Apparatus for Measurements

• 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
• Careful: Units of time often cause issues in results tables.

  • If the display on a stopwatch showed 1:30.
  • The incorrect time to record would be 1.30 minutes.
  • The correct time would be 1.5 minutes.
  • To avoid any confusion, if the time intervals are less than a minute, it is best / easire to change the recorded units to seconds
  • So, the same stopwatch display would be recorded as 90 seconds
• Temperature is measured with a thermometer or digital temperature probe

  A) thin column

  B) liquid

  C) glass capillary tube

  D) linear scale

  E) glass bulb

  F) volume

  G) liquid

  
• 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 C
• 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 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
• Mass Measuring Rule

  A) zero

  B) wait

  C) setle

  D) reading

  
• Three common pieces of apparatus for measuring the volume of a liquid are:

  • Burettes
  • Volumetric pipettes
  • Measuring cylinders
• Naming Apparatus

  A) burette

  B) measuring cylinder

  C) pipette filler

  D) volumetric

  E) bulb pipette

  
• Burettes are the most accurate way of measuring a variable volume of liquid between 0 cm^3 and 50 cm^3

  • They are most commonly used in titrations
  • Careful: Read the burette scale from top to bottom as 0.00 cm^2 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 cm^3 and 25 cm^3
• 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 cm^3 to 1 litre (1 dm^3)
  • Whichever apparatus you use, you may see markings in millilitres, ml, which are the same as a cm^3
• For some experiments, the volume of a gas produced needs to be measured
• Measuring volume of a gas 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

  A) gas syringe

  B) conical flask

  C) reaction mixture

  
• 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
  • If the gas happens to be heavier than air and is coloured, the cylinder does not need to be inverted
• Diagram of the set-up for an experiment collecting gas by downward displacement of water

  A) delivery tube

  B) measuring (graduated) cylinder

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

  A) beaker

  B) burette

  C) volumetric pipette

  D) conical flask

  E) volumetric flask

  
• 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
• Make sure you know the names of common laboratory apparatus