Nature of matter

The direct answer is B: attraction is less than in a solid and more than in a gas. Particles in every substance attract one another, but the strength of attraction differs. In a solid, particles are very close and held strongly, so they mainly vibrate in fixed positions. In a liquid, particles remain fairly close but can move past one another, showing intermediate attraction. In a gas, particles are far apart and attraction is very weak. Therefore the usual order is solid greater than liquid greater than gas. Option A reverses the comparison with a solid and is wrong. Option B gives the correct order. Option C is wrong because liquid particles do attract one another; zero attraction would not explain liquid cohesion. Option D is wrong because attraction is not always infinite; liquid particles can flow and take the shape of their container. Memory cue: stronger attraction means less freedom of movement: solid, then liquid, then gas.

Gas particles are far apart so attraction is very low. Exam tip: attraction decreases as distance increases.

Gas particles are in continuous random motion and are separated by relatively large empty spaces. When these moving particles strike the inner walls of a container, they exert forces on those walls. The force acting over an area produces pressure. A gas therefore presses on all parts of the container, not just on its bottom, and it takes the shape and volume of the container.

Option B, gas, is correct. Solids have particles that mainly vibrate around fixed positions, so they do not easily spread throughout a container. A crystal and a metal are forms of solid matter, not separate states with the described behavior. The pressure of a gas results from countless collisions of its particles with the container walls.

The direct answer is B: melting of ice. A change of state is a physical change in which the substance remains chemically the same but changes between solid, liquid, or gas. Ice is solid water. On receiving heat, its particles move more freely and the solid changes into liquid water, but the substance is still H₂O. Option B is correct because only the physical state changes. Option A, rusting of iron, is a chemical change: iron reacts with oxygen and moisture to form new iron compounds. Option C, souring of milk, is a chemical change because new substances and new tastes are produced. Option D, burning wood, is a chemical change because combustion forms ash and gases. Melting can usually be reversed by cooling, which is another useful clue, although reversibility alone is not the only test. Memory cue: ice, water, and steam are the same substance in different states.

The change of a liquid into a gas is called vaporisation. It can occur from the surface of a liquid at temperatures below its boiling point. Melting is solid to liquid, freezing is liquid to solid, and condensation is gas to liquid. Exam tip: identify a change of state by checking the initial and final states.

The governing concept is the change of physical state caused by changes in temperature or pressure. When a gas loses thermal energy or is compressed suitably, its particles move closer and form a liquid; this gas-to-liquid transition is called condensation. For example, water vapour forms liquid water on a cool surface. Hence option A is correct. Melting in option B is the change from solid to liquid, not gas to liquid. Sublimation in option C is the direct change between solid and gas, commonly solid to gas on heating. Decomposition in option D is a chemical change in which a compound breaks into simpler substances, not a physical state change. Condensation changes arrangement and energy, while the substance remains chemically the same.

When a solid absorbs heat, the kinetic energy of its particles increases and it can change into a liquid. This process is called melting, for example, ice changing into water. Freezing is the reverse process, in which a liquid becomes a solid. Exam tip: remember solid → liquid as melting.

When a liquid is cooled, the kinetic energy of its particles decreases and they can arrange into a solid state. This change is called freezing, for example, water changing into ice. Melting is the opposite process, in which a solid changes into a liquid. Exam tip: remember liquid → solid as freezing and solid → liquid as melting.

Sublimation is a change of state in which a solid changes directly into a gas without first becoming a liquid. The particles at the surface gain enough energy to leave the solid structure and enter the gaseous state. The reverse change, from gas directly to solid, is called deposition, so it should not be confused with sublimation.

Common school examples include camphor and iodine, which can form vapour when heated without producing an ordinary liquid stage under suitable conditions. Therefore option A is correct. Option B describes freezing, option C describes condensation, and option D describes melting. The essential identifying point is that the liquid state is skipped during sublimation.

The governing concept is sublimation, a physical change in which a substance changes directly from solid to vapour without passing through the liquid state under suitable conditions. Ammonium chloride is a standard school-level example: on heating, it produces vapour that can deposit again as a solid on a cooler surface. Thus option D is correct and is also relevant to sublimation-based separation of mixtures. Sugar generally melts and may decompose on strong heating, sand does not normally sublime, and common salt has a very high melting point and is not treated as a routine subliming substance in this context. The process is physical because the chemical identity of ammonium chloride is retained during deposition.

Solid particles are close so density is generally high. Exam tip: choose solids when general trend is asked.

Density is the amount of mass present in a unit volume, written as \\(\rho = m/V\\). In gases, particles are much farther apart than in liquids and solids. Therefore, a given volume of gas usually contains less mass than the same volume of a liquid or solid, so its density is generally the lowest. The exact value can vary with temperature and pressure.

Option C, gas, is correct for the usual comparison among the three states. Solids generally have closely packed particles and high density, while liquids are also much more closely packed than gases. “Generally” is important because unusual substances and special conditions can produce exceptions in detailed comparisons, but it does not change the standard school-level answer here.

When water is cooled, it changes from the liquid state to solid ice. This change of state is called freezing or solidification, so liquid to solid is correct. A change from solid to gas is sublimation, which does not occur here. Exam tip: first identify the initial and final states of the substance.

The direct answer is A: liquid to gas. Boiling occurs when a liquid is heated to its boiling point and changes throughout the liquid into a gas or vapour. Water in a vessel is liquid; on sufficient heating, it becomes water vapour. Thus the direction is liquid → gas. Option A is correct. Option B, gas to liquid, is condensation, such as steam cooling into water. Option C, solid to liquid, is melting, such as ice becoming water. Option D, liquid to solid, is freezing or solidification, such as water becoming ice. Boiling and evaporation both represent liquid-to-gas change, although boiling occurs throughout the liquid at its boiling point while evaporation can occur from the surface at many temperatures. Memory cue: heating generally gives melting and then boiling: solid → liquid → gas.

Direct answer: Option C, condensation. Condensation is the change of a gas or vapour into a liquid. Air always contains some water vapour. When a bottle is colder than the surrounding air, the water vapour touching its cold outer surface loses heat. It changes into tiny liquid-water droplets, which we see on the bottle. Step by step: water is present in air as invisible vapour; the cold bottle cools the nearby vapour; the vapour changes state from gas to liquid; liquid droplets collect on the surface. Option A, melting, means a solid changes into a liquid, such as ice becoming water, so it does not describe water vapour in air. Option B, sublimation, means a solid changes directly into a gas, such as solid carbon dioxide changing to gas; no such change is involved here. Option C is correct because gas becomes liquid. Option D, freezing, means a liquid changes into a solid, such as water becoming ice; the droplets on the bottle are liquid, not solid. Important exam cue: the water comes from the air, not usually through the bottle. Remember: cooling vapour gives condensation.

Gas particles move freely so they spread quickly. Exam tip: consider diffusion as a gas property.

A liquid has a definite volume but no definite shape. Therefore, it takes the shape of its container while its volume remains nearly constant. A solid has both definite shape and volume, whereas a gas and plasma have neither definite shape nor definite volume. Exam tip: identify a liquid by remembering—definite volume, indefinite shape.

Gas particles have large intermolecular spaces and move freely in all directions. Therefore, a gas expands to occupy the entire available volume of a container. A liquid has a fixed volume: it takes the container’s shape but fills it completely only when enough liquid is present. Exam tip: gases have neither fixed shape nor fixed volume, whereas liquids have fixed volume.

Particles in a liquid are close enough for the liquid to have a nearly fixed volume, but they can move past one another, so it flows. Thus, a liquid takes the shape of its container without filling all of it. A gas also flows, but it has neither fixed shape nor fixed volume and fills the entire container. Exam tip: fixed volume but no fixed shape identifies a liquid.

Particles in salt are arranged in a regular, fixed pattern to form crystals; therefore, salt is a solid under ordinary conditions. Unlike a liquid, a salt crystal has a definite shape and volume. Exam tip: crystalline substances are generally identified as solids.

Mixture is a chemical classification not a state. Exam tip: separate state basis from composition basis.

On chemical composition basis matter is classified as pure substances and mixtures. Exam tip: classification changes with the basis.

A pure substance contains only one kind of constituent particles, so it has a fixed composition. Elements and compounds can both be pure substances. White colour, gaseous state, and solubility in water are not tests of purity; many pure substances are coloured, solid, or insoluble in water. Exam tip: fixed composition is the key property of a pure substance.

The direct answer is C: copper. A pure substance has a fixed composition and consistent properties. It may be an element, made of one kind of atom, or a compound, made of elements chemically joined in a fixed ratio. Copper is an element, so a properly pure sample contains only copper atoms and has a definite composition. Option C is correct. Option A, sea water, is a mixture of water, dissolved salts and other substances, so its composition can vary. Option B, air, is a mixture of gases such as nitrogen, oxygen and carbon dioxide, so it is not one pure substance. Option D, soil, contains mineral particles, organic matter, water and air in varying amounts, so it is also a mixture. The key reasoning is not whether something looks uniform, but whether its composition is fixed. Remember: element or compound can be pure; air, soil and sea water are mixtures.

Air contains several gases, mainly nitrogen, oxygen, argon, carbon dioxide and water vapour; therefore, it is a mixture. Gold and oxygen are elements, while sugar is a pure compound. Exam tip: a substance formed by the physical combination of two or more substances is called a mixture.