Apparatus for Measurements (CIE IGCSE Chemistry)

Time

• Time can be measured using a stopwatch or stopclock which are usually accurate to one or two decimal places
• The units of time normally used are seconds or minutes
  • Other units may be used for extremely slow reactions (e.g. rusting)
• Remember: 1 minute = 60 seconds

Temperature

• Temperature is measured with a thermometer or digital temperature probe

• Laboratory thermometers usually have a precision of a half or one degree
  • Digital temperature probes are available which are more precise than traditional thermometers and can often read to 0.1 ^oC
• Traditional thermometers rely upon the uniform expansion and contraction of a liquid substance with temperature
  • Digital temperature probes can be just as, if not, more accurate than traditional thermometers
• The units of temperature are degrees Celsius (ºC)

Mass

• Mass is measured using a digital balance which normally gives readings to two decimal places
  • Balances should be tared (set to zero) before use
  • Balances should also be allowed time to settle on a final measurement / reading before it is recorded

• The standard unit of mass in kilograms (kg)
  • However, in chemistry grams (g) are most often used
• Remember: 1 kilogram = 1000 grams

Volumes of liquid

• The volume of a liquid can be determined using different pieces of apparatus
  • The choice of apparatus depends on the level of accuracy needed
• Three common pieces of apparatus for measuring the volume of a liquid are:
  • Burettes
  • Volumetric pipettes
  • Measuring cylinders

• Burettes are the most accurate way of measuring a variable volume of liquid between 0 cm³ and 50 cm³
  • They are most commonly used in titrations
  • Careful: Read the burette scale from top to bottom as 0.00 cm³ is at the top of the column
• Volumetric pipettes are the most accurate way of measuring a fixed volume of liquid,
  • They have a scratch mark on the neck which is matched to the bottom of the meniscus to make the measurement
  • A pipette filler is used to draw the liquid into the volumetric pipette
  • The most common volumes for volumetric pipettes are 10 cm³ and 25 cm³
• Measuring cylinders are used when approximate volumes are required (accuracy is not an important factor)
  • These are graduated (have a scale so can be used to measure)
  • Measuring cylinders typically range from 10 cm³ to 1 litre (1 dm³)
• Whichever apparatus you use, you may see markings in millilitres, ml, which are the same as a cm³

Volumes of gas

• For some experiments, the volume of a gas produced needs to be measured
• This is typically done by using one of the following methods:
  • Using a gas syringe 
  • By downward displacement of water
• A gas syringe is more precise and accurate than downward displacement of water

Diagram of the set-up for an experiment involving a gas syringe

• Downward displacement of water is where a measuring cylinder is inverted in water to collect the gas produced
  • This method does not work if the gas is soluble in water

Diagram of the set-up for an experiment collecting gas by downward displacement of water

• • If the gas happens to be heavier than air and is coloured, the cylinder does not need to be inverted

• In the lab, we often have choices of different apparatus to do the same job
• Evaluating which piece of apparatus is the best one to use is part of good experimental planning and design
• This means appreciating some of the advantages and disadvantages of laboratory apparatus

Advantages and disadvantages of lab apparatus

Apparatus	Advantage	Disadvantage
Temperature probe	More precise readings Easy to make multiple repeat readings Can be automated to run over long periods of time	Can be corroded by some reagents More expensive (to replace)
Volumetric pipette	Accurate measurement of a fixed volume	Harder to use than a normal pipette Only measures one fixed volume
Gas syringe	Easy to set up Keeps the gas dry	The syringe can stick Collects limited volumes Expensive and delicate / fragile
Microscale experiments	Less wasteful Saves energy Safer	Hard to see what's happening Lose a lot of material separating / purifying the products

Five pieces of apparatus that can be used to measure the volume of a liquid. They all have their pros and cons

Planning your method

• Good experimental design includes the answers to questions like
• Have I chosen a suitable apparatus for what I need to measure?
• Is it going to give me results in an appropriate time frame?
• Is it going to give me enough results to process, analyse and make conclusions?
• Does it allow for repetitions to check how reliable my results are?
• Does my plan give a suitable range of results?
• How can I be sure my results are accurate?
• Have I chosen an appropriate scale of quantities without being wasteful or unsafe?
• You may be asked about experimental methods in exam questions and your experience and knowledge of practical techniques in chemistry should help you to spot mistakes and suggest improvements