Chapter 9

The Amazing World of Solutes, Solvents, and Solutions

Homemade ORS (Oral Rehydration Solution) tastes the same in every sip, no matter how much you drink, because the sugar and salt you add form a mixture where the components are evenly distributed throughout the water. Chalk powder or sawdust mixed with water behaves completely differently — their components don't spread evenly, making these non-uniform mixtures, unlike the uniform mixture salt and sugar form with water.

What Are Solute, Solvent, and Solution?

A uniform mixture, like salt or sugar dissolved in water, is called a solution. When a solid is mixed with a liquid to form a solution, the solid is the solute and the liquid is the solvent — the solute dissolves in the solvent to form the solution.

When two liquids mix to form a solution, it's not always obvious which one is "doing the dissolving." In that case, whichever substance is present in the smaller amount is called the solute, and the one in the larger amount is the solvent — even when that feels a little surprising. The chashni (sugar syrup) in a Gulab jamun, for instance, is mostly sugar dissolved in a much smaller amount of water, yet water is still called the solvent and sugar the solute, because the rule is based on which substance is doing the dissolving, not which one there's more of.

How Much Solute Can a Fixed Amount of Solvent Dissolve?

Adding spoon after spoon of salt to a fixed amount of water works fine at first — each spoonful dissolves completely. But after enough spoons, a point comes where the added salt stops dissolving and settles at the bottom instead: the water has reached its limit. A solution that can still dissolve more solute at a given temperature is called an unsaturated solution; once the solute stops dissolving and begins settling out, it's called a saturated solution at that temperature.

The amount of solute present in a fixed quantity of solution (or solvent) is its concentration. A solution with less solute is dilute, and one with more is concentrated — these are relative terms, so a one-spoon salt solution is dilute compared to a two-spoon one. The maximum amount of solute that will dissolve in a fixed quantity of solvent is called its solubility.

How Does Temperature Affect the Solubility of a Solute?

Dissolving baking soda in 50 mL of water at 20°C eventually leaves some undissolved at the bottom — the solution has become saturated. Heating that same mixture to 50°C dissolves the leftover baking soda completely; adding more baking soda and heating further to 70°C dissolves even more. Water at 70°C can dissolve more baking soda than water at 50°C, which in turn dissolves more than water at 20°C — for most substances, solubility increases with temperature, so a solution that's saturated at one temperature can behave like an unsaturated solution once heated.

Solubility of Gases

Many gases, including oxygen, dissolve in water — only to a small extent, but it's exactly this small dissolved amount that sustains all aquatic life, from plants to fish. Water with dissolved oxygen is a uniform mixture, since the gas spreads evenly through it. Unlike solids, though, gas solubility generally decreases as temperature rises: cold water holds more dissolved oxygen than warm water, which is why aquatic life depends on water staying reasonably cool.

Why Do Objects Float or Sink in Water?

Husk floats on water while rice sinks; oil floats on water too. Generally, objects that float are lighter and those that sink are heavier than the liquid they're in — but weight alone isn't the whole story. A wooden stick and an iron rod of the same size feel very different in the hand, because iron is heavier for its size than wood is — a property known as density, which describes how heavy an object is for its size. The density of a substance isn't the only factor deciding whether it floats or sinks in a particular liquid, though — there's more to the story.

What Is Density?

A crowded bus with people packed together is high density; the same bus with only a few people is low density. A forest with trees growing close together is dense; one with trees spread far apart is less dense. Scientifically, density is defined as the mass present in a unit volume of a substance — expressed mathematically as Density = Mass / Volume.

Density doesn't depend on a substance's shape or size, but it does depend on temperature and pressure — pressure mainly affects the density of gases, with only a negligible effect on solids and liquids. The SI unit of density is kilogram per cubic metre (kg/m³); for liquids, gram per millilitre (g/mL) or gram per cubic centimetre (g/cm³) are also common, and 1 kg/m³ works out to 1 g/L.

Since 1 mL of water has a mass of close to 1 g at room temperature, 100 mL of water weighs approximately 100 g. If an aluminium block has a mass of 27 g and a volume of 10 cm³, its density is 2.7 g/cm³ — aluminium is 2.7 times denser than water, a ratio called its relative density with respect to water (a plain number, with no units).

Determination of Density

Finding a substance's density means measuring its mass and volume separately. Mass — the quantity of matter in an object — is measured with a balance; most balances actually measure weight (the force with which Earth attracts an object, in newtons) but display it on a scale marked in grams or kilograms, since mass and weight are closely related on Earth (a two-pan balance is the one type that measures mass directly). Weighing a stone on a digital balance — after zeroing it out with a watch glass on the pan — might show a mass of 16.400 g.

Volume — the space an object occupies — is measured with a measuring cylinder for liquids: a narrow, transparent container marked with volume readings. When reading the level, water and other colourless liquids form a curved surface called the meniscus, and the reading should be taken at the bottom of that curve — for coloured liquids, the top of the meniscus is used instead. Measuring cylinders are deliberately built narrow and tall rather than wide and short: a narrow tube means a given change in volume shows up as a larger, easier-to-read change in height, so a 100 mL cylinder can read to the nearest 1 mL, while smaller ones read even finer.

The volume of a regular solid, like a cuboid-shaped notebook, is calculated from its measured length, width, and height (Volume = l × w × h) — a notebook 25 cm × 18 cm × 2 cm has a volume of 900 cm³. For an irregularly shaped object like a stone, water displacement does the job instead: lowering the stone into a measuring cylinder partly filled with water raises the water level, and the rise in level (say, from 50 mL to 55 mL) gives the object's volume — 5 cm³ in this case. Combining a stone's mass (16.400 g) and its displaced volume (5 cm³) gives a density of 3.28 g/cm³.

Earth itself is made of layers — crust, upper mantle, lower mantle, outer core, and inner core — each with its own density. The outermost crust is the lightest, and density increases moving toward the centre, since pressure and temperature both rise significantly with depth, packing the material more tightly.

Effect of Temperature on Density

Density generally decreases with heating and increases with cooling. As temperature rises, a substance's particles — solid, liquid, or gas — move apart and spread out, increasing volume while mass stays the same; since Density = Mass / Volume, a bigger volume with the same mass means lower density. That's why hot air rises above cooler air around it — the same principle that lifts a hot air balloon.

Effect of Pressure on Density

Pressure affects density differently depending on the state of matter. For gases, increasing pressure pushes particles closer together, shrinking volume and raising density. Liquids are nearly incompressible, so pressure barely changes their density; solids, whose particles are already packed tightly, are affected even less, with density changes usually too small to matter.