Nature of matter

When salt is ground, only the size of its particles decreases; its chemical composition and identity remain unchanged. Therefore, it is a physical change. A chemical change would produce a new substance, which does not happen here. Exam tip: A change only in size, shape, or state is generally a physical change.

During ripening, enzymes convert starch in the banana into sugars, producing changes in colour, smell and taste. Since new substances are formed and the change cannot be easily reversed, it is a chemical change. A reversible change would allow the original state to be regained, which is not the case here. Exam tip: ripening, rotting and burning are generally chemical changes.

Melting ice is a physical change because solid water changes into liquid water without changing its chemical identity. The particles gain energy and move more freely, but the substance remains H₂O. Cooling the liquid can reverse the process by producing ice again. Therefore option C is correct. Rusting of iron forms iron oxides, burning paper produces ash and gases, and curdling milk involves chemical changes that produce new substances and properties. Those examples are therefore chemical changes rather than physical changes. The governing test is whether a new substance forms, not merely whether the appearance or state changes.

Burning paper is a chemical change because the paper reacts with oxygen and produces new substances, mainly ash, carbon-containing gases, and water vapour. These products have properties different from the original paper, and the original material cannot normally be recovered simply by cooling or changing its shape. Hence option D is correct. Freezing water changes only its physical state from liquid to solid. Cutting paper changes its size and shape but not its chemical composition. Dissolving salt is generally treated as a physical change in this school-level comparison because the salt can be recovered by evaporating the water.

In a physical change, the chemical composition of a substance does not change; only its state, shape, or form may change. Therefore, many physical changes, such as melting and freezing, are reversible. Formation of a new substance is a feature of a chemical change, so option C is incorrect. Exam tip: if the original substance can usually be obtained again, the change is generally physical.

A chemical change can form a substance with new properties. Exam tip: colour smell gas or heat may be clues.

Gas evolution can indicate many chemical reactions. Exam tip: connect signs with new substance formation.

The direct answer is option A: a chemical change. A permanent colour change can be evidence that a new substance has formed, which is the main idea of a chemical change. For example, a substance may react with oxygen or another substance and produce a product with a different colour. Option A is correct because a lasting colour change is a recognised possible sign of chemical reaction. It is important to say “can indicate,” not “always proves,” because some physical processes, mixtures, dyes, or changes in concentration may also alter appearance without forming a new substance. Option B is wrong because merely changing the container normally does not permanently change the substance. Option C is too narrow and unsupported: a colour change is not automatically only a measurement error. Option D is wrong because a permanent colour change is evidence that something may have changed. Check other signs too, such as gas formation, temperature change, precipitate, or difficulty of reversal. Memory cue: permanent colour change may mean a new substance, but use all evidence.

Direct answer: A, physical change. A physical change alters a substance’s shape, size, state, or appearance without producing a new substance. When an iron nail is bent, the iron remains iron; only its shape changes. The change may not be easily reversible if the nail is permanently bent, but reversibility is not the only test. The key test is whether a new chemical substance has formed. Option A is correct. Option B, chemical change, is wrong because bending alone does not create a new substance or change iron into another material. Option C, fermentation, is a biological or chemical process in which microorganisms convert substances such as sugars; it has no connection with merely bending iron. Option D, combustion, requires burning and reaction with oxygen, whereas no burning occurs here. Memory cue: changed form but same material means physical change; rusting would be chemical, but bending is not rusting.

A firecracker produces gas heat and light by chemical reaction. Exam tip: combustion-related changes are chemical.

Tearing cloth is a physical change because the cloth is divided into smaller pieces while its chemical composition remains essentially unchanged. The fibres may be separated or rearranged, and the size and shape change, but no new substance is produced. This is the governing distinction between physical and chemical changes. Therefore option A is correct. Evaporation and condensation in options C and D are specific changes of state and do not describe tearing. Option B is incorrect because tearing alone does not change the chemical identity of the fibre material. Although the original whole cloth may be difficult to restore practically, reversibility is not the only test; formation of a new substance is more important.

The direct answer is option B: chemical change. During curd formation, bacteria convert some milk sugar into lactic acid. The milk therefore develops new substances, a sour taste, a different texture, and changed properties. Because a new substance is produced and the change is not easily reversed into the original milk, it is classified as a chemical change. Option A is wrong because a physical change alters state, size, or form without producing a new substance; curd is not simply milk in another shape. Option B is correct. Option C is wrong because condensation means gas changing into liquid, whereas curd formation is not that process. Option D is wrong because freezing means liquid changing into solid due to cooling; curd formation occurs through fermentation and chemical action, not merely cooling. Memory cue: new substance and new properties mean chemical change; milk becoming curd is a standard example.

When a metal melts, only its physical state changes from solid to liquid; its chemical composition remains the same. Therefore, melting is a physical change. In a chemical change, a new substance is formed, such as rusting of iron. Exam tip: changes of state, such as melting, freezing, and boiling, are generally physical changes.

During rusting, the metal reacts with oxygen and moisture to form new substances; in iron, hydrated iron oxide is mainly formed. Since a new substance is produced and the properties of the original metal change, it is a chemical change. In a change of state, only the physical state changes and no new substance is formed. Exam tip: formation of a new substance usually indicates a chemical change.

When water freezes, only its physical state changes from liquid to solid; its chemical composition remains H₂O. Ice can be melted back into water, so this is a physical change. A chemical change forms a new substance, which does not happen here. Exam tip: changes of state such as freezing, melting and boiling are generally physical changes.

During fermentation of grapes, yeast converts sugars into new substances, mainly ethanol and carbon dioxide. Since new substances are formed, it is a chemical change. In a change of state, only the physical state changes and the chemical composition remains the same. Exam tip: formation of new substances usually indicates a chemical change.

Freezing of water is a physical change. On heating, the ice melts and becomes water again, so this change can be reversed easily. In contrast, burning wood is a chemical change; the original wood cannot be obtained back from ash and gases. Exam tip: changes of state, such as freezing and melting, are generally reversible physical changes.

Burning paper forms new substances that cannot easily become paper again. Exam tip: irreversibility can indicate chemical change.

When chalk is ground, only the size and shape of its particles change; its chemical composition, mainly calcium carbonate, remains unchanged. Therefore, it is a physical change. In a chemical change, a new substance is formed, which does not happen here. Exam tip: If only size, shape, or state changes and no new substance forms, it is a physical change.

During digestion, enzymes chemically convert complex food substances such as starch, proteins, and fats into new, simpler substances. Therefore, digestion is a chemical change. In a change of state, only the physical state changes and no new substance is formed. Exam tip: if new substances are formed, the process is generally a chemical change.

Sugar turning brown on heating indicates a chemical change, especially when heating causes caramelisation and further decomposition. Heat changes the original sugar into substances with different colour, smell, taste, and composition. The brown colour is therefore evidence that the chemical identity has changed, not merely that the sugar has changed shape. Option A is correct. Dissolving is a process in which a substance disperses in a solvent, filtration separates components, and shape change alters appearance without necessarily forming a new substance. These descriptions do not explain browning on heating. The exact products depend on temperature and heating conditions, but the observed chemical transformation is clear.

When a copper sheet is made thinner, only its shape and thickness change; its chemical composition remains copper. Hence, it is a physical change. A chemical change would form a new substance, such as the green coating that develops on copper. Exam tip: a change involving only size, shape, or state without formation of a new substance is physical.

In a physical change, only the size, state, or form of a substance may change; its chemical identity generally remains the same. For example, melting of ice into water is a physical change because both have the formula \(H_2O\). Formation of a new substance is a feature of a chemical change, so option A is incorrect. Exam tip: if the composition remains unchanged after a change, it is generally a physical change.

The direct answer is option A. A chemical change produces one or more substances whose composition or properties differ from the starting material. Examples include burning, rusting and digestion. Option A is correct because formation of a new substance is the central sign of chemical change. Option B describes only a change of position, which is physical and does not create a new substance. Option C describes a change involving the container, not necessarily the material inside it. Option D is false because chemical changes can involve energy being released or absorbed, although energy change alone is not the only test. Thus new substance formation is the strongest clue. Memory cue: ask whether the material has become a different substance.

When cardboard is cut, only its size and shape change; its chemical composition and identity remain unchanged. Therefore, it is a physical change. A chemical change forms a new substance, such as when cardboard burns. Exam tip: cutting, breaking, and grinding are generally physical changes.