Experimental Design

Experimental design is the process of turning a scientific question into a practical investigation. It is not simply about remembering the names of laboratory apparatus. You must decide what to change, what to measure, what to keep constant and how the results will answer the original question.

Imagine that you are investigating how the concentration of hydrochloric acid affects its rate of reaction with magnesium. You cannot simply mix the two substances and say that one reaction “looks faster”. You need a measurable quantity, suitable apparatus and a fair method of comparison.

A well-designed experiment therefore connects four things: the scientific question, the apparatus, the measurements and the conclusion.

Start by identifying the variables

Before choosing your apparatus, identify the variables in the investigation. This makes it much easier to decide what should be changed, what should be measured and what must be controlled.

The independent variable

The independent variable is the factor that is deliberately changed.

In an investigation of how acid concentration affects reaction rate, the independent variable is the concentration of the acid.

The dependent variable

The dependent variable is the factor that is measured to see how it responds to the independent variable.

For a reaction between magnesium and hydrochloric acid, possible dependent variables include:

• the volume of hydrogen produced after 30 seconds;
• the time taken to produce 30 cm³ of hydrogen;
• the volume of hydrogen produced at regular time intervals; or
• the decrease in mass as hydrogen escapes.

Notice that these are different ways of measuring the same underlying idea: how quickly the reaction proceeds.

The control variables

Control variables are relevant factors that are kept constant so that they do not provide another explanation for the result.

In the magnesium and hydrochloric acid investigation, suitable control variables could include:

• the volume of hydrochloric acid;
• the mass of magnesium;
• the surface area of the magnesium;
• the temperature;
• the apparatus used; and
• the method of starting and mixing the reaction.

Worked example: identifying variables

A student investigates how temperature affects the rate at which calcium carbonate reacts with hydrochloric acid.

Accuracy, precision and resolution

Students sometimes describe one piece of apparatus as simply “better” than another. In reality, the correct apparatus depends on what is being measured and how useful the measurement needs to be.

Accuracy

Accuracy describes how close a measured value is to the true or accepted value.

Precision

Precision describes how closely repeated measurements agree with one another.

For example, readings of 24.80 cm³, 24.85 cm³ and 24.80 cm³ are close together, so they are precise. However, they are only accurate if the true value is also close to these readings.

Resolution

Resolution is the smallest change that an instrument can display or that can reasonably be read from its scale.

For example, a balance displaying mass to 0.01 g has a higher resolution than one displaying mass only to the nearest gram.

Measuring time, temperature and mass

Measuring time

A stopwatch is used to measure the duration of a process. However, having a stopwatch is only part of a good method. The experiment must also have a clearly defined starting point.

For example:

Start the stopwatch immediately after the magnesium is added and the reaction flask is sealed.

Everyone repeating the experiment should use the same starting condition. Otherwise, differences in timing may be caused by the procedure rather than by the independent variable.

Measuring temperature

A thermometer or temperature probe is used to measure temperature.

The thermometer bulb should:

• be immersed in the substance whose temperature is being measured;
• not rest on the bottom of the container;
• not press against the side of the container; and
• be allowed to reach a steady reading.

If the bulb rests against hot glass, for example, the reading may be influenced by the container rather than representing the liquid accurately.

Measuring mass

An electronic balance is used to measure mass.

1. Check that the balance reads zero.
2. Place a suitable container on the balance if needed.
3. Use the tare function when the mass of only the substance is required.
4. Wait until the displayed reading is steady before recording it.

In a mass-loss rate experiment, the entire reaction apparatus may be placed on the balance. The mass decreases when a gaseous product leaves the apparatus.

Choosing apparatus for measuring liquid volume

Different pieces of apparatus are suitable for different kinds of volume measurement.

Beaker

A beaker may have volume markings, but these are intended mainly for rough estimates.

A beaker may be suitable when the instruction says:

Add approximately 50 cm³ of water.

It would not be suitable if the experiment required exactly 25.0 cm³ of a reactant solution.

Measuring cylinder

A measuring cylinder is useful when a range of volumes must be measured and moderate precision is sufficient.

Its narrower shape and finer scale make it more suitable for measuring volume than a beaker.

Where possible, choose a measuring cylinder whose capacity is reasonably close to the required volume. Measuring 8 cm³ using a 10 cm³ measuring cylinder is generally more useful than trying to read the same volume from a much larger 100 cm³ measuring cylinder.

Volumetric pipette

A volumetric pipette transfers one fixed volume accurately. A common school laboratory pipette delivers 25.0 cm³.

It is therefore suitable when the same exact volume must be transferred in every trial.

Transfer 25.0 cm³ of hydrochloric acid into a conical flask.

A 25.0 cm³ volumetric pipette cannot be used to measure any arbitrary volume such as 17.4 cm³. It is calibrated for its fixed volume.

A pipette filler must be used. Solutions must never be drawn into a pipette by mouth.

Burette

A burette is used when an accurate volume must be delivered but the required volume may vary.

This is why a burette is particularly useful in titration: the exact volume needed to reach the endpoint is not known before the experiment.

Remember that the scale on a burette increases downwards. A larger final reading therefore means that more solution has left the burette.

How do you choose between them?

• One fixed accurate volume: use a volumetric pipette.
• An accurate volume that may vary: use a burette.
• A range of reasonably measured volumes: use a measuring cylinder.
• An approximate volume only: a beaker may be sufficient.

Reading liquid volumes correctly

Even the correct apparatus can give a poor result if its scale is read incorrectly.

For most colourless aqueous solutions:

1. Bring your eye to the same level as the liquid surface.
2. Read the bottom of the meniscus.
3. Record the reading with an appropriate number of decimal places.
4. Include the correct unit.

Looking down at the liquid or looking up at it can produce a parallax error because the apparent position of the meniscus shifts relative to the scale.

Collecting and measuring gases

When a reaction produces a gas, the correct apparatus depends on what you actually need to do with that gas.

You may need to:

• measure its volume;
• collect a sample for a gas test;
• obtain a reasonably pure sample; or
• obtain a dry sample.

These purposes are related, but they do not always require the same method.

Using a gas syringe

A gas syringe is one of the most direct ways to measure the volume of gas produced during a reaction.

As gas enters the syringe, it pushes the plunger outwards. The gas volume can then be read directly from the scale.

A gas syringe is especially useful in rate experiments because the volume can be recorded repeatedly at fixed time intervals.

Before beginning:

• check that the plunger moves freely;
• make sure the apparatus is airtight;
• check that the syringe starts at the intended reading;
• use a syringe with sufficient capacity; and
• support or clamp the apparatus securely.

If the system leaks, some gas escapes without entering the syringe. The measured volume will then be lower than the actual amount produced.

Collecting a gas over water

A gas can be collected over water when it is insoluble or only slightly soluble in water.

The gas enters an inverted collecting vessel that was initially filled with water. As gas enters, it pushes the water out.

Suitable examples include:

• hydrogen;
• oxygen.

This method is unsuitable for a gas that dissolves readily in water. Ammonia, for example, is highly soluble in water and would be lost into the water rather than collected effectively.

Collecting gases by displacement of air

When collection over water is unsuitable, a gas may be collected by displacing air. The direction of collection depends on whether the gas is denser or less dense than air.

Downward delivery

A gas that is denser than air can be introduced near the bottom of an upright gas jar. It settles lower in the jar and pushes the less dense air upwards and out.

Examples include:

• carbon dioxide, CO₂;
• chlorine, Cl₂.

Upward delivery

A gas that is less dense than air rises into an inverted gas jar and displaces the denser air downwards.

Ammonia is an important example because it is both highly soluble in water and less dense than air.

Choosing a gas-collection method

When a reasonably pure sample is required, allow the gas to pass through the apparatus briefly before collecting it. The first gas leaving the apparatus contains some of the air that was already inside the flask and delivery tube.

Drying a gas

A gas prepared using aqueous reactants may leave the apparatus mixed with water vapour. Removing this water vapour is called drying the gas.

A suitable drying agent must satisfy two conditions:

1. It must remove water.
2. It must not react with or significantly absorb the gas being collected.

Quicklime: calcium oxide

Quicklime, CaO, is suitable for drying ammonia because it removes water without neutralising the alkaline ammonia gas.

Concentrated sulfuric acid

Concentrated sulfuric acid is an effective drying agent for some gases, but it cannot be used simply because it removes water strongly.

It is unsuitable for drying ammonia because sulfuric acid reacts with ammonia:

Instead of removing only water, the drying agent would remove the ammonia itself by chemical reaction.

Fused calcium chloride

Fused calcium chloride is an anhydrous solid that can absorb water. It may be used for gases with which it does not react or become significantly associated.

It is not chosen for ammonia because anhydrous calcium chloride can also absorb ammonia.

Measuring the rate of a reaction

The rate of a reaction describes how quickly reactants are used up or products are formed.

The quantity measured does not always have to be concentration. In practical O-Level Chemistry, useful measurements may include:

• gas volume;
• mass;
• colour or cloudiness;
• visibility of a mark; or
• time taken to reach a clearly defined endpoint.

The important question is whether the chosen quantity changes in a clear and measurable way as the reaction proceeds.

Method 1: measuring gas volume

If a reaction produces a gas, the flask can be connected to a gas syringe. Record the gas volume at regular time intervals.

The volume eventually becomes constant because no more gas is being produced.

For example, during the first 40 s:

On a graph of gas volume against time:

• a steeper gradient represents a faster reaction;
• a decreasing gradient shows that the reaction is slowing down; and
• a horizontal line shows that gas production has stopped.

Method 2: measuring loss of mass

If a gaseous product escapes from the reaction flask, the mass of the apparatus decreases.

The flask can therefore be placed on an electronic balance and its mass recorded at regular time intervals.

A loose plug of cotton wool may be placed in the flask neck. It:

• allows the gas to escape;
• reduces loss of liquid droplets caused by splashing; and
• helps ensure that the measured mass decrease is mainly due to gas leaving.

The cotton wool must not form an airtight seal. The gaseous product must still be able to escape.

Method 3: timing a visible change

Some reactions can be followed by timing a visible change instead of collecting a gas.

For example, when sodium thiosulfate reacts with hydrochloric acid, sulfur forms and the mixture becomes increasingly cloudy. The time taken for a cross beneath the flask to disappear can be measured.

If the same visible endpoint is used each time, a shorter time means that the reaction reached that endpoint more quickly and therefore had a greater rate.

The weakness is that deciding exactly when the cross disappears involves human judgement. The method can be made more consistent by using the same observer, lighting, viewing position, flask and cross in every trial.

Planning a complete investigation

Consider the question:

How does the concentration of hydrochloric acid affect its rate of reaction with magnesium?

A strong experimental plan can be built systematically rather than by writing a random list of apparatus.

1. State the independent variable

Use hydrochloric acid solutions of different concentrations.

2. Choose the dependent variable

Measure the volume of hydrogen produced at regular time intervals using a gas syringe.

3. State the control variables

Keep constant:

• the volume of hydrochloric acid;
• the mass and surface area of magnesium;
• the temperature;
• the gas syringe and reaction flask; and
• the method of starting and mixing the reaction.

4. Select suitable apparatus

The setup may include:

• a conical flask;
• a rubber bung;
• a delivery tube;
• a gas syringe and clamp stand;
• a stopwatch;
• a pipette or suitable measuring apparatus for the acid; and
• an electronic balance for measuring the magnesium.

5. Write a reproducible procedure

1. Transfer 25.0 cm³ of hydrochloric acid of known concentration into a conical flask.
2. Connect the flask to a gas syringe and check that the apparatus is airtight.
3. Measure a fixed mass of magnesium with the same surface condition.
4. Add the magnesium to the acid.
5. Immediately seal the flask and start the stopwatch.
6. Record the gas volume every 10 seconds until it no longer increases.
7. Repeat using other hydrochloric acid concentrations.
8. Repeat trials so that the consistency of the results can be assessed.

6. Explain how the results will be used

Plot gas volume against time for each acid concentration.

The curve with the steeper initial gradient has the greater initial reaction rate.

You could alternatively compare:

• the volume of gas produced after the same fixed time; or
• the time required to produce the same fixed volume of gas.

7. Include relevant safety precautions

Hydrochloric acid can irritate the skin and eyes, so suitable eye protection should be worn. Hydrogen is flammable, so the experiment should be kept away from naked flames.

8. Identify sources of error and improvements

Possible problems include:

• gas escaping before the bung is fitted;
• leaks around the bung or delivery tube;
• delay in starting the stopwatch;
• magnesium pieces having different surface areas;
• a gas-syringe plunger that does not move freely; and
• temperature differing between trials.

A useful improvement should address a specific problem rather than simply saying “use better apparatus”.

Making experimental results more reliable

Repeat measurements

Repeating an experiment helps you determine whether the results are consistent.

When repeated readings are sufficiently close, a mean can be calculated:

An unusual reading should not simply be deleted because it is inconvenient. First consider whether it is genuinely anomalous compared with the pattern of the other results.

Use a suitable range

The independent variable should cover a range large enough to reveal a meaningful trend while remaining safe and practical.

For example, temperatures of 20 °C, 21 °C and 22 °C may be so close that the effect is difficult to distinguish from ordinary experimental variation.

Use suitable intervals

Measurements should be taken frequently enough to show how the dependent variable changes.

If a reaction finishes in about one minute, taking readings only every two minutes would clearly miss most of the useful information.

Keep the procedure consistent

Consistency includes:

• starting the timer at the same stage;
• mixing in the same way;
• using the same endpoint;
• reading scales from the same viewing position; and
• using the same type of apparatus.

Record units in table headings

A clear results table should use headings such as:

The unit does not need to be repeated beside every individual numerical value once it has been stated clearly in the heading.

Use appropriate precision

The number of decimal places recorded should match the capability of the measuring instrument.

For example, if a balance displays mass to 0.01 g, recording 2.00000 g would imply a precision that the balance did not actually provide.

Random and systematic errors

Not all experimental errors behave in the same way. Distinguishing between random and systematic errors helps you decide how an investigation should be improved.

Random errors

Random errors cause repeated readings to vary unpredictably.

Examples include:

• small differences in human reaction time;
• judging exactly when a cross disappears; and
• small unavoidable variations between pieces of a solid.

Repeating measurements and using a mean can reduce the influence of random variation.

Systematic errors

A systematic error affects measurements in a consistent direction.

Examples include:

• a balance that does not read zero when it should;
• a thermometer that consistently gives readings that are too high; or
• a leaking gas apparatus that loses some gas in every trial.

Simply repeating the same flawed procedure does not remove a systematic error. The source of the error must be corrected.

Common experimental-design mistakes

Experimental-design checklist