Nature of matter

Correct answer: A. In distillation vapour is condensed again

Direct answer: Option A, in distillation the vapour is condensed again. Evaporation is a surface phenomenon in which a liquid changes into vapour, usually without collecting and cooling that vapour. Distillation is a separation or purification method with two linked steps: heat a liquid mixture so a component vaporises, then cool the vapour so it becomes liquid again and can be collected. Step by step: provide heat; the more volatile component enters the vapour phase; guide the vapour into a cooler part; condense it; collect the liquid. Option A is correct because re-condensation is the defining practical difference. Option B is wrong because evaporation does not require a magnet; it requires suitable temperature and exposed surface, and magnetism is irrelevant. Option C is wrong because distillation is mainly used for liquids or liquid mixtures, not only solids. Option D is wrong because the two processes are not identical: evaporation may end with vapour escaping, while distillation deliberately recovers condensed liquid. For example, salt water can be separated by evaporating water and condensing it in a distillation setup, leaving salt behind. Memory cue: evaporation lets vapour go; distillation catches and cools it.

Correct answer: A. Evaporation

In evaporation, the solution is heated so that the solvent changes into vapour, leaving the dissolved solid behind as residue. Therefore, it is the simplest method when only the solid is required and the solvent need not be recovered. Simple distillation is unnecessary here because it is used when the solvent is also to be collected. Exam tip: first identify whether the solute, the solvent, or both are required.

Correct answer: B. Distillation

Distillation recovers solvent by condensing vapour. Exam tip: choose distillation when solvent recovery is needed.

Correct answer: A. Residue

The governing concept is the terminology of filtration. When a mixture containing an insoluble solid and a liquid is poured through a porous filter, the liquid and sufficiently small particles pass through. The solid particles retained on the filter paper are called the residue, so option A is correct. The liquid collected below the paper is called the filtrate, making option B the opposite term. A solvent is the component that dissolves a solute in a solution, and vapour is the gaseous form of a substance; neither specifically describes the retained solid. Correct use of residue and filtrate helps identify which part remains and which part passes through.

Correct answer: B. Filtrate

The liquid passing through the filter is called filtrate. Exam tip: remember names above and below the filter.

Correct answer: A. Sand

Sand is insoluble in water, and its particles are too large to pass through the pores of the filter paper. Therefore, sand remains on the filter paper as the residue, while water passes through as the filtrate. Pure water does not remain on the filter paper. Exam tip: filtration separates an insoluble solid from a liquid.

Correct answer: A. Because sugar dissolves and spreads as very small particles

The governing concept is the difference between a true solution and a suspension during filtration. In sugar solution, sugar dissolves in water and becomes distributed as extremely small molecules throughout the solvent. These dissolved particles are far smaller than the pores of ordinary filter paper, so water and sugar pass through together; no visible sugar residue remains on the paper. Thus option A is correct. Sugar is not magnetic, it does not become a gas merely by dissolving, and the material of the filter paper being metal or non-metal is unrelated to the result. Sugar can be recovered later by evaporating some water and crystallising the solute.

Correct answer: A. Kerosene and water

The governing concept is separation based on immiscibility and density. A separating funnel is used for two liquids that do not mix and form separate layers. Kerosene and water remain distinct because kerosene is non-polar while water is polar; kerosene generally forms the upper layer because it is less dense. The lower layer can be drained first and the upper layer collected separately. Salt and sugar solutions are homogeneous solutions, so their components cannot be separated with this apparatus. Ink colours require techniques such as chromatography, not a separating funnel. Therefore, option A is correct.

Correct answer: A. Because they are very small and uniformly distributed

The governing concept is the particle nature and homogeneity of a solution. In a true homogeneous solution, solute particles are extremely small and become uniformly distributed among solvent particles. Their size is below the resolving power of the unaided human eye, and the uniform distribution prevents visible separate layers or patches. The particles also do not settle under gravity during ordinary standing, which distinguishes a solution from a suspension. Colour, magnetism, and settling are not defining reasons: a solution need not be black or magnetic, and its particles remain dispersed. Therefore, option A correctly explains the observation.

Correct answer: A. When the components form separate layers

In a heterogeneous mixture, the composition is not uniform in all parts, and its components may remain visibly distinct. Separate layers, as in oil and water, show unequal distribution of components; therefore, option A is correct. Options B and D describe a homogeneous mixture, while a completely dissolved solute usually forms a homogeneous solution. Exam tip: visible layers, particles, or distinct regions indicate a heterogeneous mixture.

Correct answer: A. Particles may settle down

The governing concept is sedimentation in a suspension. A suspension contains relatively large, insoluble particles dispersed through a fluid. Unlike the particles of a true solution, these particles are not permanently and uniformly distributed. When the mixture is left undisturbed, gravity acts on the dispersed particles and they may move downward. As a result, solid material can collect at the bottom as sediment, while the upper liquid may become clearer. Stirring can temporarily redistribute the particles, and filtration can often separate them from the liquid. Standing does not change the suspension into an element, a gas, or a metal; those options describe unrelated transformations. Therefore option A correctly states the possible observation.

Correct answer: A. Mixture of solid particles in a gas

The governing concept is the classification of a mixture according to the dispersed component and the dispersion medium. In dusty air, small solid dust particles are distributed through air, which is a gaseous medium. The amount of dust is not fixed, and the particles can often be removed by filtration or allowed to settle under gravity. These observations show that dusty air is not a pure substance with a fixed composition and is not a compound formed by chemical bonding in a definite ratio. It is also not a liquid-in-liquid system. Depending on particle size and concentration, it may show suspension or aerosol-like behaviour, but the essential description remains solid particles dispersed in a gas. Hence option A is correct.

Correct answer: A. Suspension particles can settle on standing

Particles in a suspension are relatively large, so they may settle down under gravity when the mixture is left undisturbed. Colloidal particles are much smaller and generally do not settle on standing, so option B is incorrect. Suspension particles can be separated by ordinary filtration, making option C incorrect as well. Exam tip: settling of particles is a key feature of a suspension.

Correct answer: A. Milk

Milk is a colloidal system. Its tiny fat particles scatter light, so the path of a light beam may become visible; this is the Tyndall effect. Clear salt and sugar solutions are true solutions, whose particles are too small to produce this effect under ordinary conditions. Exam tip: the Tyndall effect is a key characteristic of colloids.

Correct answer: A. True solution

In a clear salt solution, salt particles are uniformly distributed in water at the ionic or molecular level. Therefore, it is a homogeneous true solution; its particles are not visible and cannot be separated by ordinary filtration. A colloid may show the Tyndall effect, whereas particles in a suspension are larger and may settle on standing. Exam tip: a transparent mixture that is uniform throughout is usually identified as a true solution.

Correct answer: A. Sediment

When insoluble soil particles settle at the bottom of muddy water under gravity, the deposited solid material is called sediment. The liquid above it is the supernatant, while a filtrate is the liquid that passes through a filter. Vapour is a gas and a solute is a substance dissolved in a solvent. Thus option A is correct.

Correct answer: A. Because it is a mixture of many gases

The governing concept is the distinction between a pure substance and a mixture. A pure substance has a fixed composition and characteristic properties, whereas natural air contains several gases, mainly nitrogen and oxygen, along with argon, carbon dioxide, water vapour, and other trace substances. Their proportions can vary with location, altitude, weather, season, and pollution. Consequently, natural air does not have one fixed chemical formula and is classified as a mixture rather than a pure substance. Air still occupies space and has mass, so options B and C contradict fundamental properties of matter. It is also not always solid; under ordinary conditions it is gaseous. Therefore option A correctly identifies why natural air is not pure.

Correct answer: A. Because it shows that matter is made of particles

The direct answer is option A. The particle model says that matter is made of very small particles. These particles are separated by spaces, are always moving, and attract one another; their arrangement and motion help explain solids, liquids and gases. Option A is correct because it states the basic idea of the model. Option B is wrong because a model does not itself change colour. Option C is wrong because the particle idea applies to solids, liquids and gases, not only metals. Option D is wrong because particles do not remove mass; matter still has mass. The model is a simple way to explain observable properties using invisible particles. Memory cue: matter is particle-based; state depends on particle spacing and motion.

Correct answer: C. Solid

Particles in solids have limited motion so energy is low. Exam tip: remember energy increases from solid to gas.

Correct answer: D. Plasma

In the plasma state, particles have very high energy and exist as ions and free electrons. Therefore, plasma particles generally have more energy than particles in gases, liquids, and solids. Gas particles have more energy than those in solids and liquids, but less than plasma particles. Exam tip: remember the increasing order of particle energy as solid < liquid < gas < plasma.

Correct answer: A. They vibrate more

On heating solid particles gain energy and vibrate more. Exam tip: remember increased vibration before melting.

Correct answer: B. Motion decreases

On cooling, the average kinetic energy of gas particles decreases, so their average motion becomes slower. Motion increases on heating, not on cooling. Exam tip: a decrease in temperature generally means lower kinetic energy and slower particles.

Correct answer: C. Solid

A solid has a definite shape, so its shape remains unchanged when it is placed in a container. A liquid may have a definite volume but takes the shape of its container, while gases and vapours spread to occupy the container. Exam tip: definite shape and definite volume are key properties of solids.

Correct answer: B. They can slide over one another

Particles in a liquid are fairly close together, but the force of attraction between them is weaker than in solids. Therefore, they can slide past one another, allowing liquids to flow. Option A is more suitable for solid particles, while option C describes gas particles. Exam tip: the ability of particles to slide past each other explains the fluidity of liquids.

Correct answer: B. Far apart and randomly arranged

Gas particles have large spaces between them and are not arranged in a fixed pattern. They move freely in all directions, so a gas spreads throughout its container. Option A is closer to the arrangement of particles in a solid, where particles are closely packed. Exam tip: remember gases as having “large separation, large empty space, and random motion.”