Nature of matter

Water is a compound and has a definite formula. Exam tip: do not write a fixed formula for mixtures.

Air is a mixture so it has no definite formula. Exam tip: distinguish formulas of mixtures and compounds.

Components of mixtures are generally separated by physical methods. Exam tip: identify mixtures through separation methods.

Water components are chemically combined so filtration cannot separate them. Exam tip: compounds are not separated by physical methods.

Hand picking is a physical separation method used when an unwanted component is present in a relatively small amount and is large enough, visibly different, and easy to remove by hand. In a mixture of gram and small stones, the stones can be seen because they differ in colour, size, and appearance from the gram; they can therefore be picked out individually. Thus option A is correct. Salt water and sugar water contain dissolved particles that cannot be removed by hand, so evaporation or crystallisation is more suitable. Air is a uniform mixture of gases and its components are invisible, so hand picking cannot separate it. The method depends on visible physical differences.

Sand is insoluble in water and remains on the filter. Exam tip: choose filtration for insoluble solid and liquid.

Salt can be obtained by evaporating water from salt solution. Exam tip: choose evaporation for dissolved solid recovery.

Magnetic separation works when one component of a mixture is attracted by a magnet and the other component is not. Iron is a ferromagnetic substance, so iron filings are pulled toward a magnet, whereas sulfur is non-magnetic and remains behind. A magnet can therefore remove the iron filings from the iron–sulfur mixture, making option A correct. Salt and sugar dissolve in water and cannot be selectively collected with a magnet. Oil and water form separate layers because they are immiscible, so a separating funnel or careful decantation is appropriate instead. The governing concept is the difference in magnetic properties, not merely the fact that the substances are solids or liquids.

The direct answer is option A: oil and water. A separating funnel is used for two liquids that do not mix and form separate layers because their densities differ. Oil floats above water, so the lower water layer can be released through the tap and the oil can then be collected separately. Option A is correct because oil and water are immiscible liquids. Option B, salt and water, forms a solution; the salt is dissolved, so a separating funnel cannot separate it. Option C, sugar and water, also forms a solution and needs evaporation or crystallisation for separation. Option D, sand and water, contains an insoluble solid and a liquid; filtration or sedimentation is more suitable, not a separating funnel. The method depends on the physical nature of the mixture, not simply on whether the components look different. Memory cue: two non-mixing liquid layers, such as oil and water, suggest a separating funnel.

Direct answer: Option A, obtaining water from a salt solution. Distillation separates substances by differences in boiling points and includes vaporisation followed by condensation. Step by step: heat the salt solution; water, the volatile component, changes into vapour; dissolved salt does not vaporise under these conditions; cool the vapour; it condenses as liquid water in another container. A is correct because it recovers the solvent, water, and leaves salt behind. B, rice and pebbles, can be separated by handpicking or sieving because the solid pieces differ in size and appearance. C, iron filings and sand, are better separated with a magnet because iron is magnetic. D, cloth and paper, can be separated by hand or by suitable physical sorting, not normally by distillation. Distillation is especially useful when a liquid must be recovered from a solution. Memory cue: distillation means boil, carry vapour, cool and collect.

On heating, camphor changes directly from solid to vapour without passing through the liquid state, and it deposits again as solid camphor on a cool surface. Sand does not sublime, so a mixture of camphor and sand can be separated by sublimation. Salt and water are separated by evaporation, oil and water by a separating funnel, and sublimation is not suitable for milk and water. Exam tip: choose sublimation when one solid component sublimes and the other does not.

Chromatography is based on different movement of dyes. Exam tip: remember ink as a common example.

In a true solution, solute particles are extremely small and do not settle on standing. In a suspension, particles are comparatively larger and may settle after some time. Option B is the reverse of the correct fact: suspension particles can be separated by ordinary filtration, but true-solution particles cannot. Exam tip: settling of particles is a key sign of a suspension.

Colloidal particles are larger than particles in a true solution, so they can scatter a beam of light and show the Tyndall effect. Particles in a true solution are too small to scatter light appreciably. Option B reverses the particle-size relation, and option C is incorrect because colloidal particles are not retained by ordinary filter paper. Exam tip: associate the Tyndall effect with colloids.

Particles in a suspension are relatively large, so they settle down under gravity when the mixture is left undisturbed. Colloidal particles are much smaller and generally do not settle. Option B is incorrect because suspension particles are larger than colloidal particles. Exam tip: settling of particles is a key feature of a suspension.

Milk is commonly classified as an emulsion, which is a type of colloid: tiny droplets of one liquid are dispersed through another liquid, and the particles are larger than those in a true solution. Salt water is formed when sodium chloride dissolves uniformly in water at the molecular or ionic level, so it is a homogeneous true solution. Therefore option A is correct. Milk is not an element, and salt water is not a single compound because its salt-to-water ratio can vary and it can be separated by physical methods. Neither substance is a gas under ordinary conditions, so options B, C and D confuse the states or classifications of matter.

A colloid contains a dispersed phase distributed through a continuous dispersion medium. Smoke is generally an aerosol in which very fine solid particles, such as soot, are dispersed in a gas, usually air. Fog is also an aerosol, but its dispersed phase consists of tiny liquid water droplets in air. Thus both are aerosol-type colloids, although the nature of their dispersed particles differs; option A is correct. They are not pure metals or solid compounds. The claim that they have no mass is incorrect because both contain matter with measurable mass. The important governing concept is classification by the physical state of the dispersed phase and dispersion medium.

At a given temperature, a saturated solution contains the maximum amount of solute that can dissolve, so no additional solute dissolves in it at that temperature. An unsaturated solution contains less than this maximum amount and can dissolve more solute. Option B is incorrect because both types of solutions contain solute. Exam tip: always relate saturation to the maximum solubility of a solute at a specified temperature.

The difference between concentrated and dilute solutions depends on the relative amount of solute present in a fixed amount of solvent. A solution with more solute is concentrated, whereas one with less solute is dilute. Temperature may affect solubility, but it is not the basis for describing a solution as concentrated or dilute. Exam tip: link ‘concentrated’ and ‘dilute’ with a comparison of the amount of solute.

The governing concept is the composition of a solution. The solute is the substance that gets dissolved, while the solvent is the component that dissolves it and usually forms the larger part of the solution. For example, when salt is mixed with water, salt is the solute and water is the solvent; together they form salt solution. The solvent need not always be water, liquid, or even the larger component in every special case, so options B and C use absolute and incorrect claims. Option D is also false because solutes and solvents may be solids, liquids, or gases, and they are not necessarily metals. Therefore, option A correctly states the essential distinction.

In a salt-water solution, salt is the substance that dissolves in a smaller amount, so it is the solute. Water is the medium that dissolves the salt, so it is the solvent. Option B reverses the roles of solute and solvent. Exam tip: the substance that dissolves another substance in a solution is generally the solvent.

Direct answer: A, residue stays on the filter and filtrate passes through below. Filtration separates an insoluble solid from a fluid by using a porous filter. The filter has tiny openings: the fluid and dissolved substances that can pass through collect below as the filtrate. The insoluble solid particles that are too large to pass through remain on the filter paper as the residue. Thus option A correctly identifies both terms and their positions. Option B is wrong because residue is not always a gas; in the usual school example of sand and water, it is a solid. Option C is wrong because filtrate is generally a liquid in ordinary filtration, not always a solid. Option D is wrong because the two words name different parts obtained after filtration. For example, filtering muddy water leaves mud as residue and gives clearer water as filtrate. Memory cue: residue remains; filtrate has filtered through.

In sedimentation, heavier insoluble particles settle at the bottom under gravity. The clearer liquid above is then carefully poured into another container; this is decantation. Therefore, sedimentation occurs before decantation. Exam tip: settling is sedimentation, while pouring off the upper liquid is decantation.

The direct answer is option A: crystallisation focuses on obtaining pure crystals. Evaporation removes solvent, commonly water, by changing it into vapour. It may leave a solid residue, but rapid or complete evaporation can give impure or poorly formed solids. Crystallisation uses a suitable solution, often concentrates it and then cools it carefully, so a relatively pure solid forms as crystals while many impurities remain in the solution. Option A is correct because the main purpose of crystallisation is purification and recovery of a solid in crystal form. Option B is wrong: evaporation does not always use a magnet; magnets are unrelated to ordinary evaporation. Option C is wrong because crystallisation is commonly used for dissolved solids, not only gases. Option D is wrong because the methods have different purposes and are not identical. Memory cue: evaporation removes solvent; crystallisation aims at pure crystals.

Pure water is a fixed composition compound while sea water has dissolved salts. Exam tip: natural water samples are mixtures.