Experimental DesignCambridge IGCSE Chemistry: Revision notes
Section 1
What apparatus is used to measure time in chemistry experiments?
Stop-watches are the standard apparatus for measuring time in chemistry experiments.
- Analogue stop-watches: read to the nearest 0.1 second; have a second hand and dial
- Digital stop-watches: read to the nearest 0.01 second; display time on a screen
Advantages of digital stop-watches:
- More precise (0.01 s precision vs 0.1 s for analogue)
- Easier to read quickly without parallax error
- Can be started and stopped with one hand
Disadvantages of digital stop-watches:
- May be more expensive
- Require batteries which can fail
- Human reaction time still introduces uncertainty when starting/stopping
Examiners reward you for stating both the apparatus name AND the precision it measures to. For example: 'digital stop-watch measuring to 0.01 s' is better than just 'stop-watch'.
Section 2
Which apparatus measures temperature and how do they compare?
Thermometers are used to measure temperature in chemistry experiments.
Types of thermometers:
| Thermometer Type | Precision | Range | Advantages | Disadvantages |
|---|---|---|---|---|
| Liquid-in-glass (mercury or alcohol) | ±1 °C | −10 to +110 °C (typical) | Inexpensive; no batteries needed; reliable | Slow to respond; parallax error possible; mercury is toxic |
| Digital thermometer | ±0.5 °C | Usually −20 to +120 °C | Very precise; fast response time; easy to read | Requires batteries; more expensive |
| Thermometer with 0.1 °C graduations | ±0.1 °C | Limited range | High precision for accurate work | Fragile; limited temperature range |
Key considerations:
- The bulb of the thermometer must be fully submerged in the solution being measured
- Avoid parallax error by reading at eye level
- Allow sufficient time for the thermometer to reach thermal equilibrium with the liquid
Many students forget to specify the precision of the thermometer (e.g. '±1 °C'). The specification requires you to name the apparatus AND state its precision for full marks.
When comparing apparatus, always link your answer to the context. For an experiment tracking slow temperature change, a digital thermometer is better; for quick readings, response time matters.
Section 3
How is mass measured and which balances are used in experiments?
Balances are used to measure the mass of substances in chemistry.
Types of balances:
| Balance Type | Precision | Use | Advantages | Disadvantages |
|---|---|---|---|---|
| Two-pan balance | ±0.1 g | Measuring larger masses (>10 g) | Simple; no batteries; robust | Less precise; slower readings |
| Beam balance | ±0.1 g or ±0.01 g | General chemistry work | Reliable; good for solid masses | Requires careful handling |
| Electronic/digital balance | ±0.01 g or ±0.001 g | Precise work; small masses | Very accurate; fast; easy to read; tare function | Requires batteries; more expensive; can be affected by vibration |
Best practice:
- Always tare the balance (zero it) before adding a substance
- Use a weighing boat or beaker to avoid contaminating the balance pan
- Place solids gently to avoid spillage
- Record the reading as soon as it stabilises on a digital balance
If measuring 2.34 g of a powder, use an electronic balance reading to ±0.01 g. State: 'Electronic balance (±0.01 g)'. If you only have a two-pan balance available, use it but note the lower precision (±0.1 g) and accept you can only measure to 2.3 g.
Section 4
Which apparatus measures volume and how do they differ?
Volume measurement apparatus are essential for precise liquid and gas measurements.
| Apparatus | Precision | Use | Advantages | Disadvantages |
|---|---|---|---|---|
| Measuring cylinder | ±1 cm³ (typical) | Approximate volume of liquids | Quick; suitable for rough measurements | Less precise; parallax error common; not for accurate work |
| Volumetric pipette (pipet) | ±0.05 cm³ | Delivering a fixed, precise volume of liquid | Highly accurate for accurate transfer; narrow tube reduces parallax | Only measures one specific volume; slower to use |
| Burette | ±0.05 cm³ | Titrations; delivering variable volumes | Very precise; can measure any volume in its range; accurate graduations | Requires careful handling; takes time to use; slower delivery |
| Gas syringe | ±0.5 cm³ | Measuring volumes of gases | Allows measurement of gas volumes at constant pressure; removable needle for flexibility | Less precise than liquid apparatus; can leak if worn |
Reading measuring cylinders:
- Always read at the bottom of the meniscus at eye level
- For aqueous solutions, the meniscus curves downward (read at the lowest point)
- Parallax error occurs if you read from above or below
Examiners test whether you choose the correct apparatus for the task. For 'measuring 25 cm³ of sodium hydroxide accurately', the correct answer is 'volumetric pipette' or 'burette'—not measuring cylinder. Matching apparatus to precision required is key.
Students often say 'beaker' or 'test tube' for volume measurement. These are not appropriate for precise measurements. Only use measuring cylinder, volumetric pipette, burette, or gas syringe.
Section 5
What are the key terms for solutions and separation?
Understanding solution chemistry vocabulary is essential for describing experiments and processes.
Key definitions:
- Solvent: The substance present in the largest amount that dissolves the solute; typically water in aqueous solutions
- Solute: The substance being dissolved in the solvent; present in smaller amount
- Solution: A homogeneous mixture formed when a solute dissolves completely in a solvent; the particles are distributed evenly at the molecular level
- Saturated solution: A solution that contains the maximum amount of dissolved solute at a given temperature; no more solute can dissolve
- Unsaturated solution: A solution containing less solute than the maximum possible; more solute could still dissolve
- Residue: The solid material that remains on filter paper after filtration; the insoluble part
- Filtrate: The clear liquid that passes through filter paper during filtration; the soluble part
Example process:
- A salt dissolves in water → solute in solvent → solution forms
- If more salt is added and some remains undissolved → saturated solution
- Filtering the mixture separates residue (solid salt) from filtrate (salt solution)
When filtering sand and salt solution: the sand is the residue (insoluble solid left on filter paper), and the salt solution passing through is the filtrate. In the original mixture, water is the solvent and salt is the solute.
Examiners often test whether you know residue and filtrate are different things. Residue = solid; filtrate = liquid. Both terms must appear in your answer when describing filtration results.
Section 6
How do you evaluate advantages and disadvantages of experimental methods?
When asked to suggest advantages and disadvantages, link your answer to the context of the experiment and the properties of the apparatus.
Advantages typically relate to:
- Accuracy/precision: How close the measurement is to the true value
- Speed: How quickly the measurement can be taken
- Reliability: Consistency and freedom from errors
- Cost-effectiveness: Expense and availability
- Safety: Reduced risk of contamination or hazard
- Practicality: Ease of use in the specific experiment
Disadvantages typically relate to:
- Limited precision: Cannot measure to required accuracy
- Slow response time: Delays in obtaining readings
- Parallax error: Potential for reading errors
- Fragility: Risk of breakage or damage
- Cost: Expense of apparatus
- Limited range: Cannot measure outside certain bounds
- Practical issues: Difficulty of use or contamination risk
Example comparison: Using a measuring cylinder (±1 cm³) vs. a burette (±0.05 cm³) to measure 25 cm³ of acid:
- Measuring cylinder advantage: Faster, simpler to use
- Measuring cylinder disadvantage: Less precise; error of ±1 cm³ is too large for accurate titration
- Burette advantage: Much more precise; ideal for titrations
- Burette disadvantage: Slower; requires more careful handling
Always justify your advantages/disadvantages by reference to the experiment. Instead of 'digital stop-watches are more precise', say 'digital stop-watches measure to ±0.01 s, which is essential for monitoring fast reactions accurately'.
Must Know
- Apparatus for measurement: Stop-watch (time), thermometer (temperature), balance (mass), and volumetric pipette/burette/measuring cylinder (volume) must be named with their precision stated (e.g. digital balance ±0.01 g)
- Volume apparatus hierarchy: Measuring cylinder (±1 cm³) for rough work; burette or volumetric pipette (±0.05 cm³) for accurate work; gas syringe for gases
- Solution vocabulary: Solvent (largest component), solute (dissolved substance), solution (homogeneous mixture); saturated solution has maximum dissolved solute; residue = solid left after filtration; filtrate = liquid passing through
- Comparing apparatus: Always link advantages/disadvantages to the experiment context and the apparatus properties (precision, speed, accuracy, cost, safety, practicality)
- Reading errors: Parallax error occurs when reading scales at an angle; avoid by reading at eye level. Meniscus must be read at the bottom for aqueous solutions
- Best practice: Tare balances before use; allow thermometers to reach thermal equilibrium; always submerge thermometer bulbs; read measuring cylinders at the meniscus at eye level
That's the notes covered.
Carry on to the next subtopic.