Implementing

Practical work is the backbone of chemistry. In your written examinations, OCR will assess your understanding of how experiments are actually executed in the laboratory. This area of the specification is called Implementing. It covers selecting the correct apparatus, measuring physical quantities using appropriate units, and recording observations and data with absolute precision.

What you'll learn

How to select and use high-precision laboratory apparatus (such as burettes, pipettes, and volumetric flasks).
The correct units, conversions, and significant figure rules required for quantitative calculations.
How to construct professional data tables and record precise qualitative and quantitative observations.

1. Selecting and Using Practical Apparatus

To carry out a successful chemical experiment, you must choose apparatus that provides the correct level of precision. Using a measuring cylinder when you require the high precision of a volumetric pipette is a classic experimental error that introduces unnecessary uncertainty.

Definition

Resolution

The resolution of a measuring instrument is the smallest change in the quantity being measured that gives a perceptible change in the reading on the instrument's scale.

The table below summarises the typical apparatus you will use and their associated precision in A-Level Chemistry:

Apparatus	Typical Capacity	Typical Resolution / Uncertainty	When to Use
Volumetric Pipette	25.0 cm³ (fixed)	$cm3\pm 0.06\text{ cm}^3$	To deliver highly accurate, fixed volumes of a solution.
Burette	50.0 cm³ (variable)	$cm3\pm 0.05\text{ cm}^3$ (per reading)	To deliver variable, precise volumes of a solution (e.g., in titrations).
Volumetric Flask	250.0 cm³ (fixed)	$cm3\pm 0.15\text{ cm}^3$	To prepare standard solutions of exact concentration.
Measuring Cylinder	10 cm³ to 100 cm³	$cm3\pm 0.5\text{ cm}^3$ to $cm3\pm 1.0\text{ cm}^3$	For measuring approximate volumes of reagents in excess.

Reading a Burette Scale

When using a burette, you must read the scale at the bottom of the curved liquid surface, known as the meniscus. Your eye must be completely level with the meniscus to avoid parallax error.

How to Read a Burette Meniscus

Because a burette has markings every 0.1 cm³, you are expected to interpolate (estimate) between the lines to the nearest 0.05 cm³. This means every single burette reading you write down must end in either .00 or .05.

Tip

Burette Reading Precision

Even if the meniscus sits perfectly on a line, you must record it to two decimal places. For example, a reading exactly on the 22 cm³ mark must be written as 22.00 cm³, not 22 cm³ or 22.0 cm³.

2. Appropriate Units for Measurements

In chemistry calculations, mismatching your units is one of the fastest ways to lose marks. You must be comfortable converting between units seamlessly.

Volume

While the standard SI unit of volume is the cubic metre (m³), chemists typically work with much smaller volumes:

Cubic decimetres (dm³): Equivalent to litres.
Cubic centimetres (cm³): Equivalent to millilitres.

The conversion factors are:

1\text{ dm}^3 = 1000\text{ cm}^3

1\text{ m}^3 = 1000\text{ dm}^3 = 1,000,000\text{ cm}^3

To convert from cm³ to dm³, you divide by 1000:

\text{Volume in dm}^3 = \frac{\text{Volume in cm}^3}{1000}

Temperature

Most laboratory thermometers measure temperature in degrees Celsius (°C), but thermodynamic calculations (such as those involving gas laws or Gibbs free energy) require temperature in Kelvin (K).

The scale conversion is:

T/\text{K} = \theta/^\circ\text{C} + 273

Common Mistake

Temperature Changes

A change in temperature ( $ΔT\Delta T$ ) has the same numerical value in both Celsius and Kelvin. If a reaction mixture warms up by 15 °C, it has also warmed up by 15 K. Do not add 273 to a temperature difference!

Pressure

Pressure is measured in Pascals (Pa) or kilopascals (kPa).

$Pa1\text{ kPa} = 1000\text{ Pa}$
In gas calculations using the ideal gas equation ( $p V = n R T$ ), pressure ( $p$ ) must always be converted into Pascals (Pa).

Key Idea

Unit Consistency in Calculations

Always check that your units match the units of the constant you are using. In $p V = n R T$ , the gas constant $R$ is $mol−18.314\text{ J K}^{-1}\text{ mol}^{-1}$ . Because $R$ contains Joules (which is based on SI base units like metres and kilograms), you must use volume in $m3\text{m}^3$ , pressure in $Pa\text{Pa}$ , and temperature in $K\text{K}$ .

3. Presenting Observations and Data

When you record raw data during a practical activity, it must be laid out systematically so that any other scientist can understand and repeat your work.

Quantitative Data Tables

When constructing tables for numerical data:

Headers: Every column header must contain both the physical quantity and its unit, separated by a forward slash. For example: Volume / cm³ or Temperature / °C or Time / s.
Raw Data Precision: All raw data in a given column must be recorded to the same number of decimal places or significant figures, matching the resolution of the apparatus used.
Processed Data: Calculations derived from raw data (like averages or rate calculations) should be presented in separate columns, rounded to an appropriate number of significant figures (typically matching the least precise piece of raw data).

Common Mistake

Units inside the data rows

Do not write units next to individual numbers inside the cells of a table (e.g. writing 12.5 cm³ in a cell). The unit belongs only in the column header. The cells should contain pure numbers.

Qualitative Observations

Qualitative data describes physical properties that do not involve numbers, such as colour changes, gas evolution, or precipitate formation. Your descriptions must be precise and unambiguous:

Colourless vs Clear: A solution can be "clear" (not cloudy) but still have a colour (like copper sulfate solution, which is blue and clear). If it has no colour, describe it as colourless, never "clear".
Precipitates: If a solid forms when two solutions are mixed, do not just state "it turned cloudy". State the colour of the solid and use the term precipitate (e.g., "a white precipitate formed").
Gases: If a reaction produces gas bubbles, record effervescence or "fizzing". If you test the gas, record both the test and the result (e.g., "the gas turned damp red litmus paper blue").

Worked Example

Example

Recording and processing titration data

A student performs a titration to find the concentration of a sample of hydrochloric acid using $dm−30.100\text{ mol dm}^{-3}$ sodium hydroxide solution.

Their raw burette readings are shown in the table below:

Titration Run	Rough	Run 1	Run 2	Run 3
Final Reading / cm³	24.50	23.40	46.95	24.15
Initial Reading / cm³	0.00	0.00	23.40	0.80
Titre / cm³	24.50	23.40	23.55	23.35

Calculate the mean titre that the student should use for their subsequent concentration calculations.

Step-by-step Solution:

Understand the rule for concordant titres: In volumetric analysis, concordant titres are those that agree within $cm30.10\text{ cm}^3$ of each other. Only concordant titres can be used to calculate the mean.
Evaluate and select the concordant values:
- Rough run: $cm324.50\text{ cm}^3$ (always exclude the rough titre, as it is only a guide).
- Run 1: $cm323.40\text{ cm}^3$
- Run 2: $cm323.55\text{ cm}^3$
- Run 3: $cm323.35\text{ cm}^3$
Let's check the differences between the real runs:
- Difference between Run 1 and Run 3: $0.05\text{ cm}^3$ (concordant).
- Difference between Run 1 and Run 2: $0.15\text{ cm}^3$ (not concordant).
- Difference between Run 2 and Run 3: $0.20\text{ cm}^3$ (not concordant).
Therefore, the concordant titres are Run 1 ( $cm323.40\text{ cm}^3$ ) and Run 3 ( $cm323.35\text{ cm}^3$ ). Run 2 must be excluded.
Calculate the mean of the selected concordant titres: Sum the concordant values and divide by the number of values (2):

\text{Mean Titre} = \frac{23.40 + 23.35}{2} = 23.375\text{ cm}^3

Round to the appropriate precision: Burette readings are measured to two decimal places (ending in .00 or .05). When calculating a mean of values ending in .00 or .05, the result must be rounded to two decimal places.

23.375\text{ cm}^3 \approx 23.38\text{ cm}^3

Exam technique

In the exam

Always use 2 decimal places for burette volumes: Ensure every single burette reading and calculated titre ends in .00 or .05.
State colour changes clearly: Avoid writing "the solution changed colour". Instead, write "the solution changed from yellow to orange".
Double-check unit conversions: In physical chemistry questions (especially ideal gas, enthalpy, and rate calculations), pause and check if you need to convert $cm3→dm3\text{cm}^3 \to \text{dm}^3$ , $kPa→Pa\text{kPa} \to \text{Pa}$ , or $∘C→K^\circ\text{C} \to \text{K}$ .

Self review

Check yourself

Why is it incorrect to include a rough titration value when calculating a mean titre?
A gas syringe has markings every $cm31\text{ cm}^3$ . To what precision should you record gas volumes measured with this syringe?
Convert $cm324.5\text{ cm}^3$ into $m3\text{m}^3$ for use in the ideal gas equation.

What you'll learn

How to select and use high-precision laboratory apparatus (such as burettes, pipettes, and volumetric flasks).
The correct units, conversions, and significant figure rules required for quantitative calculations.
How to construct professional data tables and record precise qualitative and quantitative observations.

1. Selecting and Using Practical Apparatus

Definition

Resolution

The resolution of a measuring instrument is the smallest change in the quantity being measured that gives a perceptible change in the reading on the instrument's scale.

The table below summarises the typical apparatus you will use and their associated precision in A-Level Chemistry:

Apparatus	Typical Capacity	Typical Resolution / Uncertainty	When to Use
Volumetric Pipette	25.0 cm³ (fixed)	$cm3\pm 0.06\text{ cm}^3$	To deliver highly accurate, fixed volumes of a solution.
Burette	50.0 cm³ (variable)	$cm3\pm 0.05\text{ cm}^3$ (per reading)	To deliver variable, precise volumes of a solution (e.g., in titrations).
Volumetric Flask	250.0 cm³ (fixed)	$cm3\pm 0.15\text{ cm}^3$	To prepare standard solutions of exact concentration.
Measuring Cylinder	10 cm³ to 100 cm³	$cm3\pm 0.5\text{ cm}^3$ to $cm3\pm 1.0\text{ cm}^3$	For measuring approximate volumes of reagents in excess.

Reading a Burette Scale

When using a burette, you must read the scale at the bottom of the curved liquid surface, known as the meniscus. Your eye must be completely level with the meniscus to avoid parallax error.

How to Read a Burette Meniscus

Tip

Burette Reading Precision

2. Appropriate Units for Measurements

In chemistry calculations, mismatching your units is one of the fastest ways to lose marks. You must be comfortable converting between units seamlessly.

Volume

While the standard SI unit of volume is the cubic metre (m³), chemists typically work with much smaller volumes:

Cubic decimetres (dm³): Equivalent to litres.
Cubic centimetres (cm³): Equivalent to millilitres.

The conversion factors are:

1\text{ dm}^3 = 1000\text{ cm}^3

1\text{ m}^3 = 1000\text{ dm}^3 = 1,000,000\text{ cm}^3

To convert from cm³ to dm³, you divide by 1000:

\text{Volume in dm}^3 = \frac{\text{Volume in cm}^3}{1000}

Temperature

The scale conversion is:

T/\text{K} = \theta/^\circ\text{C} + 273

Common Mistake

Temperature Changes

Pressure

Pressure is measured in Pascals (Pa) or kilopascals (kPa).

$Pa1\text{ kPa} = 1000\text{ Pa}$
In gas calculations using the ideal gas equation ( $p V = n R T$ ), pressure ( $p$ ) must always be converted into Pascals (Pa).

Key Idea

Unit Consistency in Calculations

3. Presenting Observations and Data

When you record raw data during a practical activity, it must be laid out systematically so that any other scientist can understand and repeat your work.

Quantitative Data Tables

When constructing tables for numerical data:

Headers: Every column header must contain both the physical quantity and its unit, separated by a forward slash. For example: Volume / cm³ or Temperature / °C or Time / s.
Raw Data Precision: All raw data in a given column must be recorded to the same number of decimal places or significant figures, matching the resolution of the apparatus used.
Processed Data: Calculations derived from raw data (like averages or rate calculations) should be presented in separate columns, rounded to an appropriate number of significant figures (typically matching the least precise piece of raw data).

Common Mistake

Units inside the data rows

Do not write units next to individual numbers inside the cells of a table (e.g. writing 12.5 cm³ in a cell). The unit belongs only in the column header. The cells should contain pure numbers.

Qualitative Observations

Qualitative data describes physical properties that do not involve numbers, such as colour changes, gas evolution, or precipitate formation. Your descriptions must be precise and unambiguous:

Colourless vs Clear: A solution can be "clear" (not cloudy) but still have a colour (like copper sulfate solution, which is blue and clear). If it has no colour, describe it as colourless, never "clear".
Precipitates: If a solid forms when two solutions are mixed, do not just state "it turned cloudy". State the colour of the solid and use the term precipitate (e.g., "a white precipitate formed").
Gases: If a reaction produces gas bubbles, record effervescence or "fizzing". If you test the gas, record both the test and the result (e.g., "the gas turned damp red litmus paper blue").

Worked Example

Example

Recording and processing titration data

A student performs a titration to find the concentration of a sample of hydrochloric acid using $dm−30.100\text{ mol dm}^{-3}$ sodium hydroxide solution.

Their raw burette readings are shown in the table below:

Titration Run	Rough	Run 1	Run 2	Run 3
Final Reading / cm³	24.50	23.40	46.95	24.15
Initial Reading / cm³	0.00	0.00	23.40	0.80
Titre / cm³	24.50	23.40	23.55	23.35

Calculate the mean titre that the student should use for their subsequent concentration calculations.

Step-by-step Solution:

Understand the rule for concordant titres: In volumetric analysis, concordant titres are those that agree within $cm30.10\text{ cm}^3$ of each other. Only concordant titres can be used to calculate the mean.
Evaluate and select the concordant values:
- Rough run: $cm324.50\text{ cm}^3$ (always exclude the rough titre, as it is only a guide).
- Run 1: $cm323.40\text{ cm}^3$
- Run 2: $cm323.55\text{ cm}^3$
- Run 3: $cm323.35\text{ cm}^3$
Let's check the differences between the real runs:
- Difference between Run 1 and Run 3: $0.05\text{ cm}^3$ (concordant).
- Difference between Run 1 and Run 2: $0.15\text{ cm}^3$ (not concordant).
- Difference between Run 2 and Run 3: $0.20\text{ cm}^3$ (not concordant).
Therefore, the concordant titres are Run 1 ( $cm323.40\text{ cm}^3$ ) and Run 3 ( $cm323.35\text{ cm}^3$ ). Run 2 must be excluded.
Calculate the mean of the selected concordant titres: Sum the concordant values and divide by the number of values (2):

\text{Mean Titre} = \frac{23.40 + 23.35}{2} = 23.375\text{ cm}^3

Round to the appropriate precision: Burette readings are measured to two decimal places (ending in .00 or .05). When calculating a mean of values ending in .00 or .05, the result must be rounded to two decimal places.

23.375\text{ cm}^3 \approx 23.38\text{ cm}^3

Exam technique

In the exam

Always use 2 decimal places for burette volumes: Ensure every single burette reading and calculated titre ends in .00 or .05.
State colour changes clearly: Avoid writing "the solution changed colour". Instead, write "the solution changed from yellow to orange".
Double-check unit conversions: In physical chemistry questions (especially ideal gas, enthalpy, and rate calculations), pause and check if you need to convert $cm3→dm3\text{cm}^3 \to \text{dm}^3$ , $kPa→Pa\text{kPa} \to \text{Pa}$ , or $∘C→K^\circ\text{C} \to \text{K}$ .

Self review

Check yourself

Why is it incorrect to include a rough titration value when calculating a mean titre?
A gas syringe has markings every $cm31\text{ cm}^3$ . To what precision should you record gas volumes measured with this syringe?
Convert $cm324.5\text{ cm}^3$ into $m3\text{m}^3$ for use in the ideal gas equation.

What you'll learn

1. Selecting and Using Practical Apparatus

Reading a Burette Scale

2. Appropriate Units for Measurements

Volume

Temperature

Pressure

3. Presenting Observations and Data

Quantitative Data Tables

Qualitative Observations

Worked Example

Recap questions

Review Implementing by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Implementing

What you'll learn

1. Selecting and Using Practical Apparatus

Reading a Burette Scale

2. Appropriate Units for Measurements

Volume

Temperature

Pressure

3. Presenting Observations and Data

Quantitative Data Tables

Qualitative Observations

Worked Example

Recap questions

Review Implementing by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Implementing

What you'll learn

1. Selecting and Using Practical Apparatus

Reading a Burette Scale

2. Appropriate Units for Measurements

Volume

Temperature

Pressure

3. Presenting Observations and Data

Quantitative Data Tables

Qualitative Observations

Worked Example

Recap questions

Review Implementing by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Implementing Revision Guide

Implementing

What you'll learn

1. Selecting and Using Practical Apparatus

Reading a Burette Scale

2. Appropriate Units for Measurements

Volume

Temperature

Pressure

3. Presenting Observations and Data

Quantitative Data Tables

Qualitative Observations

Worked Example

Recap questions

Review Implementing by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Implementing Revision Guide