Nature of matter

In evaporation higher energy particles escape. The average energy of remaining liquid decreases.

Fog forms when moist air close to the ground cools sufficiently for water vapour to change into extremely small liquid-water droplets suspended in the air. This gas-to-liquid change is called condensation, so option C is correct. During winter, cooling may reduce the amount of water vapour that air can hold. When the air reaches its dew point, excess vapour condenses around tiny particles and becomes visible as fog. Melting is a solid-to-liquid change, freezing is a liquid-to-solid change, and sublimation is a solid-to-gas change. None of those transformations describes the usual formation of fog. The droplets remain suspended, which explains why fog looks like a cloud near the ground.

Evaporation can occur at any temperature from surface. Boiling occurs throughout liquid at fixed boiling point.

Evaporation is a surface phenomenon in which higher-energy molecules escape from the exposed surface of a liquid into the gas phase. When the same amount of liquid is spread out, its exposed surface area becomes larger, so more molecules can escape at the same time. Consequently, the rate of evaporation increases, making option A correct, provided temperature, wind, and humidity are otherwise comparable. Increasing surface area does not stop evaporation, convert the liquid into a solid, or change its chemical identity into an element. This explains why water spreads or dries faster in a shallow dish than in a narrow container.

Humidity measures the amount of water vapour already present in the air. When humidity is high, the air is closer to saturation, so the difference between the vapour concentration at a liquid surface and that in the surrounding air becomes smaller. Consequently, fewer molecules leave the liquid per unit time and evaporation generally slows; option B is correct. It does not necessarily become zero, because evaporation can continue if the air is not fully saturated or if temperature and air movement support the process. High humidity does not make evaporation faster, and evaporation remains a physical change rather than becoming a chemical reaction. Wind and temperature can also affect its rate.

Boiling point is the temperature at which liquid changes rapidly into vapour. It is a physical property.

Melting point is the temperature at which solid changes into liquid. It can help identify a substance.

A pure substance has a definite, sharp melting point because it contains only one kind of chemical substance. A homogeneous mixture may look uniform, but it contains more than one substance and usually melts over a range of temperatures. Exam tip: A sharp melting point is an important indication of purity.

Gas particles have a large amount of empty space between them. When pressure is applied, the particles can move closer into this empty space, so the volume of a gas can decrease considerably and gases are highly compressible. Option A is more characteristic of solids, where particles are closely packed. Exam tip: greater interparticle space generally means greater compressibility.

Particles in liquids are slightly farther than in solids. Still compressibility is much less than gases.

Gas particles have large intermolecular spaces and move rapidly and freely. Hence, particles of one substance spread into another most quickly in gases, so diffusion is fastest in the gaseous state. Diffusion also occurs in liquids, but it is slower because their particles are closer together. Exam tip: remember the usual order: gas > liquid > solid.

In a solid, particles are closely packed and have strong forces of attraction between them. Therefore, they cannot move freely from one place to another; they mainly vibrate about fixed positions. This limited motion makes diffusion very slow in solids. Options B and C describe properties more closely associated with gases. Exam tip: diffusion is generally fastest in gases, slower in liquids, and slowest in solids.

Gas particles spread in available space due to random motion. Therefore gas has no fixed shape.

A change of state, such as ice melting into water or water changing into vapour, is a physical change. The chemical nature or composition of the substance remains the same; only the arrangement, separation, and motion/energy of its particles change. Therefore, option A is correct. Shape generally changes during a change of state because a solid has a definite shape, whereas liquids and gases do not. Exam tip: If no new substance is formed, the change is generally physical.

Melting of ice is a physical change. Both ice and liquid water have the chemical formula \(H_2O\); only the physical state changes from solid to liquid. Therefore, the chemical composition remains the same. Formation of oxygen or sodium chloride would require a chemical change. Exam tip: In a change of state, the composition of a substance generally remains unchanged.

In air, gases such as nitrogen and oxygen are uniformly mixed, so its composition is nearly the same throughout. Milk and smoke are heterogeneous colloids. Exam tip: a mixture that appears uniform is generally classified as homogeneous.

The direct answer is D: dissolving salt in water is generally a physical change. A physical change alters the form, distribution, or state of a substance without producing a new substance. When salt dissolves, its ions become distributed among water molecules. Under suitable conditions, water can be removed by evaporation and the salt can be obtained again, so the change is usually treated as reversible and physical at school level. Option A is wrong because a nuclear change involves the atomic nucleus, which does not happen when ordinary salt dissolves. Option B is wrong because dissolving salt does not turn it into a metal or represent a metallic change. Option C is wrong because optical change concerns light-related behaviour, not the basic classification of salt dissolving. Option D is correct because the substances can be recovered and no new chemical identity is normally required to explain the process. A helpful memory cue is: recoverable by evaporation usually points to a physical change, though unusual chemical reactions must be judged separately.

When paper burns, new substances such as ash, smoke, and gases are formed, and the original paper cannot be obtained back. Hence, it is a chemical change. In a physical change, the chemical composition remains unchanged, as in cutting paper. Exam tip: Formation of a new substance usually indicates a chemical change.

Distilled water contains only
H_2O molecules, so it is a pure compound with fixed composition. Air and brass are mixtures, while soil is heterogeneous. Exam tip: a compound always has a definite composition.

In a chemical change, the chemical composition changes and one or more new substances are formed. For example, rusting of iron produces iron oxide. A change in shape or cutting is usually a physical change because no new substance is formed. Exam tip: formation of a new substance is a key indicator of a chemical change.

Air has gases such as nitrogen and oxygen uniformly distributed, so its composition is the same throughout. Milk is a colloid and smoke is heterogeneous. Exam tip: a mixture with a single visible phase is generally homogeneous.

When wax melts, only its physical state changes from solid to liquid. Its chemical composition remains the same, and it can solidify again on cooling; therefore, it is a physical change. In a chemical change, a new substance is formed. Exam tip: Changes of state, such as melting and freezing, are generally physical changes.

In the flame, wax vapour reacts with oxygen to form new substances such as carbon dioxide and water vapour, releasing energy. Therefore, burning of a candle is mainly a chemical change. Melting of wax is a physical change, but it occurs alongside burning and is not the main change asked here. Exam tip: Formation of new substances during combustion indicates a chemical change.

States of matter are understood by particle model. Distance attraction and motion are key factors.

Properties of solid liquid and gas are explained by particle arrangement. State changes are mostly physical changes.