Solutions: Class 9 Chemistry

Key points

A solution is a homogeneous mixture of two or more substances. The component present in the larger amount is the solvent, and the one dissolved in it is the solute. In sugar solution, sugar is the solute and water is the solvent. A solution has the same composition and properties throughout, and its particles cannot be seen or filtered out.

Solutions can be formed in all three states. Examples:

• Gas in gas: air (oxygen and other gases in nitrogen).
• Gas in liquid: carbonated water (carbon dioxide in water).
• Liquid in liquid: vinegar (ethanoic acid in water).
• Solid in liquid: salt in water.
• Solid in solid: brass (zinc in copper), an alloy.
• Gas in solid: hydrogen in palladium.

Water is called the universal solvent because it dissolves more substances than any other common liquid. Its molecule is polar, and water molecules surround and separate ions and polar molecules. The rule is that like dissolves like. Polar and ionic substances dissolve in polar solvents such as water. Non-polar substances, such as oil, wax and grease, dissolve in non-polar solvents such as petrol, benzene and carbon tetrachloride.

Types of solutions by amount of solute:

• Unsaturated: contains less solute than the maximum it can dissolve at that temperature. More can still dissolve.
• Saturated: contains the maximum solute at that temperature. Extra solute stays undissolved and is in dynamic equilibrium with the dissolved solute.
• Supersaturated: contains more solute than a saturated solution at that temperature. It is unstable, and a small crystal or a shake makes the extra solute crystallise.

Solubility is the maximum mass of solute in grams that dissolves in 100 g of solvent at a given temperature to give a saturated solution. For most solids, solubility increases with temperature, for example potassium nitrate. For a few, such as sodium chloride, it changes very little. For gases, solubility decreases when temperature rises and increases when pressure rises. This is why a soft drink fizzes when the bottle is opened and pressure falls.

Dilute and concentrated are descriptive words. A dilute solution has a small amount of solute relative to solvent and a concentrated solution has a large amount. They do not give numerical values, so chemists use concentration units.

Common concentration units:

• Percentage by mass (w/w) = mass of solute / mass of solution × 100.
• Percentage mass by volume (w/v) = mass of solute in g / volume of solution in cm³ × 100.
• Percentage by volume (v/v) = volume of solute / volume of solution × 100.
• Parts per million (ppm) = mass of solute / mass of solution × 10⁶.
• Molarity (M) = moles of solute / volume of solution in dm³. Unit: mol dm⁻³.

A 1 molar (1 M) solution contains one mole of solute in 1 dm³ of solution. The number of moles is mass divided by molar mass. For preparing a standard solution, the measured mass of solute is dissolved in a little water in a volumetric flask, and water is then added up to the mark.

Dilution means adding solvent to reduce concentration. The amount of solute does not change, so M₁V₁ = M₂V₂. To dilute a stock solution, measure V₁ with a pipette, put it in a flask and add water until the volume is V₂.

A suspension has large particles (bigger than about 1000 nm) which are visible. It is cloudy, the particles settle on standing and can be filtered. Muddy water and chalk in water are suspensions. A colloid has medium-size particles (about 1 to 1000 nm). It does not settle and cannot be filtered with ordinary paper. Milk, fog, smoke and ink are colloids.

The Tyndall effect is the scattering of light by colloidal particles, which makes a light beam visible through the colloid. True solutions do not show it. Brownian motion is the continuous zig-zag motion of colloidal particles caused by random collisions with the molecules of the medium. Colloids and suspensions are heterogeneous or look so under a microscope, while a true solution is homogeneous.

Study tip: Write the formula, the unit conversions and the molar mass calculation as separate lines in every numerical. For the colloid, suspension and true solution comparison, use a table with particle size, appearance, settling, filtration and the Tyndall effect, and give one example of each.

Definitions and flashcards

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Short questions with model answers

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Numericals with worked steps

Common mistakes

• Using the volume of the solvent instead of the volume of the whole solution for molarity.
• Forgetting to convert cm³ to dm³ by dividing by 1000.
• Using the mass of solvent alone in the denominator of percentage by mass.
• Saying that a dilute solution is the same as an unsaturated one. They are different ideas.
• Stating that gas solubility rises with temperature.

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