Reversible and Irreversible Changes

Introduction

Ice can melt and freeze again as many times as you like. A burnt piece of toast can never become bread again. The difference is reversibility : whether you can get the starting materials back. Reversibility is a useful way to sort changes, and it links closely to the ideas of physical and chemical change, but the link is not perfect. This page explains what reversible and irreversible mean and where the exceptions lie.

Core explanation

Reversible changes. A change is reversible if the original substances can be recovered, usually by changing the conditions, such as the temperature. Most physical changes are reversible because no new substances are made; the particles are simply rearranged or moved further apart.

- Melting and freezing: chocolate melts in the sun and sets again in the fridge. - Boiling and condensing: water vapour from a kettle condenses on a cold window. - Dissolving: salt dissolved in water can be recovered by evaporating the water. - Stretching elastic material: a rubber band returns to its original shape.

Irreversible changes. A change is irreversible if the original substances cannot easily be recovered. Most chemical changes are irreversible in practice, because the atoms have been rearranged into new substances that do not readily turn back.

- Burning wood or paper produces ash, carbon dioxide and water vapour. - Cooking an egg permanently changes its proteins. - Baking a cake turns batter into a new spongy solid. - Rusting turns iron into hydrated iron(III) oxide.

The exceptions. The rule "physical is reversible, chemical is irreversible" is a helpful starting point, but it is not always true.

Change Type Reversible? --- --- --- Melting ice Physical Yes Burning paper Chemical No Heating blue hydrated copper(II) sulfate Chemical Yes: adding water turns it blue again Ammonium chloride decomposing on heating Chemical Yes: the gases recombine on cooling Breaking a glass or shredding paper Physical Not in practice, though no new substance forms

Some chemical reactions can run in both directions; these are called reversible reactions and are shown with the symbol ⇌. Some physical changes, such as smashing a plate, cannot easily be undone even though the pieces are still the same substance.

The better test. For deciding physical versus chemical, always ask whether a new substance has formed. Use reversibility as supporting evidence, not as the definition.

Step-by-step reasoning

To decide whether a change is reversible:

1. Identify the starting materials. 2. Identify what is present after the change. 3. Ask whether a simple change of conditions (cooling, heating, evaporating, adding water) could give back the starting materials. 4. If yes, the change is reversible; if not, it is irreversible. 5. Separately, check whether a new substance formed to decide if it is chemical or physical.

Visual explanation

Picture two arrows. Between "ice" and "water", a double-headed arrow shows the change going both ways as the temperature goes down and up. Between "paper" and "ash + gases", a single arrow points one way only, with a cross drawn over any attempt to go back.

Real-world analogy

A reversible change is like rearranging the furniture in a room: you can always move it back. An irreversible change is like sawing the furniture up for firewood and burning it. The wood is still made of atoms, but you cannot get the chair back.

Real-world example

Recycling glass relies on a reversible physical change. Broken bottles are melted in a furnace and the liquid glass is moulded into new bottles as it cools and sets. Paper is harder to recycle because its fibres shorten each time, and burnt paper cannot be recycled at all, because burning is an irreversible chemical change.

Why?

Why are most chemical changes hard to reverse? Many reactions release energy and form very stable products, such as carbon dioxide and water from burning. To turn these back into the starting materials would need a large input of energy and special conditions, and the products have often escaped as gases.

Common misconception

"All chemical changes are irreversible." Some reactions, such as the dehydration of hydrated copper(II) sulfate or the decomposition of ammonium chloride, can easily be reversed by changing the conditions. Reversibility and chemical change are separate ideas.

Worked example

Question: Classify each change as reversible or irreversible, and physical or chemical: (a) water freezing; (b) a match burning; (c) sugar dissolving in tea.

Reasoning: (a) Ice melts back to water and no new substance forms. (b) The match forms ash and gases that cannot be turned back into wood. (c) Evaporating the tea would leave the sugar; no new substance forms.

Answer: (a) reversible, physical; (b) irreversible, chemical; (c) reversible, physical.

Quick check

1. What does it mean to say that a change is reversible? Answer: The original substances can be recovered by changing the conditions.

Exam focus

Be ready to give examples of each type and to explain the link: most physical changes are reversible and most chemical changes are irreversible, but there are exceptions. Heating hydrated copper(II) sulfate is a favourite example of a reversible chemical change, and ⇌ is the symbol to recognise.

Advanced insight

In principle, every chemical reaction is reversible at the level of particles: product molecules can sometimes react to reform reactants. In practice, when the reverse reaction is extremely slow, or when a product escapes as a gas, the change behaves as if it goes in one direction only. Chemists call such reactions effectively "complete".

Summary

A reversible change is one where the starting materials can be recovered; an irreversible change is one where they cannot. Most physical changes, such as melting and dissolving, are reversible, and most chemical changes, such as burning and cooking, are irreversible. There are important exceptions, so the formation of a new substance remains the true test of a chemical change.

Practice questions

1. Give two examples of reversible changes. Answer: Melting ice and dissolving salt in water; the ice can be refrozen and the salt recovered by evaporation. 2. Why is burning wood irreversible? Answer: New substances (ash, carbon dioxide and water vapour) form and cannot easily be turned back into wood. 3. Name a chemical change that is reversible. Answer: Heating hydrated copper(II) sulfate to form the white anhydrous solid; adding water reverses it. 4. Smashing a window is a physical change. Why is it not reversible in practice? Answer: The glass pieces cannot be put back into a single sheet without melting and remoulding them, even though no new substance forms.