A Burning Candle Shows Two Kinds of Change at Once

Both, in different places at the same time. The wax melting near the flame is a physical change — cooled, it becomes solid wax again. The wax vapour burning in the flame is a chemical change, producing new substances that can never be turned back into wax.

One object, two kinds of change. This page covers everything in the ICSE Class 7 Chemistry chapter on changes: telling the two types apart, classifying changes, the signs of a chemical change, and the standard examples.

In a physical change no new substance is formed; in a chemical change one or more new substances are.

Four tests distinguish them:

- New substance — not formed in a physical change; formed in a chemical change.
- Reversibility — a physical change is usually easily reversed; a chemical change is usually not.
- Composition — unchanged in a physical change; changed in a chemical change.
- Energy — a physical change involves little energy change; a chemical change usually absorbs or gives out a noticeable amount of heat, light or sound.

Physical changes: ice melting, water boiling, sugar dissolving, a wire being stretched, glass breaking, iron being magnetised.

Chemical changes: milk turning into curd, iron rusting, paper burning, a chapati browning on the tawa, food being digested, cooking gas burning.

Curd is a good test case. You cannot turn curd back into milk, and it tastes and behaves quite differently — so a new substance has clearly formed.

Two cases regularly cause arguments. Dissolving sugar in water is physical, because evaporating the water gives the sugar back unchanged. Breaking glass is physical too, however permanent it feels — irreversibility alone does not make a change chemical. The test that decides it is always whether a new substance exists afterwards.

The same change can be sorted in several different ways, and questions ask for more than one label at a time.

Reversible or irreversible. Reversible: melting of ice, stretching of a rubber band, magnetising iron. Irreversible: burning of paper, cooking of rice, curdling of milk.

Periodic or non-periodic. Periodic changes repeat at regular intervals: day and night, the phases of the Moon, the swing of a pendulum, the beating of the heart. Non-periodic changes have no fixed interval: rainfall, an earthquake, a bus arriving.

Natural or man-made. Natural: ripening of fruit, growth of a plant, rusting of iron left in the rain. Man-made: burning of a candle, manufacture of cement, cooking food.

Desirable or undesirable. Desirable: ripening of mangoes, curdling of milk into curd, growth of crops. Undesirable: souring of milk, rusting of an iron gate, decay of teeth.

Slow or fast. Slow: rusting, the weathering of rocks, the ripening of fruit. Fast: burning of a matchstick, an explosion, lightning.

Curdling of milk shows why the labels are separate. It is chemical, irreversible, and desirable when you want curd — but exactly the same process is undesirable when it happens to milk left out overnight. Desirability depends on what you wanted, not on the chemistry.

Five observable clues tell you a new substance has formed.

- Change in colour — shiny iron turns reddish-brown on rusting; a cut apple browns in air.
- Evolution of a gas — baking soda added to lemon juice fizzes as carbon dioxide escapes.
- Formation of a precipitate — an insoluble solid appearing in a clear solution, as when limewater turns milky with carbon dioxide.
- Change in smell — milk going sour, or food spoiling.
- Evolution or absorption of heat and light — quicklime added to water becomes hot; a burning magnesium ribbon gives a bright light.

Cooking is full of them at once. Mustard seeds popping in hot oil, onions turning brown, and dough rising with the gas from yeast are all chemical changes showing different clues.

One clue by itself can mislead, and this is worth knowing. Water boiling gives off a gas, and water freezing releases heat, yet both are physical changes — steam and ice are still water.

So treat the five signs as evidence, not proof. The final test remains whether a new substance with new properties has been formed.

Because each one produces substances that were not there before, with properties quite unlike the starting materials.

Rusting. Iron reacts with oxygen and moisture from the air to form hydrated iron oxide — rust. It is reddish-brown, flaky and crumbly, while iron is grey, hard and strong. Rusting needs both air and moisture, which is why iron tools rust quickly in the monsoon and why gates are painted or greased.

Burning of a candle. The wax vapour combines with oxygen to give carbon dioxide and water vapour, with heat and light released. Neither product is wax, and neither can be turned back into it.

Photosynthesis. Green leaves use carbon dioxide and water, in the presence of sunlight and chlorophyll, to make glucose and release oxygen. Gases and water become food — clearly new substances.

Digestion of food. Complex substances in food are broken down into simpler ones the body can absorb, so the chapati you eat no longer exists as a chapati.

The candle deserves a second look, because it is the classic exam question. The melting of wax is physical, the solidifying of dripped wax is physical, and only the burning in the flame is chemical. An answer that calls the whole candle a chemical change misses the part the examiner is testing.