Measuring Variables in Chemistry (DP IB Chemistry)

Measuring Variables in Chemistry

• You need to know how to accurately measure variables to allow the collection of valid and high-quality data

• Sometimes, you will be required to make a decision as to what piece of equipment to use based on which is the most appropriate for that particular task

Measuring mass

• Mass is measured using a digital balance which normally gives readings to two decimal places

• Balances must be tared (set to zero) before use

• The standard unit of mass is kilograms (kg) but in chemistry, grams (g) are most often used

  • 1 kilogram = 1000 grams

Measuring the volume of liquids

• The volume of a liquid can be determined using several types of apparatus, depending on the level of accuracy needed

• For approximate volumes where high accuracy is not an important factor, measuring (or graduated) cylinders are used

  • These are graduated (have a scale so can be used to measure) and are available typically in a range of sizes from 10 cm³ to 1 litre (1 dm³)

• Volumetric pipettes are the most accurate way of measuring a fixed volume of liquid, usually 10 cm³ or 25 cm³

  • They have a scratch mark on the neck which is matched to the bottom of the meniscus to make the measurement

• Burettes are the most accurate way of measuring a variable volume of liquid between 0 cm³ and 50 cm³ (e.g. in a titration)

  • The tricky thing with burettes is to remember to read the scale from top to bottom as 0.00 cm³ is at the top of the column

• Whichever apparatus you use, you may see markings in ml (millilitre) which is the same as a cm³

Equipment used to measure the volume of liquids

Diagram of a burette, a measuring cylinder, a pipette filler and a volumetric pipette

Measuring the volume of gases

• The volume of a gas sometimes needs to be measured and is done by collecting it in a graduated measuring apparatus

• A gas syringe is usually the apparatus used

• A graduated measuring cylinder or burette inverted in water may also be used, provided the gas is not water-soluble

• If the gas happens to be heavier than air and is coloured, the cylinder can be used upright

Measurement of the volume of gas using a gas syringe

Diagram of the set-up for an experiment involving gas collection

Measuring time

• Time can be measured using a stopwatch or stop-clock which are usually accurate to one or two decimal places

• The units of time normally used are seconds or minutes although other units may be used for extremely slow reactions (e.g. rusting)

  • 1 minute = 60 seconds

• An important factor when measuring time intervals is human reaction time

  • This can have a significant impact on measurements when the measurements involved are very short (less than a second)

Measuring temperature

• Temperature is measured with a thermometer or digital 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)

Measuring length

• Rulers can be used to measure small distances of a few centimetres (cm).

  • They are able to measure to the nearest millimetre (mm)

• The standard unit of length is metres (m)

• Larger distances can be measured using a tape measure

• Many distances in chemistry are on a much smaller scale, for example, a typical atomic radius is around 1 x 10^-10 m, so cannot be measured in this way

 Measuring length

A ruler can measure distances to the nearest mm

Measuring the pH of a solution

• pH can be measured using an indicator or a digital pH meter

• pH meters contain a special electrode with a thin glass membrane that allows hydrogen ions to pass through; the ions alter the voltage detected by the electrode

• An indicator is a substance which changes colour depending on the pH of the solution to which it is added

• There are natural indicators and synthetic indicators which have different uses

  • Generally, natural indicators are wide range indicators that contain a mixture of different plant extracts and so can operate over a broad range of pH values

  • Synthetic indicators mostly have very narrow pH ranges at which they operate

    • They have sharp colour changes meaning they change colour quickly and abruptly as soon as a pH specific to that indicator is reached

• Indicators are intensely coloured and very sensitive so only a few drops are needed

• Universal indicator is a wide range indicator and can give only an approximate value for pH

  • It is made of a mixture of different plant indicators which operate across a broad pH range and is useful for estimating the pH of an unknown solution

  • A few drops are added to the solution and the colour is matched with a colour chart which indicates the pH which matches with specific colours

  • Universal indicator colours vary slightly between manufacturer so colour charts are usually provided for a specific indicator formulation

Colours of universal indicator

pH scale with the universal indicator colours used to determine the pH of a solution

Measuring electric current

• Current is measured using an ammeter

• Ammeters should always be connected in series with the part of the circuit you wish to measure the current through

An ammeter can be used to measure the current around a circuit

Digital or Analogue?

• Ammeters can be either

  • Digital (with an electronic display)

  • Analogue (with a needle and scale)

 Analogue Ammeters

• Typical ranges are 0.1 - 1.0 A and 1.0 - 5.0 A for analogue ammeters

  • Always double-check exactly where the marker is before an experiment

  • If the marker is not at zero, you will need to subtract this from all your measurements

• They should be checked for zero errors before using

• They are also subject to parallax error 

  • Always read the meter from a position directly perpendicular to the scale

An analogue ammeter

Analogue ammeters have a needle and scale for measuring electric current

Digital Ammeters

• Digital ammeters can measure very small currents, in mA or µA

• Digital displays show the measured values as digits and are more accurate than analogue displays

• They’re easy to use because they give a specific value and are capable of displaying more precise values

• However, digital displays may 'flicker' back and forth between values and a judgement must be made as to which to write down

  • Make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results

  • Digital ammeters should be checked for zero errors

A digital ammeter

Digital ammeters have an electric read-out for measuring electric current

Measuring the electric potential difference

• Electric potential difference is measured using a voltmeter, which can be either

  • Digital (with an electronic display)

  • Analogue (with a needle and scale)

• Voltmeters are connected in parallel with the component being tested

  • The potential difference is the difference in electrical potential between two points, therefore the voltmeter has to be connected to two points in the circuit 

Analogue or Digital?

• Analogue voltmeters are subject to 

  • Always read the meter from a position directly perpendicular to the scale parallax errors 

• Typical ranges are 0.1-1.0 V and 0-5.0 V for analogue voltmeters although they can vary

  • Always double-check exactly where the marker is before an experiment, if not at zero, you will need to subtract this from all your measurements

  • They should be checked for zero errors before using

An analogue and digital voltmeter

Voltmeters can be either analogue (with a scale and needle) or digital (with an electronic read-out) for measuring the electric potential difference

• Digital voltmeters can measure very small potential differences, in mV or µV

• Digital displays show the measured values as digits and are more accurate than analogue displays

• They’re easy to use because they give a specific value and are capable of displaying more precise values

  • However, digital displays may 'flicker' back and forth between values and a judgement must be made as to which to write down

• Digital voltmeters should be checked for zero errors

  • Make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results