Nature of matter

A pure substance has a uniform, definite composition and consistent properties throughout the sample. On the basis of chemical composition, pure substances have two main classes: elements and compounds. An element contains only one kind of atom and cannot be decomposed into simpler substances by ordinary chemical means. A compound contains two or more elements chemically combined in a fixed ratio and can be decomposed chemically. Therefore option A is correct. Solid, liquid, and gas in option B are physical states, not chemical classes, while option C mixes unlike categories and includes examples rather than a valid classification.

An atom is the smallest particle of an element that retains the chemical properties of that element. Therefore, atom is the correct answer. A compound is formed by the chemical combination of two or more elements, so it cannot be the simplest particle of a single element. Exam tip: To distinguish an element from a compound, check whether it contains only one kind of atom.

In a compound elements chemically combine in a fixed ratio. Therefore properties can differ from elements.

A mixture contains two or more substances that need not be present in a fixed ratio, so the proportion of its components can vary. Hence, a mixture does not have a fixed composition. In contrast, elements and compounds are pure substances with fixed composition. Exam tip: variable composition generally indicates a mixture.

Hydrogen is an element because it consists of only one type of atom: hydrogen atoms. A compound contains two or more elements chemically combined in a fixed ratio, which is not the case for hydrogen. Exam tip: A substance made of only one kind of atom is an element.

Distilled water contains only H₂O molecules, so it is a pure compound. Air, brass and milk contain more than one substance. Exam tip: a pure substance has a fixed composition.

Brass is an alloy of copper and zinc. The proportion of its components can vary, and they are not chemically combined in a fixed ratio to form a new compound; therefore, brass is a homogeneous mixture. A pure compound has a fixed composition, whereas brass does not. Exam tip: Alloys are generally classified as homogeneous mixtures.

Steel is an alloy made mainly by mixing iron with carbon and sometimes other elements. An alloy is a mixture of metals or a metal with other elements, so steel is neither a pure element nor a pure compound with a fixed composition. Option C is close because most steel is homogeneous, but alloy is the more specific classification. Exam tip: For a mixture involving a metal, look for the term ‘alloy’.

Solvent is the component present in larger amount in a solution. Water is a common solvent in many solutions.

A solute is the substance present in a relatively smaller amount that dissolves in the solvent to form a solution. For example, in salt water, salt is the solute and water is the solvent. A solvent is the component present in the larger amount, so option A is incorrect. Exam tip: remember that the substance that gets dissolved is the solute.

Salt water is a solution formed when salt dissolves uniformly in water. The solvent is the component that dissolves the other component and is commonly present in the larger amount; the solute is the substance that is dissolved. In this example, water dissolves the salt and is normally present in greater quantity, so water is the solvent and option C is correct. Salt is the solute, not a precipitate, because it remains distributed at the molecular or ionic level rather than settling. Water in this solution is not being classified as an element; the question asks for its role in the solution.

In salt water, salt is the substance that dissolves, so it is called the solute. Water is the medium in which salt dissolves and is therefore the solvent. A colloid has particles that are not truly dissolved, whereas salt water is a homogeneous solution. Exam tip: the substance that dissolves is the solute, and the substance that dissolves it is the solvent.

In a solution, solute particles are uniformly distributed in the solvent, so its composition is the same throughout. Therefore, a solution is a homogeneous mixture. In a heterogeneous mixture, the composition is not uniform in all parts. Exam tip: A mixture with uniform composition is homogeneous.

A suspension is a heterogeneous mixture containing relatively large particles. On standing, these particles may settle at the bottom due to gravity, so option B is correct. Particles that remain dispersed without settling are more characteristic of a colloid, not a suspension. Exam tip: A mixture whose particles can be filtered and settle on standing is a suspension.

In muddy water, relatively large soil particles are temporarily dispersed in water, making the mixture heterogeneous. These particles settle on standing and can be separated by filtration; therefore, muddy water is a suspension. In a homogeneous solution, particles do not settle in this way. Exam tip: A mixture that forms sediment on standing is generally a suspension.

Particle size provides a useful physical distinction among mixtures. True-solution particles are molecular or ionic in size and are the smallest; suspension particles are much larger and may settle under gravity; colloidal particles have an intermediate size between these two ranges. Because of this intermediate size, a colloid can scatter light and generally does not settle rapidly, although its particles may not be visible individually. Therefore option D is correct. Option A reverses the comparison, option B incorrectly makes colloids larger than suspensions, and option C has no physical meaning because dispersed particles are not zero-sized.

Milk is a colloid, specifically an oil-in-water emulsion. Very fine fat droplets are dispersed in water and do not normally settle on standing. In a suspension, particles are comparatively larger and may settle after some time. Exam tip: If tiny fat or oil droplets are dispersed in water, identify the mixture as an emulsion colloid.

The direct answer is B: smoke is a colloid. Smoke is made when very small solid particles, such as soot or ash, are dispersed throughout a gas, usually air. In colloids, the dispersed particles are larger than ordinary molecules but small enough to remain spread through the medium for a considerable time. Smoke can scatter light, which is a familiar colloidal behaviour. Option B is correct because its dispersed phase is solid and its dispersion medium is gas. Option A is wrong: smoke is not a pure gas because it contains solid particles. Option C is wrong: it is not a pure element because smoke is a combination of different substances. Option D is wrong: it is not a pure metal, and its particles may contain several materials. Do not confuse a colloid with a true solution or a coarse suspension; school-level classification identifies smoke as a solid-in-gas colloid. Memory cue: smoke = solid particles floating in gas.

The Tyndall effect is the scattering of a beam of light by colloidal particles, making the path of the beam visible. Therefore, a colloid is the correct answer. Particles in a true solution are too small to produce visible scattering. Exam tip: A visible path of light is a key test for a colloid.

Particles of a true solution are very small and do not scatter light. Therefore Tyndall effect is not seen.

Colloidal particles scatter the incident light, making the path of the light beam visible. This phenomenon is called the Tyndall effect. Brownian motion refers to the random motion of colloidal particles, not the visibility of the light path. Exam tip: In colloids, visible light scattering indicates the Tyndall effect.

Sand particles are large and can be retained on filter paper. This is a common example of filtration.

In filtration, a mixture is passed through filter paper. Insoluble solid particles remain on the paper as residue, while the liquid passes through as filtrate. Therefore, option C is correct for a mixture such as sand and water. Two miscible liquids cannot be separated by filtration; a method such as distillation is used instead. Exam tip: Remember that filtration is used for an insoluble solid–liquid mixture.

Salt forms a solution when it dissolves in water. On evaporation, water escapes as vapour and salt is left behind as a solid residue. Filtration cannot separate dissolved salt because the salt particles pass through the filter paper. Exam tip: Evaporation is commonly used to obtain a dissolved solid from a solution.

The governing concept is separation of a homogeneous liquid mixture by exploiting a difference in boiling points. In distillation, the liquid with the lower boiling point vaporises more readily when the mixture is heated. The vapour is then cooled in a condenser and collected as liquid, while the less volatile component remains behind or distils later. Thus option A, distillation, is correct. Filtration cannot separate miscible liquids because both pass through the filter, magnetic separation requires a magnetic component, and handpicking is useful only for visibly distinct solid pieces.