Nature of matter

When an apple is cut, its tissues come into contact with oxygen in air. In the presence of enzymes, oxidation produces new brown-coloured substances, so browning is a chemical change. In a physical change or a change of state, the chemical composition of the substance does not change. Exam tip: A colour change accompanied by formation of a new substance usually indicates a chemical change.

Density is an intensive property because it remains the same for smaller or larger samples of the same substance. Using density = mass/volume, doubling a sample doubles both mass and volume. Mass and volume are extensive properties. Exam tip: properties dependent on amount are extensive.

The direct answer is A: new substances are formed. A chemical change is a change in which the original substance changes into one or more different substances with new properties. During burning, the material reacts with oxygen in air. For example, wood or a fuel can form carbon dioxide, water vapour, ash, smoke, and heat. These products are not simply the original material in a different shape or state. Option A is correct because formation of new products is the main sign of a chemical change. Option B is wrong because changing only shape, such as cutting paper, does not make a new substance. Option C is wrong because changing only state, such as melting ice, is a physical change; the substance remains water. Option D is wrong because burning involves an actual reaction with oxygen. Memory cue: new substance means chemical change; only shape or state means physical change.

In melting solid becomes liquid but identity of substance remains same. Therefore it is physical change.

The governing concept is reversibility without a change in chemical identity. Ice melts when heat is supplied and becomes liquid water; on removing heat, the same water freezes back into ice. The process is therefore a reversible physical change, and option C is correct. Only the state and particle arrangement change, while the chemical formula remains H2O. Burning wood and converting paper to ash produce new substances through chemical reactions, so they are not reversible physical changes. Souring milk also involves chemical and biological changes that cannot be reversed simply by cooling or reheating.

When wood burns new substances form and original wood is not easily recovered. It is an irreversible chemical change.

Particulate nature tells that matter is made of particles. It explains diffusion and state properties.

Sugar particles can fit into spaces between water particles. This gives idea of interparticle space.

The governing concept is the random motion of very small suspended particles caused by continuous collisions with rapidly moving molecules of the surrounding medium. This irregular, zigzag movement is called Brownian motion and provides evidence for the particulate nature and kinetic motion of matter. Therefore option A is correct. The motion is not itself a melting point, chemical formula, or measurement of molecular mass. Its speed can be influenced by factors such as temperature, particle size, and the viscosity of the medium, but its defining feature remains random particle movement. This explains why dust or pollen particles may appear to move unpredictably in a fluid.

Diffusion is evidence of continuous motion of particles. It is easily seen in gases and liquids.

The direct answer is A: interparticle attraction affects the state and shape of matter. Particles in every substance attract one another, but the strength of attraction and the amount of particle motion differ. In a solid, strong attraction keeps particles close in a fixed arrangement, so the solid has a fixed shape and volume. In a liquid, attraction is weaker relative to particle movement, so particles remain close but can slide, giving a fixed volume and no fixed shape. In a gas, particles are far apart and move freely, so the gas has neither fixed shape nor fixed volume. Option A is correct because attraction helps determine these physical properties. Option B is wrong because student marks depend on learning and assessment, not particle attraction. Option C is irrelevant; notebook colour is not determined by this molecular idea. Option D is wrong because sound pitch mainly depends on frequency, not simply on interparticle attraction. A useful cue is: strong attraction means particles stay close; weaker effective attraction allows more movement.

Gas particles have large intermolecular spaces between them. On applying pressure, these spaces decrease, so gases are highly compressible. A fixed shape is a property of solids; a gas neither has a fixed shape nor particles fixed in position. Exam tip: Remember that large spaces between particles mean high compressibility in gases.

A liquid has a definite volume because its particles are much closer together than those in a gas. However, the particles can slide past one another, so a liquid takes the shape of its container and has no fixed shape of its own. Option B describes a solid, while options C and D are closer to the properties of a gas. Exam tip: liquid = fixed volume, no fixed shape.

In solid particles are close and strongly attracted. Therefore shape and volume remain definite.

The governing concept is the particle model of matter: temperature is related to the average kinetic energy of particles, while the state of matter describes how freely those particles can move. In a gas, particles are separated by relatively large spaces and move rapidly in random directions with the greatest freedom. Therefore, among the listed states, gas generally has the highest particle motion and hence the highest average kinetic energy under comparable conditions. Solid particles mainly vibrate about fixed positions, and liquid particles can move but remain relatively close together. A crystal is an ordered solid, so it is not a separate higher-energy state. Thus option A is correct.

In solids particles mainly vibrate around fixed positions. Therefore kinetic energy is generally lowest.

Temperature measures the average kinetic energy of the particles of a substance. When temperature increases, particles move faster, so their average kinetic energy increases. The option stating that kinetic energy remains constant is incorrect because a rise in temperature means a rise in average kinetic energy. Exam tip: Remember that temperature is directly linked to the average kinetic energy of particles.

On cooling, the average kinetic energy of the particles decreases, so their motion becomes slower. Their motion does not stop completely because particles still possess some motion at ordinary temperatures. Exam tip: particle kinetic energy increases on heating and decreases on cooling.

The governing concept is the particle model of matter: all matter is made of very small particles, the particles are in continuous motion, and spaces exist between them. This model connects microscopic behaviour with observations. In a solid, closely packed particles mainly vibrate, so the solid keeps a definite shape. In a liquid, particles can move past one another, so the liquid flows. In a gas, particles are far apart and move freely, allowing the gas to fill its container and be compressed. Random particle motion also explains diffusion, such as the spreading of perfume through a room. Therefore option A is correct. The other choices have no scientific basis and do not describe what the model explains.

The direct answer is A: ice, water and steam are different states of the same substance, water. Ice is solid water, liquid water is the liquid state, and steam is water in the gaseous state. In a state change, the arrangement and movement of particles change, but the chemical identity remains \\(H_2O\\). Melting changes ice to water, boiling changes water to vapour, and cooling can reverse these changes. Option A is correct. Option B is wrong because iron and wood are different substances with different compositions. Option C is wrong because salt and sugar are different compounds, not states of one substance. Option D is wrong because sand and soil are mixtures with different compositions. The important test is not whether the materials look related, but whether their chemical composition remains the same. Memory cue: ice, water and steam are all \\(H_2O\\).

The direct answer is A: in a state change, the chemical identity generally remains the same. A state change alters the physical form or arrangement of particles. For example, melting ice, boiling water and freezing water change the state of \\(H_2O\\), but do not normally create a different chemical substance. Option A is correct because the change is physical when composition remains unchanged. Option B is wrong: a new element does not always form; ordinary melting and boiling do not form elements. Option C is wrong: atoms do not vanish during a normal state change; they remain present and are rearranged or move differently. Option D is wrong: matter does not always become energy; that is not the description of an ordinary phase change. The word “generally” is important because unusual extreme conditions can involve chemical or nuclear processes, but standard school examples are physical changes. Memory cue: change in state, not identity.

Matter is defined as anything that has mass and occupies space, so option A is correct. Matter can exist as a solid, liquid, or gas; therefore, option B is incorrect. Also, particles of matter have spaces between them. Exam tip: Remember “mass + occupies space” as the key identification of matter.

In Nature of matter physical chemical properties and changes are asked. Identification becomes easy through examples.

Definition states classification properties and changes of matter are the basic foundation. In exams classify through examples.

The most general definition of matter is that it has mass and occupies space, so option B is correct. Colour is not essential because many substances are colourless; matter is also not always solid or necessarily visible. Exam tip: remember both conditions—mass and occupied space—when identifying matter.