Nature of matter

The direct answer is option A: physical change. Sublimation is the direct change of a solid into vapour without first becoming a liquid. Camphor changes its state and gradually disappears into vapour, but its basic substance remains camphor; no new substance is required in this process. Therefore it is a physical change. Option A is correct because change of state without formation of a new substance is physical. Option B is wrong because a chemical change forms a new substance with new composition or properties. Option C, rusting, is a chemical process in which iron compounds form, not sublimation. Option D, fermentation, is also a chemical process involving conversion of substances by microorganisms. The word “sublimation” describes a physical state change, although the vapour should not be inhaled unnecessarily. Memory cue: solid directly to gas, with no new substance, means physical sublimation.

Boiling of water is a physical change in which water changes from the liquid state to the gaseous state (water vapour). No new substance is formed. Burning paper, curd formation, and rusting of iron are chemical changes because new substances are produced. Exam tip: in a change of state, the chemical composition of the substance remains unchanged.

Burning paper forms new substances, so it is the correct answer: option C. During combustion, paper reacts with oxygen and produces ash, carbon dioxide, water vapour, and other gases. These products have a different composition and different properties from paper, which demonstrates a chemical change. Melting wax changes its physical state, freezing water changes liquid water into solid water, and grinding salt changes only the size of its crystals. In those three cases, the chemical identity of the material is retained. The governing rule is that formation of one or more new substances indicates a chemical change, whereas a change in state or size alone is physical.

When a tomato rots, microorganisms and enzymes form new substances, causing changes in its colour, smell and taste. It cannot be easily changed back into a fresh tomato, so it is a chemical change. In a change of state, no new substance is formed. Exam tip: rotting, burning and cooking are generally chemical changes.

Crushing ice is a physical change because only the size and shape of the solid water change. Each smaller piece is still ice made of H₂O, and no new substance is produced during crushing. The process may change the surface area and appearance, but it does not alter chemical composition. Therefore option A is correct. Chemical change is not the best classification because no reaction occurs. Fermentation is a biological chemical process that converts substances such as sugars into new products, while combustion is a reaction with oxygen that releases heat and usually forms new substances. Neither process is involved in simply crushing ice.

The direct answer is option A: respiration is a chemical change. During respiration, oxygen helps break down food molecules, and carbon dioxide and water are formed while energy is released for the body. Because new substances form and the original materials are chemically changed, this is not merely a physical process. Option A is correct. Option B is wrong because physical change does not create new chemical substances, whereas respiration does. Option C, filtration, is a separation method based on particle size and does not describe respiration. Option D, sublimation, is the direct change of a solid into a gas and is unrelated to breathing. Memory cue: oxygen used plus carbon dioxide formed means a chemical reaction is occurring.

Water becoming ice is only a state change. Exam tip: if form changes but substance remains same it is physical.

Burning paper changes the chemical identity of matter because paper reacts with oxygen and is converted into new substances such as ash and gases. The products no longer have the composition or properties of the original paper, so the process is a chemical change. Thus option B is correct. Boiling water and melting ice are physical state changes: in both cases the substance remains H₂O. Tearing cloth changes its size and shape but normally does not change the chemical composition of its fibres. The essential governing concept is not simply a visible change; it is whether the original substance is transformed into a different substance through a chemical reaction.

Baking brings new properties and chemical changes. Exam tip: permanent changes during cooking are chemical.

Direct answer: A, physical change. A physical change changes the form or state of a substance while its chemical identity remains the same. Many such changes can be reversed, so the original substance can often be recovered. For example, ice melts into liquid water and water can freeze back into ice; the substance remains H2O. This is why option A is the best answer. Option B, chemical change, generally produces one or more new substances, such as rust from iron, so the original material cannot usually be recovered by a simple physical step. Option C, combustion, is a particular chemical change involving burning and is not the general answer. Option D, fermentation, is another chemical or biological process that forms new products, such as alcohol or acids, so it is not the intended category. The word “often” matters: not every physical change is perfectly reversible, but recovery is commonly possible. Memory cue: same substance after melting, freezing, cutting, or dissolving usually indicates physical change.

Chemical change can involve energy change and new substances. Exam tip: match signs with the reason.

Density is a physical property related to mass and volume. Exam tip: use density for identifying matter.

Odour can be sensed without changing chemical identity. Exam tip: sensory properties can be physical.

Melting point is a physical property related to change of state. Exam tip: melting point helps identify pure substances.

Direct answer: A, physical property. A physical property is a characteristic that can be observed or measured without changing a substance into a new substance. The boiling point is the temperature at which a liquid’s vapour pressure equals the surrounding pressure and the liquid boils; during ordinary boiling, the liquid changes to gas but its chemical identity remains the same. Therefore boiling point is a physical property, and A is correct. Option B, chemical change, is wrong because boiling does not normally create a new substance; water vapour is still H2O. Option C, only smell, is wrong because boiling point is a measurable temperature, not an odour. Option D, only colour, is wrong because colour is a different physical property and does not define boiling point. In distillation, substances are separated partly because they have different boiling points. Memory cue: melting point and boiling point are physical properties because they describe state changes, not new substances.

Solubility tells how much a substance dissolves in a solvent. Exam tip: remember salt and water example.

Electrical conductivity is a measurable property of matter. Exam tip: remember metals as good conductors.

Thermal conductivity is a physical property because it describes how readily a substance transfers heat without requiring the substance to become a different substance. For example, metals generally conduct heat well, while materials such as wood and plastic conduct it poorly. Measuring this property does not by itself alter the chemical identity of the sample. Therefore option A is correct. A chemical change is a process, not a property, and a biological process refers to activity in living systems. Smell is another physical property, but it is not the meaning of thermal conductivity. The governing idea is that physical properties can be observed or measured without changing chemical composition.

Combustibility relates to burning and forming new substances. Exam tip: properties related to burning are chemical.

Hardness describes how strongly a material resists scratching, cutting, pressing or wearing. It can be observed or measured without changing the substance into a different substance. For example, comparing which material scratches another concerns a physical feature, even though the result may help identify the material. A physical property can be studied without changing chemical composition, while a chemical property concerns the ability to undergo a chemical reaction.

Therefore, option A, physical property, is correct. Hardness is not a chemical or nuclear property, and it is certainly not an optical illusion. The key test is whether a new substance is formed during the observation. Since testing hardness does not require formation of a new substance, it belongs to physical properties.

Brittleness is a physical property in which a substance breaks or shatters under force or impact without bending much. Therefore, option A is correct. Ability to burn is a chemical property, whereas brittleness is a mechanical/physical property. Exam tip: brittle materials are generally not ductile or malleable.

Volatility describes the tendency of a substance to change into vapour easily. A more volatile liquid generally evaporates faster and has a relatively higher vapour pressure. Metallic character, magnetism, and hardness are different properties and do not define volatility. Exam tip: a liquid that evaporates quickly or spreads a strong smell is usually more volatile.

The governing concept is solubility, which is the ability of a substance, called the solute, to dissolve uniformly in a particular solvent under specified conditions. Here, water is the solvent, so the relevant question is whether the substance forms a uniform solution in water. Therefore, option A is correct. Ductility describes the ability of a material to be drawn into wires, and sonority concerns the sound produced by certain materials, especially metals. Magnetism describes attraction or repulsion in a magnetic field; it does not generally tell whether a substance dissolves. Solubility can depend on temperature and, for gases, pressure.

The melting point is the characteristic temperature at which a solid begins to change into a liquid under specified pressure. At this temperature, particles in the solid gain enough energy to overcome the orderly arrangement that holds them in fixed positions. The substance then changes state, although the temperature can remain constant during the change while heat is being absorbed. Thus, melting point directly tells us when the solid-to-liquid change occurs.

Option A is correct because it names the relevant property and temperature. Density tells us how much mass is present in a given volume, but it does not identify the melting temperature. Odour describes a sensory characteristic, and sound is not the property used to mark this state change. The exact melting point can vary with pressure and with the substance, but among the given choices only melting point answers the question directly.

Boiling point is the temperature at which a liquid’s vapour pressure equals the external pressure, so the liquid boils throughout and changes into a gas. Melting point is the related but incorrect distractor because it refers to a solid changing into a liquid. Exam tip: remember liquid → gas as boiling point and solid → liquid as melting point.