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Experimental Methods

Choose suitable apparatus, record measurements clearly and plan a fair, safe investigation.

  • SEC G2 Science Chemistry component 2027
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Learning objectives

  • name appropriate apparatus for the measurement of time, temperature, mass and volume; including burettes, pipettes, measuring cylinders and gas syringes
  • suggest suitable apparatus, given relevant information, for a variety of simple experiments, including collection of gases.
  • select and use techniques, apparatus and materials
  • take readings and record observations
  • interpret and evaluate experimental data and observations
  • evaluate methods and suggest possible improvements.

An experimental method earns marks when every apparatus choice has a reason. Name the quantity, choose an instrument with a suitable range and precision, and state how the result will be read or collected.

1. Definition

An experimental method is an ordered, safe procedure for making observations or measurements that answer a question. A measurement is a numerical value recorded with its unit.

2. Key Ideas

  • Measure time with a stopwatch or data logger, temperature with a thermometer, mass with a balance, and volume with suitable volumetric apparatus.
  • A measuring cylinder is suitable for approximate liquid volumes; a pipette or burette is used when greater precision is required.
  • A gas syringe measures gas volume directly.
  • Collect a gas over water only if it is not very soluble in water.
  • Use displacement of air when water is unsuitable, choosing the jar orientation from the gas density.
  • Read a liquid scale at eye level and record the instrument’s precision.
  • In an acid–alkali titration, use a pipette for a fixed volume, a burette for a measured variable volume and the given indicator to show the end-point.

3. Detailed Explanations

Selecting an instrument

First identify the quantity and expected range. Then choose the smallest useful scale divisions without exceeding the instrument range. For a change found from two readings:

change = final reading-initial reading

For reaction-rate measurements, keep time intervals consistent and measure a quantity such as gas volume or mass loss.

Correct and incorrect viewing positions for a concave meniscusThe correct panel shows an eye level with the bottom of a concave meniscus and a horizontal sight line. The parallax panel shows eyes above and below the meniscus with diagonal sight lines, which shift the apparent reading.Correct: eye levelRead the lowest point of the curve.Wrong: parallaxThe apparent scale position shifts.
Bring your eye level with the liquid surface and read the correct part of the meniscus. Record every digit justified by the scale rather than inventing extra precision.

Collecting gases

Collection over water is convenient when the gas has low solubility and does not react with water. If water is unsuitable, use density to choose the jar orientation:

  • upward delivery collects a gas less dense than air in an inverted gas jar; the gas rises and pushes air downwards;
  • downward delivery collects a gas denser than air in an upright gas jar; the gas sinks and pushes air upwards.

A gas syringe avoids relying on density and gives a volume reading, but connections must be gas-tight.

Gas collection over water, by upward delivery and by downward deliveryThree labelled panels compare an inverted gas jar over a water trough, an inverted gas jar collecting a less-dense gas that rises, and an upright gas jar collecting a denser gas that sinks. Arrows also show displaced water or air.1. Over waterInsoluble or slightly soluble gasGas displaces water from the jar.2. Upward deliverySoluble gas less dense than airGas rises; air exits below.inverted gas jar3. Downward deliverySoluble gas denser than airGas sinks; air exits at the top.upright gas jar
Choose the method from the task and gas properties: collect insoluble gases over water; use upward delivery for soluble gases less dense than air and downward delivery for soluble gases denser than air.

Carrying out an acid–alkali titration

Rinse the pipette with the solution it will transfer, then use it to place a fixed volume in a conical flask. Add the suitable given indicator. Rinse and fill the burette with the other solution, remove the funnel, record the initial reading and add solution while swirling the flask. Near the end-point, add it drop by drop until the indicator changes colour permanently. Record the final burette reading and calculate the volume delivered. Repeat until close results are obtained. Focus here on the apparatus, safe method, readings and end-point; use the Chemical Calculations lesson for any calculation based on titration data.

Labelled titration setupA vertical burette is held by a clamp stand above a conical flask on a white tile. The burette funnel is removed, the meniscus is read at eye level, and a separate volumetric pipette with a pipette filler is shown.clamp standburettetapfilling funnel removedread meniscus at eye levelconical flaskwhite tilefillervolumetric pipetteNever pipette by mouth.

Main titration setup

Transfer the measured volume safely

Titration setup: clamp the burette vertically, remove the filling funnel, read the meniscus at eye level and deliver into a conical flask on a white tile.

4. Common Mistakes

Make the method specific
  • Do not write “measure the liquid” without naming volume and the apparatus.
  • Do not read a scale from above or below eye level.
  • Do not collect a water-soluble gas over water.
  • Do not ignore gas leaks, which make a collected volume too small.
  • Do not give a reading without its unit.
  • Do not use a measuring cylinder in place of a pipette when an accurate fixed volume is required for titration.
  • Do not add large portions near the end-point; add solution dropwise while swirling.

5. Exam Tips

Exam question 1: Match precision to purposeCore

A student must transfer exactly 25.0 cm³ of solution. Name the most suitable apparatus and explain the choice.

Show Answer

Use a 25.0 cm³ volumetric pipette. It transfers one fixed volume more precisely than a measuring cylinder.

Exam question 2: Approach a titration end-pointCore

Explain two changes to the method as the indicator begins to change colour near the end-point.

Show Answer

Add the burette solution drop by drop and swirl the conical flask after each addition. This prevents one large addition from passing the end-point before the colour is mixed throughout the solution.

Use the pattern quantity → apparatus → reading or collection method → unit.

6. Worked Examples

Modelled example 1

Choose a Gas-Collection Method

Core

Problem

A gas is very soluble in water and denser than air. Suggest a collection method and justify both parts of the choice.
Study the worked solution

  1. Reject collection over water

    Method

    Do not collect the gas over water.

    Reason

    A very soluble gas would dissolve, so little of it would remain in the collection vessel.

    Working

    Water collection: unsuitable.

  2. Use density

    Method

    Use downward delivery into an upright gas jar, with the delivery tube entering low in the jar.

    Reason

    The denser gas sinks and displaces the less-dense air upwards.

    Working

    Method: downward delivery (upward displacement of air).

Guided practice 2

Calculate a Delivered Volume

About 5 min

Problem

A burette reading changes from 1.20 cm³ to 24.65 cm³. Calculate the delivered volume and report it to the justified precision.

Try this before viewing the solution

Operation
Unit: cm³

Hints

Hint 1: change rule
Use final reading minus initial reading.
Hint 2: precision
Both readings are recorded to two decimal places, so preserve two decimal places in the difference.
View solution step by step

  1. Find the change

    Method

    Subtract the initial reading from the final reading.

    Reason

    Delivered volume is the change between the two scale readings.

    Working

    24.65-1.20 = 23.45.

  2. Attach unit and precision

    Method

    Report the value in cubic centimetres to two decimal places.

    Reason

    The original readings justify that precision.

    Working

    23.45 cm³.

Common misconception 3

A Leak Does Not Increase the Syringe Reading

Find and correct the mistake

Learner claim

“If a gas-syringe connection leaks, extra air enters and the measured gas volume is too large.” Diagnose the error for a reaction producing gas.

Try this before viewing the solution

Path of product gas
Measured volume

View solution step by step

  1. Follow the product gas

    Method

    State that some newly formed gas escapes through the loose connection.

    Reason

    The leak gives the gas another path instead of forcing all of it into the syringe.

    Working

    Gas reaching syringe < gas produced.

  2. State the error direction

    Method

    Conclude that the measured collected volume is too small.

    Reason

    The syringe records only the gas that reaches it.

    Working

    Leak → underestimated gas volume.

Examiner practice 4

Match Volume Apparatus to Purpose

4 marks

Examination question

A learner must (i) transfer exactly 25.0 cm³ of solution and (ii) measure approximately 35 cm³ of water. Name suitable apparatus for each task and justify each choice. [4 marks]

Try this before viewing the solution

View solution step by step

  1. Choose for the fixed exact volume

    2 marks

    Method

    Use a 25.0 cm³ volumetric pipette.

    Reason

    It transfers one calibrated fixed volume more precisely than a measuring cylinder.

    Working

    Exact fixed transfer: volumetric pipette.

  2. Choose for the approximate variable volume

    2 marks

    Method

    Use a suitably sized measuring cylinder.

    Reason

    It can measure approximately 35 cm³ and its precision is sufficient when an exact fixed volume is not required.

    Working

    Approximate volume: measuring cylinder, read at eye level.

Challenge 5

Collect a Lighter Water-Soluble Gas

Minimal support

Problem

Gas X is very soluble in water and less dense than air. Select a collection method, describe the gas-jar orientation and explain why collection over water is unsuitable.

Try this before viewing the solution

Water collection
Air-displacement method

Hints

Hint 1: solubility first
Decide whether the gas would remain available if bubbled through water.
Hint 2: density direction
A gas less dense than air rises, so the jar must let displaced air leave below.
View solution step by step

  1. Reject water

    Method

    Reject collection over water.

    Reason

    Gas X is very soluble and would dissolve instead of accumulating.

    Working

    Use displacement of air.

  2. Orient the jar

    Method

    Use upward delivery into an inverted gas jar.

    Reason

    The less-dense gas rises into the jar and displaces denser air downwards.

    Working

    Less dense than air → upward delivery.

7. Mind Stretchers

Mind stretcher 1: Improve a gas-volume experimentExtension

Why should the stopper be fitted before the reactants are mixed in a gas-volume experiment?

Show Answer

Gas may form immediately. Sealing first prevents early gas from escaping before it reaches the gas syringe.

Mind stretcher 2: Balance range and precisionExtension

Why is the instrument with the finest scale not automatically the best choice?

Show Answer

Its range may be too small for the expected value. The instrument must cover the whole range while still giving suitable precision.

8. Quiz

Before attempting the assessment, check that you can:

  • select apparatus for time, temperature, mass and volume;
  • explain a choice using range and precision;
  • read a scale correctly with units;
  • select gas collection from solubility and density;
  • carry out an acid–alkali titration using a pipette, burette and given indicator;
  • identify and prevent gas loss.

Try this next: choose an unfamiliar gas from supplied solubility and density data, then write a one-sentence justification for its collection method.