Physical and Chemical Changes Notes
Cambridge IGCSE Chemistry: Revision notes
Key facts
- A chemical change makes a new substance; a physical change does not.
- Chemical changes are usually hard to reverse and often have a noticeable energy change.
- Rate can be measured by mass lost or gas volume against time.
- Rate increases with higher concentration, pressure, surface area, temperature or a catalyst.
- A catalyst speeds up a reaction but does not change the amount of product.
Physical or chemical
A chemical change makes a new substance; a physical change only changes state or form.
In a physical change the particles or state change, but no new substance forms. In a chemical change atoms are rearranged into one or more new substances with different properties.
- 1
Look for a new substance
bubbles, a colour change, a precipitate, heat or light
- 2
No new substance
physical change: only state or form changes
- 3
New substance formed
chemical change: atoms are rearranged
| Physical change | Chemical change | |
|---|---|---|
| New substance formed? | No | Yes |
| Reversible? | Usually easily reversed | Usually difficult or impossible |
| Energy change | Usually small | Often noticeable (heat, light) |
| Example | Melting ice, dissolving salt | Burning wood, rusting iron |
Physical change
- New substance formed?:
- No
- Reversible?:
- Usually easily reversed
- Energy change:
- Usually small
- Example:
- Melting ice, dissolving salt
Chemical change
- New substance formed?:
- Yes
- Reversible?:
- Usually difficult or impossible
- Energy change:
- Often noticeable (heat, light)
- Example:
- Burning wood, rusting iron
Which is a chemical change?
Measuring rate
Measure how fast a product forms or a reactant is lost, using mass or gas volume against time.
Two common methods: record the mass on a balance as a gas escapes, or measure the volume of gas produced using a gas syringe or by collecting over water. Plotting the results against time shows how the rate changes as the reaction proceeds.
Which apparatus measures the volume of gas produced?
Factors that change rate
Concentration, pressure, surface area, temperature and a catalyst all speed up a reaction when increased or added.
Higher concentration of solutions, higher pressure of gases, larger surface area of solids (smaller pieces), higher temperature and adding a catalyst (including enzymes) all increase the rate. Removing a catalyst decreases it.
| Change | Effect on rate | |
|---|---|---|
| Concentration | Higher | Faster |
| Pressure (gases) | Higher | Faster |
| Surface area | Larger (smaller pieces) | Faster |
| Temperature | Higher | Faster |
| Catalyst | Added | Faster |
Change
- Concentration:
- Higher
- Pressure (gases):
- Higher
- Surface area:
- Larger (smaller pieces)
- Temperature:
- Higher
- Catalyst:
- Added
Effect on rate
- Concentration:
- Faster
- Pressure (gases):
- Faster
- Surface area:
- Faster
- Temperature:
- Faster
- Catalyst:
- Faster
Which change makes a reaction between marble chips and acid faster?
Catalysts
The same amount of product forms in a shorter time, and the catalyst is unchanged.
A catalyst increases the rate but is chemically unchanged at the end, so you compare the reaction with and without it. With a catalyst the curve is steeper early on. Catalysts do not change the amount of product, only how quickly it forms.
What does a catalyst change?
Try an exam question
Describe two differences between a physical change and a chemical change.
[3 marks]
- [1]In a chemical change a new substance forms; in a physical change no new substance forms.
- [1]A chemical change is usually difficult to reverse; a physical change is usually easily reversed.
- [1]A chemical change often has a noticeable energy change; a physical change usually has a small one.
That's the notes covered.
Carry on to the next subtopic.