Experimental Design Notes

Cambridge IGCSE Chemistry: Revision notes

Key facts

  • Name the apparatus and its precision, such as "digital balance, ±0.01 g".
  • Time: stop-watch. Temperature: thermometer. Mass: balance. Volume: measuring cylinder, pipette, burette or gas syringe.
  • For accurate volumes use a burette or volumetric pipette (±0.05 cm³), not a measuring cylinder (±1 cm³).
  • Read scales at eye level to avoid parallax error; read the bottom of the meniscus.
  • Residue is the solid left on the filter paper; the filtrate is the liquid that passes through.
  • Link every advantage and disadvantage to the experiment.

Measuring time

A stop-watch measures time, and a digital one reads more precisely than an analogue one.

Examiners reward you for stating both the apparatus and its precision: "digital stop-watch measuring to 0.01 s" is better than "stop-watch". Human reaction time still adds uncertainty when starting and stopping any stop-watch.

00.020.040.060.080.1AnalogueDigitalStop-watchPrecision (s)
Typical smallest reading of each stop-watch.

Analogue

Precision:
Nearest 0.1 s
Display:
Second hand and dial
Advantage:
No batteries needed
Disadvantage:
Harder to read precisely

Digital

Precision:
Nearest 0.01 s
Display:
Numbers on a screen
Advantage:
More precise, easy to read
Disadvantage:
May cost more; batteries can fail

Which is a better answer to "name the apparatus to measure time"?

Measuring temperature

Thermometers measure temperature, and precision differs between types.

Fully submerge the bulb, read at eye level to avoid parallax error, and allow time for the thermometer to reach thermal equilibrium with the liquid.

00.20.40.60.81Liquid-in-glassDigital0.1 °C graduationsThermometerPrecision (± °C)
Smaller bars are more precise

Liquid-in-glass

Precision:
±1 °C
Advantage:
Inexpensive, no batteries
Disadvantage:
Slow to respond; mercury is toxic

Digital

Precision:
±0.5 °C
Advantage:
Fast response, easy to read
Disadvantage:
Needs batteries; more expensive

0.1 °C graduations

Precision:
±0.1 °C
Advantage:
High precision
Disadvantage:
Fragile; limited range

What should you do to avoid parallax error?

Measuring mass

Balances measure mass, and an electronic balance is the most precise.

Tare the balance (zero it) before adding a substance, use a weighing boat so the pan stays clean, place solids gently and record the reading once it stabilises. To measure 2.34 g use an electronic balance reading to ±0.01 g.

00.020.040.060.080.1Two-panElectronicBalancePrecision (g)
Typical precision of two-pan and electronic balances.

Two-pan

Precision:
±0.1 g
Use:
Larger masses
Advantage:
Simple; no batteries

Beam

Precision:
±0.1 g or ±0.01 g
Use:
General chemistry
Advantage:
Reliable for solids

Electronic

Precision:
±0.01 g or ±0.001 g
Use:
Small masses
Advantage:
Accurate and fast; has tare

A student needs to measure 2.34 g of powder. Which balance is best?

Measuring volume

Choose the apparatus to match the accuracy needed: a measuring cylinder for rough work, a pipette or burette for accurate work.

Read a measuring cylinder at the bottom of the meniscus at eye level. To measure 25 cm³ of sodium hydroxide accurately use a volumetric pipette or burette, not a measuring cylinder.

00.20.40.60.81Measuring cylinderVolumetric pipetteBuretteGas syringeApparatusPrecision (± cm³)
Smaller bars are more precise

Measuring cylinder

Precision:
±1 cm³
Use:
Approximate liquid volumes

Volumetric pipette

Precision:
±0.05 cm³
Use:
One fixed volume

Burette

Precision:
±0.05 cm³
Use:
Titrations; any volume

Gas syringe

Precision:
±0.5 cm³
Use:
Volumes of gases

Which apparatus is best to deliver exactly 25.0 cm³ of alkali?

Solutions and separation

A solute dissolves in a solvent to make a solution; filtering leaves a residue and a filtrate.

The solvent is present in the largest amount. The solute is dissolved. A saturated solution holds the maximum solute at that temperature, so no more dissolves. When filtering sand and salt solution, the sand is the residue and the salt solution is the filtrate.

  1. 1

    Solute in solvent

    salt dissolves in water to form a solution

  2. 2

    Add more salt

    when some stays undissolved, the solution is saturated

  3. 3

    Filter

    undissolved salt is the residue; the salt solution passing through is the filtrate

Salt, water and filtering

What is the clear liquid that passes through the filter paper called?

Evaluating methods

Link every advantage and disadvantage to the experiment and the properties of the apparatus.

Advantages usually relate to accuracy, speed, reliability, cost, safety and practicality. Disadvantages relate to limited precision, slow response, parallax error, fragility, cost and limited range. Say "digital stop-watches measure to 0.01 s, essential for fast reactions", not just "more precise".

00.20.40.60.81Measuring cylinderBuretteApparatusUncertainty (± cm³)
The smaller the uncertainty, the more precise the apparatus.

Measuring cylinder (±1 cm³)

Advantage:
Faster and simpler
Disadvantage:
±1 cm³ is too large for an accurate titration

Burette (±0.05 cm³)

Advantage:
Much more precise; ideal for titrations
Disadvantage:
Slower; needs careful handling

Which is the best justification for using a burette?

Try an exam question

A student must measure 25.0 cm³ of dilute acid accurately for a titration. Name suitable apparatus and give one advantage over a measuring cylinder.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.