Nature of matter

Oil and water are immiscible liquids and form layers. Exam tip: use separating funnel for immiscible liquids.

When muddy water is kept undisturbed, heavier insoluble particles such as soil settle at the bottom due to gravity. This process is called sedimentation. Filtration separates particles using a filter, whereas in this case the particles settle on their own. Exam tip: if particles “settle down on standing,” choose sedimentation.

The governing concept is decantation, a physical separation method used after a denser, insoluble solid has settled at the bottom. First, the mixture is left undisturbed so sedimentation can occur. Then the clear liquid above the sediment is gently poured into another container without disturbing the solid. This second step is called decantation, making option A correct. Melting changes a solid into a liquid, crystallisation forms crystals from a solution or melt, and evaporation removes a liquid as vapour. Decantation does not require boiling or a filter and is especially useful when the solid particles have already settled.

The governing concept is the nature of a suspension, a heterogeneous mixture containing relatively large, insoluble particles dispersed in a fluid. Because of their size and density, these particles scatter light and can settle at the bottom when the mixture is left undisturbed. Therefore option A is the defining characteristic. A suspension is not permanently uniform, so option B is incorrect. Its particles may be visible directly or with simple observation, contradicting option C. It is a mixture rather than a pure substance, so option D is also incorrect. Stirring can temporarily redistribute the particles, but gravity eventually causes settling again.

A true solution is a homogeneous mixture, meaning its composition is uniform throughout. The solute particles are extremely small and are distributed at the molecular or ionic level. Because of this, the particles cannot normally be seen separately with the naked eye, do not settle at the bottom on standing, and generally pass through ordinary filter paper.

For example, salt dissolved completely in water forms a true solution. Its sample has the same composition in every part, unlike a suspension whose larger particles may settle. Therefore, “it is homogeneous” is the defining characteristic among the choices. The other statements are incorrect because true-solution particles do not settle easily, components are not always visible, and a true solution can certainly be liquid.

Colloid particles have size between solution and suspension particles. Exam tip: connect intermediate size with colloids.

Milk is an emulsion because tiny liquid fat droplets are dispersed in a liquid medium, mainly water. Therefore, it is a liquid-in-liquid colloid. Foam is different because it contains gas dispersed in a liquid. Exam tip: associate milk and cream with emulsions.

Smoke contains fine solid particles dispersed in a gaseous medium. A colloidal dispersion of solid particles in a gas is called an aerosol, so smoke is an example of a colloidal system. In a true solution, solute particles are molecular or ionic in size, unlike the larger dispersed particles in smoke. Exam tip: remember that solid or liquid particles dispersed in a gas form an aerosol.

Direct answer: Option A, a colloid of liquid droplets in gas. Fog is an aerosol, which is a type of colloid in which very small liquid droplets are dispersed through a gas, usually air. Step by step: the dispersed substance is water; water is present as tiny liquid droplets; the continuous medium is air, a gas; therefore the system is liquid in gas, called an aerosol. Option A is correct. Option B, solid in solid solution, describes systems such as some solid mixtures or alloys, not suspended water droplets in air. Option C, pure gas, is wrong because fog contains liquid water droplets as well as air, so it is not a single pure gas. Option D, metallic alloy, is a mixture of metals or a metal with another element in a solid state, such as brass; fog has neither the required metallic nature nor a solid structure. Fog may scatter light because its droplets are large enough to interact with light, but it does not become a true solution merely because the droplets are very small. In classification questions, identify both phases: liquid dispersed in gas means aerosol. Memory cue: mist, fog and spray are liquid aerosols; smoke is generally solid particles in gas.

In Tyndall effect colloidal particles scatter light. Exam tip: if light path is visible think colloid.

The Tyndall effect is the visible scattering of a beam of light by particles in a mixture. In a true solution, the solute particles are extremely small and are distributed at the molecular or ionic scale. They are too small to scatter ordinary light strongly enough for the path of the beam to become visible under normal conditions.

Therefore option A is correct: the particles are very small. The explanation is about particle size, not about the solution being black or solid. Heavy particles are not the defining reason, and a true solution is generally a homogeneous liquid, although solutions can exist in other states. Colloids show the Tyndall effect more clearly because their particles are larger than those in true solutions.

A suspension is a heterogeneous mixture containing relatively large, insoluble particles dispersed in a fluid. Because these particles are large enough, they can be trapped by the pores of ordinary filter paper. Filtration therefore separates the suspended solid from the liquid, although the mixture may need to be allowed to settle or poured carefully.

Option A, suspension, is correct. Particles of a true solution are too small to be retained by normal filter paper and pass through with the solvent. A homogeneous gas mixture has particles mixed at the molecular level, and an alloy is a solid solution or homogeneous metallic mixture; neither is separated by ordinary filtration. The deciding clue is the presence of large suspended particles.

The governing concept is particle size in a true solution and the operation of ordinary filtration. A true solution is homogeneous: solute particles are distributed at the molecular or ionic scale and are much smaller than the pores of normal filter paper. Consequently, the solvent and dissolved solute pass through the paper together, so filtration does not separate them. Option A is correct. The particles are not necessarily heavy or magnetic, and they do not form solid sheets; therefore options B, C, and D do not explain the observation. A dissolved substance can instead be recovered by evaporation, crystallisation, or another suitable method.

Salt is the dissolved substance so it is the solute. Exam tip: the dissolved component is solute.

Water is the dissolving medium so it is the solvent. Exam tip: the larger liquid component is often solvent.

The substance that dissolves in a solution is called the solute. The substance in which it dissolves is the solvent, so solvent is not the correct answer. For example, in salt water, salt is the solute and water is the solvent. Exam tip: remember: solute dissolves; solvent does the dissolving.

The substance in which a solute dissolves to form a homogeneous mixture is called the solvent. For example, in salt water, water is the solvent and salt is the solute. Do not confuse the solute with the solvent: the solute is the substance that dissolves. Exam tip: the component present in a larger amount is usually considered the solvent.

A saturated solution contains the maximum amount of solute that can dissolve in a given amount of solvent at a specified temperature. If more solute is added after this limit is reached, it will remain undissolved or settle at the bottom, unless the temperature or another condition changes. Thus, saturation is always understood with a temperature condition.

Option A is correct: no more solute can dissolve at that temperature. A saturated solution must contain solvent and dissolved solute, so it is not a solute-free solution. It is not necessarily a gas and need not be colourless; its physical state and colour depend on the substances present. The key idea is the solubility limit at a fixed temperature.

An unsaturated solution can dissolve more solute at the same temperature. Exam tip: distinguish using dissolving capacity.

The direct answer is A: a concentrated solution contains a relatively large amount of solute compared with the amount of solvent or solution being considered. A solute is the substance that dissolves, such as salt, and the solvent is the substance that dissolves it, such as water. If more salt is dissolved in the same amount of water, the solution becomes more concentrated, provided the salt has not exceeded its solubility. Option A is correct because concentration describes how much solute is present in a given amount of solution or solvent. Option B is wrong: a solution with no solute is not a concentrated solution; it is simply the solvent. Option C is wrong because solutions may be solids, liquids, or gases, not only gases. Option D is wrong because a concentrated solution can be homogeneous, such as salt water; it is not always heterogeneous. The useful cue is: concentrated means more solute, while dilute means less solute.

A dilute solution contains a small amount of solute compared with the amount of solvent, so it has low concentration. Option B describes a more concentrated solution, not a dilute one. Exam tip: remember that dilute means low concentration, whereas concentrated means high concentration.

Crystallisation is used to obtain pure crystals. Exam tip: remember it for purification of impure solids.

Crystallisation gives a solid in pure crystal form. Exam tip: think crystallisation when pure solid is required.

Chromatography is based on different movement of soluble colours. Exam tip: remember ink dyes as an example.

The direct answer is A: paper chromatography. Chromatography separates substances because different components dissolve in a moving solvent to different extents and travel at different speeds. To separate ink, a small spot is placed on paper; the lower edge of the paper is put in a suitable solvent; the solvent rises through the paper; different dyes move different distances and appear as separate coloured spots. Option A is correct. Option B, simple distillation, is mainly used to separate a liquid from dissolved non-volatile material or liquids with a suitable boiling-point difference, not normally to display the several dyes in ink. Option C, sublimation, is used when a solid changes directly into vapour, so it is not the usual method for ink colours. Option D, filtration, separates insoluble solid particles from a liquid; dissolved dyes pass through the filter. Remember: a moving solvent carrying different colours through paper means chromatography.