Measuring Variables in Chemistry (DP IB Chemistry): Revision Note

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 typically measured with a digital balance accurate to two decimal places.

• Always tare (zero) the balance before weighing

• In chemistry, mass is usually recorded in grams (g), though the SI unit is kilograms (kg):

  • 1 kg = 1000 g

Measuring the volume of liquids

• The method used depends on how accurate the measurement needs to be.

• Measuring cylinders

  • Used for approximate volumes.

  • Graduated with volume markings.

  • Typically range from 10 cm³ to 1 dm³

• Volumetric Pipettes

  • Most accurate for measuring a fixed volume (e.g. 10 cm³ or 25 cm³)

  • A calibration mark indicates the volume; align with the bottom of the meniscus

• Burettes

  • Most accurate for measuring a variable volume (e.g. 0–50 cm³)

  • The scale runs top to bottom (0.00 cm³ is at the top), so always read from the top down

• 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

Measuring the volume of gases

• The volume of a gas can be measured by collecting it in a graduated measuring device.

  A gas syringe is the most common apparatus used

• Alternatively, a measuring cylinder or burette inverted over water can be used if the gas is insoluble in water

• If the gas is denser than air and coloured, it may be collected upright in a cylinder by downward displacement of air

Measurement of the volume of gas using a gas syringe

Measuring time

• Time is measured using a stopwatch or stop-clock, usually accurate to 1–2 decimal places

• The most common units are seconds and minutes, though hours may be used for very 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 using a thermometer or digital temperature probe

• Laboratory thermometers:

  • Use the thermal expansion of a liquid (e.g. alcohol or mercury) in a capillary tube

  • Commonly give readings to the nearest 1 °C or 0.5 °C

  • Are simple, robust, and inexpensive

  • May take longer to equilibrate and are less precise than digital options

  • Units are typically recorded in degrees Celsius (°C)

• Digital temperature probes:

  • Use electronic sensors (e.g. thermistors or thermocouples) to detect temperature

  • Often have a higher precision, reading to ±0.1 °C or even more accurate

  • Provide fast, real-time readings

  • Often used in data logging and continuous monitoring

  • Can reduce human error and are ideal for remote or automated experiments

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

Measuring the pH of a solution

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

• Digital pH meters work by:

  • Using an electrode with a thin glass membrane that allows hydrogen ions to pass through

  • These ions affect the voltage, which is converted into a pH value

  • This provides precise and quantitative results, often to two decimal places

  • They are suitable for accurate pH monitoring in research or industrial settings

• Indicators are substances that change colour depending on the pH of the solution.

  • Only a few drops are needed as they are intensely coloured and highly sensitive.

  • Indicators may be natural or synthetic

• Natural (e.g. litmus, red cabbage extract):

  • Contain a mixture of plant extracts

  • Useful across a broad range of pH

  • Provide approximate pH values

• Synthetic indicators (e.g. phenolphthalein, methyl orange):

  • Have sharp, defined colour changes at specific pH ranges

  • Suitable for titrations where clear endpoints are required

• Universal Indicator

  • A wide range indicator made from a blend of indicators

  • Gives approximate pH readings across the full pH scale (pH 1–14)

  • Colour change is matched to a colour chart

  • Colours can vary slightly between manufacturers, so the correct chart must be used

Colours of universal indicator

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

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

• 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

Measuring the electric potential difference

• Potential difference (voltage) is measured using a voltmeter, which can be:

  • Analogue: scale and needle display

  • Digital: electronic numerical readout

• Voltmeters are always connected in parallel with the component being tested

  • They measure the difference in electrical potential between two points in a circuit

Analogue or digital?

• Analogue voltmeters are subject to parallax errors so need to be read at eye level to avoid misreading

• The common ranges are from 0.1–1.0 V or 0–5.0 V, though this may vary

• They should be checked for zero errors before use:

  • 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

An analogue and digital voltmeter

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

• Digital displays show the measured values as digits

  • This reduces parallax error

• Highly accurate and easy to use, but may:

  • Flicker between values; a judgment may be needed to record a stable value

• 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