A Change That Repeats on Schedule Can Be Predicted

Because the type tells you what you can do about it.

A change that repeats at fixed intervals can be predicted — you can publish a tide table or a calendar. One that does not repeat cannot be predicted the same way, which is exactly why earthquakes still catch people out.

A change that is reversible can be undone; an irreversible one cannot, so it must be prevented instead. That is the practical difference between a dented car panel and a rusted one.

So these classifications are not a labelling exercise. Each pair answers a different useful question. This page covers the first part of the ICSE Class 8 Chemistry chapter on changes: natural and man-made, slow and fast, periodic and non-periodic, desirable and undesirable, reversible and irreversible.

A natural change happens on its own, without human involvement. A man-made change is deliberately brought about by people.

Natural changes — sunrise and sunset, rainfall, growth of a plant, ripening of a mango on the tree, formation of clouds, flowing of a river, souring of milk left out, an earthquake.

Man-made changes — cooking food, making a chapati, weaving cloth, building a house, burning a gas stove, melting metal in a furnace, setting curd by adding a starter.

The overlap that trips students. Ripening of fruit is natural on the tree, but ripening it deliberately in a covered basket is man-made. Rain is natural; irrigation is man-made. The same chemical process can fall in either group depending on who started it, so the question is always who or what caused it, not what happened.

Slow and fast changes are about the rate at which a change takes place.

Slow changes take a long time and cannot be observed as they happen — rusting of iron, weathering of rocks, growth of a tree, formation of coal, curdling of milk, ageing of a person.

Fast changes are over in seconds or less — burning of a matchstick, bursting of a balloon, lightning, an explosion, switching on a bulb, breaking of glass.

Speed is relative, and questions exploit that. The bursting of a firecracker is fast, but the weathering of the rock it was lit on is slow. And some changes can be made faster or slower: milk curdles in hours at room temperature and takes days in a refrigerator, so temperature is what shifted it between the two groups.

How a slow change is studied at all. Since you cannot watch rusting happen, you look at the same object at intervals — a nail in damp air checked every few days shows progressively more rust. This is how any slow change is measured: not by watching, but by comparing.

A periodic change repeats itself at regular, equal intervals of time. A non-periodic change does not repeat, or repeats at irregular intervals.

The interval between two successive repetitions is the time period of the change — the same quantity met in the chapter on sound waves.

Periodic changes — day and night, the seasons, the phases of the Moon, the swinging of a pendulum, the beating of the heart, breathing, the ticking of a clock, high and low tides, the rising and setting of the Sun.

Non-periodic changes — an earthquake, a volcanic eruption, a landslide, bursting of a tyre, rainfall, souring of milk, winning a game, a road accident.

Finding the time period. Worked example — a simple pendulum completes $30$ complete swings in $60\text{s}$. Its time period is

$$T=\frac{\text{total time}}{\text{number of repetitions}}=\frac{60}{30}=2\text{s}$$

so the change repeats every $2$ seconds.

A heart beating $72$ times a minute has

$$T=\frac{60}{72}\approx 0.83\text{s}$$

and a day-and-night cycle has a time period of $24$ hours, while the Earth's revolution around the Sun takes about $365$ days.

Why rainfall is the tricky one. The monsoon arrives roughly at the same time each year, so students often call rainfall periodic. But it does not repeat at equal and fixed intervals — a shower can come today, next week, or not at all for a month. The word that matters is regular, and rainfall is not.

And why this classification is useful. Because a periodic change is predictable. Knowing the time period of a tide lets fishermen plan a day's work; knowing the phases of the Moon gives a calendar. Nothing equivalent is possible for an earthquake, which is why prediction remains so difficult — not because earthquakes are mysterious, but because they are genuinely non-periodic.

Two further questions, and they are independent of each other.

Desirable and undesirable is about whether the change is useful to us.

- Desirable — ripening of fruit, setting of curd, cooking of food, germination of a seed, hardening of cement, drying of washed clothes, digestion of food.
- Undesirable — rusting of iron, souring of milk, spoiling of food, a landslide, cracking of a wall, rotting of vegetables, decay of a tooth.

Some changes are both at once. Burning fuel in a stove is desirable for cooking and undesirable for the smoke it produces. Rain is desirable for a farmer and undesirable when it floods a city. So the honest answer to is this desirable? is often for whom, and for what purpose?

Reversible and irreversible is about whether the original substance can be got back.

- Reversible — melting of ice, boiling of water, stretching of a rubber band, inflating a balloon, dissolving salt in water, magnetising a nail, folding paper.
- Irreversible — burning of paper, cooking of an egg, rusting of iron, setting of curd, ripening of fruit, burning a candle wick, baking a cake, tearing paper.

The pattern behind it. Reversible changes are almost always physical, in which the substance itself is unchanged and only its state or shape has altered. Irreversible changes are almost always chemical, in which a new substance has formed. Ice melting gives you water — still ${\text{H}}_2\text{O}$, so freezing brings the ice back. Milk setting into curd gives something genuinely different, and no treatment turns curd back into milk.

That link is what the second part of this chapter develops in full.

A boundary case for each. Dissolving salt in water looks irreversible because the salt disappears — but evaporate the water and the salt crystals return, so it is reversible. And melting a candle is reversible, while burning the same candle is not: melting only changes the wax's state, whereas burning destroys the wax and produces new gases.

Desirable does not mean reversible. Setting curd is desirable and irreversible; rusting is undesirable and irreversible; melting ice is reversible and may be either. The two pairs of labels answer different questions and must be applied separately.