Physical and Chemical Changes (Leaving Cert Chemistry): Revision Notes

Physical and Chemical Changes

Understanding the difference between physical and chemical changes is fundamental to chemistry. These two types of changes occur all around us every day, and recognising which type is happening helps us understand how matter behaves.

What are physical changes?

A physical change occurs when a substance changes its appearance or physical properties without forming any new substances. Think of it this way - the material remains chemically the same, but looks or feels different.

During a physical change, you can observe differences in properties like shape, size, texture, or state (solid, liquid, gas) without the substance becoming something entirely different. The key point is that no new chemical substances are created.

Common examples of physical changes include:

• Ice melting - solid water becomes liquid water (still $H_2O$)
• Water boiling - liquid water becomes steam (still $H_2O$)
• Breaking glass - the glass fragments are still glass
• Stretching rubber - the rubber band remains rubber
• Chopping wood - the wood pieces are still wood
• Crushing metal - the compressed metal is still the same metal
• Shredding paper - the paper strips are still paper

What are chemical changes?

A chemical change results in the formation of one or more completely new substances. During these changes, the original materials undergo chemical reactions to produce different compounds with new properties.

You can often identify chemical changes by observing signs such as colour changes, gas production (bubbles or fizzing), heat or light being given off, or the formation of precipitates.

Examples of chemical changes include:

• Fireworks exploding - chemicals react to produce light, heat, and new compounds
• Rusting of iron - iron combines with oxygen to form iron oxide (rust)
• Baking a cake - flour, eggs, and other ingredients react to form new compounds
• Fizzing of indigestion mixture - acid and base react to produce gas
• Photosynthesis - plants convert carbon dioxide and water into glucose and oxygen
• Burning fuel - wood or other materials combine with oxygen to produce ash and gases

Key differences between physical and chemical changes

Understanding the main differences helps you classify changes correctly:

Physical changes:

• No new substances are formed - the material remains chemically identical
• Usually no energy changes take place (though some energy may be needed, like heating ice)
• Easy to reverse - you can often get back to the original state

Chemical changes:

• New substances are formed - completely different materials are created
• Energy changes occur - heat, light, or sound are often produced
• Difficult to reverse - it's hard or impossible to get back the original materials

Identifying changes in everyday life

When you encounter a change, ask yourself these questions:

Exam tips

• Learn the key characteristics: Physical changes don't create new substances; chemical changes do
• Practice with examples: Make lists of physical and chemical changes you observe daily
• Look for clues: Gas production, colour changes, and energy release often indicate chemical changes
• Remember reversibility: Physical changes are usually easy to undo; chemical changes are not

Remember!