Physical and Chemical ChangesCambridge IGCSE Chemistry: Revision notes
Section 1
What Is the Difference Between a Physical and a Chemical Change?
Changes to substances can be classified into two types.
| Physical change | Chemical change | |
|---|---|---|
| New substance formed? | No | Yes |
| Reversible? | Usually easily reversible | Usually difficult or impossible to reverse |
| Energy change | Usually small | Often a noticeable energy change (heat, light) |
| Example | Melting ice, dissolving salt in water | Burning wood, rusting iron |
- In a physical change, the substance's particles or state change, but no new substance is formed — the same chemical substance is still present
- In a chemical change, the atoms are rearranged into one or more new substances with different properties from the starting materials
Melting ice into water is a physical change — it is still H2O. Burning magnesium in oxygen to form magnesium oxide is a chemical change — a new substance with different properties is formed.
Section 2
How Can You Investigate the Rate of a Chemical Reaction?
Rate of reaction experiments look at how quickly reactants are used up or products are formed during a chemical change. Two common practical methods are:
- Change in mass: if a gas is released during the reaction, the reaction flask can be placed on a balance and the decrease in mass recorded over time as gas escapes
- Formation of a gas: the volume of gas produced can be measured over time using a gas syringe, or by collecting the gas over water and measuring how much collects in a set time
Both methods allow you to plot how much reactant/product has changed against time, to see how the rate changes as the reaction proceeds.
A steep gradient on a graph of mass/volume against time means a fast reaction; a flat, horizontal line means the reaction has finished.
Section 3
What Factors Change the Rate of a Reaction?
Several conditions can be changed to speed up or slow down a chemical reaction:
- Concentration of solutions — a higher concentration generally increases the rate
- Pressure of gases — a higher pressure generally increases the rate
- Surface area of solids — smaller pieces (larger surface area) generally increase the rate
- Temperature — a higher temperature generally increases the rate
- Catalysts (including enzymes) — adding a catalyst increases the rate; removing one decreases it
A catalyst is not 'used up' in the reaction — it remains chemically unchanged, even though it speeds up the reaction, so its mass is the same before and after.
Section 4
How Do We Know if a Catalyst Has Worked?
Because a catalyst increases the rate of reaction but is chemically unchanged at the end, its effect is measured by comparing how the reaction proceeds with and without it.
- With a catalyst present, the same amount of product forms in a shorter time (a steeper graph line early on)
- The catalyst's mass, and its chemical identity, are the same at the start and end of the experiment
- Catalysts do not change the amount of product eventually formed — only how quickly it forms
Manganese(IV) oxide catalyses the decomposition of hydrogen peroxide into water and oxygen — the same total volume of oxygen is produced, but much faster, and the manganese(IV) oxide can be recovered unchanged afterwards.
Must Know
- A physical change forms no new substance (e.g. change of state); a chemical change forms one or more new substances
- Rate of reaction can be measured by monitoring the change in mass of a reactant/product, or the volume of gas formed over time
- Increasing concentration, pressure, surface area or temperature generally increases the rate of reaction
- Adding a catalyst increases the rate of reaction; removing one decreases it
- A catalyst increases the rate of a reaction but remains chemically unchanged at the end
- Physical changes are usually easily reversible; chemical changes are usually difficult to reverse
That's the notes covered.
Carry on to the next subtopic.