Changes Around Us: Physical and Chemical · Lesson 4 of 6
More Ways to Classify Changes
“Separate the stages of a burning candle and compare changes by reversibility and usefulness.”
• Explain how a burning candle involves both physical and chemical changes. • Distinguish reversible and irreversible changes using everyday examples. • Explain why a physical change is not necessarily reversible. • Compare desirable and undesirable changes and explain why context matters. • Connect everyday changes with composting and environmental effects.
A process can contain more than one kind of change
“A candle burns” describes a whole process, not one single step. Several things happen around the flame: solid wax becomes liquid, liquid moves along the wick, wax becomes vapour, and the vapour reacts with oxygen. To classify the process accurately, follow each step rather than forcing the entire event into only one category.
Heat from the flame melts wax close to the wick. Liquid wax travels up the wick, whose fibres help draw it upward. Near the hot flame, the wax vaporises: it changes from liquid to gas. The wax vapour then burns. Melting and vaporisation leave the substance as wax in different states; burning forms new substances.
Some liquid wax runs down the candle and later cools into a solid. This solidification is another physical change. The part that burns does not simply become solid wax again: its reaction with oxygen produces new substances, including carbon dioxide. Both categories therefore belong in an explanation of a burning candle.
Problem
One learner says “the wax melts,” and another says “the wax burns.” Must one of them be wrong?
- 1.The first learner describes solid wax turning into liquid wax, a physical change.
- 2.The second describes wax vapour reacting with oxygen, a chemical change.
- 3.Both descriptions can be correct because they refer to different stages of the same process. Add vaporisation and cooling wax to complete the explanation.
Michael Faraday used a series of nineteenth-century lectures, known as The Chemical History of a Candle, to explain science through this familiar object. Following a candle’s wax through melting, vaporisation, and burning makes an everyday scene into an investigation.
Can the original form be brought back?
Classifying a change as physical or chemical asks whether a new substance formed. Asking whether it can be reversed asks something different: can we bring back the original material or object in the relevant form? The distinction matters because some changes of shape are physical but are difficult or impossible to undo by ordinary means.
A change for which the original state or form can be brought back under suitable conditions.
A change for which the original state or form cannot be brought back by ordinary means in the situation being considered.
Melted ice can be frozen again. Water vapour can be collected and condensed into liquid water. These state changes are reversible when the appropriate conditions are provided. Cooling an empty pan after its water has boiled away, however, does not bring the escaped water back; the vapour has to be collected for that reversal.
Chopping vegetables does not usually create a new substance, but you cannot turn all the pieces back into the original whole vegetable. Crushed chalk and a punctured balloon make the same point. Popcorn also does not return to its original corn kernel when cooled. We should state what “bringing it back” would actually require, rather than answering from appearance alone.
| Change | Can the relevant original form be restored? | Reason |
|---|---|---|
| Melting ice | Yes | Freeze the liquid water again. |
| Chopping vegetables | No, by ordinary means | The original whole vegetable is not restored by placing pieces together. |
| Boiling water | Yes, if vapour is collected | Condense the vapour to recover liquid water. |
| Making popcorn | No, by ordinary means | Cooling does not restore the original kernel. |
| Folding paper | Yes, approximately | Unfold it; creases may remain. |
| Inflating an intact balloon | Yes | Let the air escape without tearing the rubber. |
| Puncturing a balloon | No, by ordinary means | Deflation does not repair the torn rubber. |
| Crushing chalk | No, by ordinary means | Loose powder does not restore the original stick. |
| Dissolving sugar in water | Yes, with a suitable separation | Water can be removed by evaporation to recover sugar. |
| Rolling a mat | Yes | Unroll the mat. |
Problem
A learner calls boiling irreversible because the pan becomes empty. How can you improve this answer?
- 1.Distinguish the water changing state from the water escaping into the surroundings.
- 2.Water vapour can condense into liquid, so the state change itself is reversible.
- 3.In an open pan, escaped vapour is not recovered automatically. Explain the need to collect and condense it rather than claiming cooling the pan is enough.
Problem
Can stitching cloth into a shirt always be called reversible?
- 1.Ask whether the cloth was only stitched or was also cut into shaped pieces.
- 2.If stitching alone held an uncut piece, removing stitches might restore much of the original cloth.
- 3.If the cloth was cut, removing stitches does not restore the original sheet. State the conditions instead of assuming all shirt-making situations are identical.
“It cannot be undone, so it is chemical” is incorrect. Cutting paper and crushing chalk are physical changes even though the original object is not restored easily. Ask whether a new substance formed and whether the original form can be restored as separate questions.
Is the change desirable in this situation?
A change may be useful, harmful, or useful in one setting but harmful in another. This judgment depends on our purpose and the surroundings. It does not replace the scientific classification. A useful change can be chemical, and an unwanted change can be physical.
A change considered useful or wanted in a particular situation.
A change considered harmful or unwanted in a particular situation.
Cutting fruit into pieces for a meal is a desirable physical change. Cooking food, turning milk into curd, and ripening fruit can be desirable chemical changes. Rusting a bicycle part and food decaying during storage are usually unwanted because they damage something we wish to preserve.
When suitable bacteria act on milk to make curd, they produce substances that change its sourness and cause its proteins to gather into a thicker structure. The result is more than milk taking a different shape. Chemical processes change its composition and properties. Likewise, cooking can cause new substances to form and change the texture, flavour, and smell of food; cutting a raw ingredient before cooking is a separate physical step.
Now change the purpose. Decomposition spoils stored food, but the decomposition of collected food scraps can produce useful compost. Bacteria and fungi bring about chemical changes in this material. The same general process is undesirable when a meal is spoiled and desirable when waste is deliberately converted into compost for plants.
| Event | Physical/chemical question | Reversibility question | Usefulness question |
|---|---|---|---|
| Cutting fruit for lunch | Physical: fruit becomes pieces. | The whole fruit is not restored. | Desirable for serving. |
| Ice melting in a drink | Physical: water changes state. | Water can be frozen again. | May be desirable for cooling a drink. |
| Wax vapour burning | Chemical: new substances form. | Cooling does not restore the original wax. | Useful for providing light. |
| Food scraps decomposing | Chemical: new substances form. | Ordinarily irreversible. | Useful for compost; unwanted in stored food. |
Problem
Why can decomposition be undesirable in the kitchen but desirable in a compost pot?
- 1.For stored food, the aim is to keep it suitable for eating; decomposition interferes with that aim.
- 2.For collected waste, the aim is to make compost; decomposition helps accomplish that aim.
- 3.Its usefulness depends on context. In both cases, formation of new substances makes the decomposition chemical.
Think about longer-term effects and exploratory changes
A change that is useful immediately can still have unwanted wider effects. Burning fuels provides energy for transport, but increased fuel use adds carbon dioxide to the atmosphere. Drying paint can be useful when finishing a wall, yet substances that evaporate from paint may contribute to air pollution. Consider both the immediate purpose and effects on the surroundings.
The lemon-juice writing project also separates stages. A message written with lemon juice becomes difficult to see as it dries: water evaporates, a physical change. Gentle adult-controlled warming can cause components in the dried juice to change chemically and produce brown material, revealing the letters. Cooling does not undo those chemical changes or simply make the browned letters disappear again. Record drying and browning separately; the explanation involves more than just the fact that a colour changed. This is an adult demonstration, not an invitation to heat paper over a flame yourself.
A chameleon can change its body colour in response to conditions, including its surroundings and emotional state. A previous colour can return, so this colour change can be reversible. Explore observations of the same animal under different conditions. This is a question about reversibility. Observing a colour return would not, by itself, tell you the full physical and chemical processes inside a living animal; avoid deciding those processes from colour alone.
Try a kitchen observation record with four entries for each event: what changed, whether new substances formed, whether the original form can be restored, and whether the change is useful in that situation. Compare chopping, dissolving, boiling, cooking, ripening, and composting. If evidence is insufficient, state what you would need to know instead of guessing.
Quiz
Which candle stage is chemical?
Which observation disproves “all physical changes are reversible”?
How can water that has changed to vapour be recovered as liquid?
Which comparison correctly describes decomposition?
Which pair of questions must be answered separately?
Why can drying paint have both a useful outcome and an unwanted effect?
Practice Problems
- Trace wax from the solid candle to its burning vapour. Classify each stage and include wax that drips and solidifies.
- For melting ice, chopping vegetables, boiling water, and making popcorn, explain whether the original form can be recovered.
- Compare physical/chemical classification with reversible/irreversible classification using crushed chalk.
- Classify twisting a string, rolling a mat, grinding wheat, making idlis, and ripening fruit by reversibility. Explain any conditions that matter.
- Explain why drawing water from a well is a change of location and describe what would count as reversing that event.
- Give one desirable physical change and one desirable chemical change with reasons.
- Explain how food decay and composting show the importance of context.
- For the lemon-juice writing project, predict which observations would remain after cooling and identify what extra evidence would help classify the stages.
- Plan a four-column kitchen observation record: event, type of change, reversibility, and usefulness.
Key Takeaways
• A burning candle combines physical changes in wax with chemical change when wax vapour burns. • Reversibility asks whether the original state or form can be restored under suitable conditions. • An irreversible change can be physical; cut paper and crushed chalk are examples. • Desirability depends on the situation and our purpose. • Composting can make decomposition useful, while the same process spoils stored food. • Consider environmental effects alongside the immediate usefulness of fuel use and paint drying.