Types of changes in nature

Physical changes in nature alter only the state, shape or form of a substance — its chemical identity, and usually its mass, stays exactly the same. The clearest natural stage for this is the water cycle: mountain-top ice melts, sea water evaporates into vapour, vapour condenses into clouds, and clouds return the water as rain or dew, sometimes freezing it back into ice. At every single stage the substance is chemically $\text{H}_2\text{O}$; only the arrangement and motion of its molecules differ between solid, liquid and vapour. Wind and running water grinding a boulder into pebbles and sand is the same idea applied to rock — smaller pieces of the identical mineral, nothing new formed. Sublimation of naphthalene balls or camphor left in the open, and the slow evaporation of a puddle after rain, belong to this family too.

Chemical changes in nature transform a substance into one or more new substances with different properties, and the change cannot usually be undone by simple physical means. Iron left out in moist air slowly rusts; cut fruit browns and eventually rots; milk left in the open curdles and turns sour; a fallen leaf decomposes into compost; food is chemically broken down during digestion. In every one of these, colour, texture, smell or composition change in a way that melting or freezing never does, because the original molecules have rearranged into different ones. Whichever type of change occurs, matter is never created or destroyed — only rearranged. This is the Law of Conservation of Mass, and it applies just as strictly in a forest or a pond as it does inside a lab flask:

where: $m_{\text{substances before}}$ is the total mass of everything that goes into the change (the original substance, plus anything it absorbs from its surroundings, such as oxygen from the air), and $m_{\text{substances after}}$ is the total mass of everything that comes out (the new substance formed, plus anything released into the surroundings). Nothing about "physical" or "chemical" changes this rule — it only tells you that mass is conserved overall, never that the two sides of a chemical change must weigh the same as the single starting substance alone, because a chemical change with the outside air can add or remove mass from the visible object.

There is no separate rule book for "natural" changes — nature simply presents physical and chemical changes in the same weather, water and living things you already study, so the classroom classification test applies unchanged. Ask two questions: has a new substance with different properties been formed, and can the original be recovered by ordinary physical steps such as cooling, drying or evaporating? If the substance is chemically unchanged and the process is reversible, it is physical — the water cycle and physical weathering of rock both pass this test cleanly, because ice, liquid water and vapour are all $\text{H}_2\text{O}$, and pebbles are simply smaller pieces of the same rock. If a new substance appears — rust instead of iron, brown mushy tissue instead of firm apple flesh, curds instead of milk — the change is chemical, and no simple physical trick restores the original. Nature often runs both together (a rock physically weathers into gravel while its surface also chemically weathers with rainwater) only because several processes act on the same object at once; each step still answers the "new substance?" question independently, and mass is conserved across the whole process either way.

Question: Apples left exposed to air after being cut slowly turn brown, become soft, and eventually smell rotten if left for many days. Classify this natural change, justify your answer, and state whether the total mass of the fruit and the air around it changes.

Step 1 — Identify what is changing. The freshly cut, pale apple flesh gradually turns brown, then soft and mushy, and finally develops an unpleasant odour different from a fresh apple.

Step 2 — Apply the new-substance test. Is the brown, rotten apple flesh the same substance as the fresh apple flesh? No — its colour, texture, taste and smell are all different from the original, showing that new substances have formed as the apple's compounds are altered on exposure to air over time.

Step 3 — Check recoverability. Once rotten, the apple cannot be turned back into fresh apple flesh by any simple physical method such as cooling or drying, confirming the change is not physical.

Step 4 — Apply conservation of mass. The apple reacts with oxygen from the air and loses some matter as water vapour, so the visible apple's mass can change; but the total mass of the apple's original matter plus everything it exchanges with the air stays exactly constant, by the Law of Conservation of Mass.

Conclusion: This is a chemical change — new substances, with distinctly different properties from fresh apple flesh, have been formed, the original state cannot be recovered by physical means, and total mass is still conserved once the surrounding air is included in the count.

Competitive stretch

A clean iron nail of mass $11.2\ \text{g}$ is left outdoors in moist air until it rusts completely, forming a flaky coating of rust of total mass $14.4\ \text{g}$. Find the mass of oxygen the nail absorbed from the air.

Step 1. Identify the two sides of the mass balance: the mass before is the iron nail plus the oxygen it absorbs, and the mass after is the rust that forms, by $m_{\text{substances before}} = m_{\text{substances after}}$.

Step 2. Substitute the known masses: $11.2\ \text{g (iron)} + m_{\text{O}_2} = 14.4\ \text{g (rust)}$.

Step 3. Solve for the unknown: $m_{\text{O}_2} = 14.4 - 11.2 = 3.2\ \text{g}$.

The nail absorbed 3.2 g of oxygen from the surrounding air, which is why a rusted nail always weighs more than the original clean nail — visible confirmation that rusting is a chemical change with the surrounding air, not a loss of matter.

• Assuming "natural" and "slow" changes must be physical. Rusting and fruit-rotting both happen slowly, outdoors, from exposure to air and moisture alone, yet both are chemical changes because new substances are formed.
• Treating the water cycle as partly chemical. Every stage — evaporation, condensation, freezing, melting — is a physical change; water remains $\text{H}_2\text{O}$ throughout, however different ice, liquid water and vapour look or feel.
• Confusing physical weathering (wind or water breaking rock into smaller pieces of the same rock) with chemical weathering (rock reacting with rainwater to form new minerals). Nature often shows both side by side — always check whether a new substance formed.
• Forgetting that a rusted object can gain mass from the air. A chemical change with the surroundings can make the visible object heavier or lighter; conservation of mass applies to the total, not the object alone.

Ask the tutor: Spotted a natural change you're curious about — like why bread goes mouldy, why a nail left in a damp shed rusts faster, or why fallen leaves turn brown and crumble — ask your AI tutor to classify it step by step using the new-substance test.