of matter.

Solids in liquids make up the familiar family: common salt, sugar, alum, glucose and copper sulphate in water. Copper sulphate is worth a moment of your attention, because its solution is a deep blue and yet you can read print straight through it. Clear and colourless are not the same thing. A true solution must be clear, but it is perfectly allowed to be strongly coloured.

Liquids in liquids give solutions only when the two liquids are miscible. Vinegar, which is a solution of acetic acid in water to begin with, mixes with more water in any proportion. Mustard oil poured into water refuses, floats on top, and settles back into a separate layer soon after you stop shaking, so oil and water never make a solution.

Gases in liquids are easy to forget because you cannot see them, and they matter enormously. Oxygen from the air dissolves into ponds, rivers and the sea, and it is this dissolved oxygen that fish and water plants breathe. An aquarium keeps an air pump bubbling for precisely this reason. Carbon dioxide dissolved under pressure is what puts the fizz into a soft drink.

Gases in gases bring us to the solution you are sitting inside. Air is a homogeneous mixture of gases. Counted by volume it is roughly 78 percent nitrogen and 21 percent oxygen, with argon, carbon dioxide, other gases and water vapour sharing the small remainder. It behaves exactly as a solution should: it is uniform, it is clear, and the heavier gases do not settle into a layer near the floor.

Solids in solids are the surprise of the list. An alloy is made by melting a metal together with one or more other substances — usually another metal, sometimes a small amount of a non-metal — and letting the mixture cool into one uniform solid, which makes it a solid solution. Brass is copper and zinc. Bronze is copper and tin. Steel is iron with a small amount of carbon in it. Each of these is harder or more useful than the metals it came from, which is why so much of the metal around you is an alloy rather than a pure element.

Add salt to water and it disappears. Add chalk powder to water and it does not: the water turns cloudy, and if you leave the glass alone the chalk sinks to the bottom. Add a few drops of milk instead and something in between happens, because the water turns white and then stays white for days without anything settling out. These three behaviours belong to three different kinds of mixtures, and what separates them is how big the scattered particles are.

A true solution sits at the fine end. Its particles are so small that no ordinary microscope will show them to you. They pass straight through filter paper along with the solvent, they never settle no matter how long the glass stands, and the mixture is clear. Salt solution, sugar solution and copper sulphate solution are true solutions.

A suspension sits at the coarse end. Its particles are large enough to make the liquid cloudy and sometimes large enough to be seen outright. A suspension is heterogeneous, meaning it is not the same all through. Leave it standing and the particles settle into a layer at the bottom. Pour it through filter paper and the particles are trapped while clear liquid runs out below. Chalk powder in water, sand in water, muddy river water and distemper paint are all suspensions, and so is any medicine whose bottle carries an instruction to shake well before use.

A colloid lies between the two. Its particles are bigger than those in a true solution but still far too small to pick out by eye, so a colloid looks perfectly uniform even though it is really heterogeneous. Colloid particles do not settle even after very long standing, and ordinary filter paper does not hold them back. Milk, fog, mist, smoke, jelly, cheese, butter, whipped cream and shaving foam are all colloids.

The most convincing test in the whole set is a beam of light. Take a glass of clear salt solution into a darkened room and shine a torch through it. You see the bright spot where the beam lands on the far side, but you cannot see the beam crossing the liquid at all. Now shine the same torch through a glass of very dilute milk and the entire path of the beam lights up, because the colloid particles scatter light sideways into your eye. This scattering of a light beam by colloid particles is called the Tyndall effect, after the scientist John Tyndall. You have seen it many times without naming it: sunlight streaming through a gap in a curtain into a dusty room, the beams of a vehicle headlight in fog, and shafts of light coming through the leaves of a tree in early morning haze.

Put together, three simple bench tests sort all three mixtures out. Does it settle on standing? Only a suspension does. Does filter paper hold the particles back? Again only a suspension. Does a beam of light show its path through the liquid? A colloid does, a true solution does not. Milk, which looks as uniform as any solution, fails the light test at once and gives itself away.

Solutions are not a laboratory idea. You drink them, cook with them, breathe one, and depend on several more without ever noticing.

Start with the sea. Sea water is a solution of common salt and several other salts in water, and along the coast people have used the sun to take it apart for centuries. Shallow beds called salt pans are filled with sea water, the sun and wind evaporate the water away, and solid salt is left behind as a crust that can be raked up. This is the standard way of separating a dissolved solid from its solution, because filtration is completely useless here. The dissolved particles are so small that they slip through the pores of filter paper along with the water itself. To recover a dissolved solid you must take away the solvent, not try to strain out the solute.

Cooling does a related job more neatly. If a hot, nearly saturated solution is allowed to cool slowly, the solvent can no longer hold everything it was holding, and the excess comes out as well formed crystals. This is crystallisation, and it is how sugar candy, alum crystals and blue copper sulphate crystals are grown in a school laboratory. Slow cooling gives bigger and cleaner crystals than fast cooling, so patience is rewarded.

In the kitchen you make solutions all day. Salt disappearing into dal, sugar into kheer and sherbet into a jug of cold water are all solutions being prepared. A pickle sits in a concentrated salt solution called brine, and the very high concentration is what keeps it from spoiling.

Medicine leans on the same ideas. When a body has lost a lot of water and salts, a doctor may advise oral rehydration solution, which is nothing more exotic than a measured amount of sugar and salt dissolved in clean water. Many syrups are solutions. Bottles that must be shaken before use are usually suspensions instead, and the shaking is there to spread the settled particles evenly again for a moment.

Outdoors, dissolved substances keep whole ecosystems running. Fish and water plants use the oxygen dissolved in ponds, rivers and the sea, which is why an aquarium bubbles air through the tank. Farmers depend on solutions too, since a fertiliser is of no use at all until it dissolves in soil water and travels to the roots. Plants take in minerals only in dissolved form. Even the air you are breathing at this moment is a solution of gases. Once you start looking for them, the amazing world of solutes and solvents turns out to be the ordinary world.