UNIT OVERVIEW
Practical Skills in Chemistry I
Unit 3 develops the practical skills and experimental understanding required throughout Units 1 and 2.
Unit 3 is a written practical-skills examination. You must apply practical knowledge to both familiar and unfamiliar experimental situations.
What You Need To Be Able To Do
- Understand experimental procedures.
- Select appropriate apparatus and techniques.
- Record observations and measurements correctly.
- Process and interpret experimental data.
- Identify sources of error.
- Suggest improvements to experimental methods.
- Use practical knowledge in unfamiliar situations.
- Perform relevant chemical calculations.
TOPIC 01
Laboratory Equipment & Measurements
Choosing suitable apparatus is an important practical skill. Different pieces of equipment provide different levels of accuracy and precision.
Choosing Apparatus
The apparatus should be appropriate for the required measurement and level of accuracy.
Do not simply name an apparatus. Be prepared to explain why it is suitable for the measurement being made.
TOPIC 02
Accuracy, Precision & Uncertainty
Accuracy
Accuracy describes how close a measurement is to the accepted or true value.
Precision
Precision describes how closely repeated measurements agree with one another.
Random Error
Causes readings to vary unpredictably. Repeating measurements and calculating a mean can reduce its effect.
Systematic Error
Produces a consistent shift in measurements and is not removed simply by repeating the experiment.
Repeats can reduce the effect of random error, but they do not eliminate systematic error.
TOPIC 03
Variables & Experimental Design
Independent
The variable deliberately changed by the experimenter.
Dependent
The variable measured as a result of changing the independent variable.
Control
Variables kept constant so the investigation is a fair test.
Planning an Investigation
- Identify the independent variable.
- Identify the dependent variable.
- Control relevant variables.
- Use an appropriate range of values.
- Repeat measurements where appropriate.
- Record results clearly.
TOPIC 04
Safety & Risk Assessment
Practical chemistry requires hazards to be identified and appropriate precautions to be considered.
Hazard
A property of a substance or procedure that can cause harm.
Risk
The likelihood that a hazard will actually cause harm under the circumstances.
Risk Reduction
- Use appropriate personal protective equipment.
- Use suitable quantities.
- Use appropriate ventilation where necessary.
- Follow the correct experimental procedure.
- Dispose of chemicals appropriately.
TOPIC 05
Titration
Titration is used to determine the concentration of a solution by reacting it with a solution of known concentration.
Typical Apparatus
Burette Readings
The initial and final burette readings are recorded and the titre is calculated from their difference.
Concordant Results
Repeated titres should be sufficiently close together before a mean titre is calculated.
Always pay attention to the burette scale and record readings to the appropriate precision.
TOPIC 06
Preparing Solutions
A solution of known concentration can be prepared using a known mass of a substance and a volumetric flask.
Measure
Accurately measure the required mass.
Dissolve
Dissolve the solid in an appropriate amount of solvent.
Transfer
Transfer the solution quantitatively.
Make Up
Make the solution up to the calibration mark.
Remember that volume must be converted into dm³ when using concentration in mol dm⁻³.
TOPIC 07
Calorimetry
Calorimetry can be used to investigate energy changes associated with chemical reactions.
where q is the energy transferred, m is the mass, c is the specific heat capacity and ΔT is the temperature change.
Experimental Considerations
- Measure temperature accurately.
- Use an appropriate volume of solution.
- Reduce heat loss to the surroundings.
- Stir the solution where appropriate.
- Record the maximum or minimum temperature reached.
TOPIC 08
Investigating Reaction Rates
Practical investigations can be used to determine how changing conditions affects the rate of a reaction.
Possible Measurements
- Volume of gas produced.
- Mass lost.
- Time taken for a visible change.
- Concentration change.
Where the same endpoint is used, comparing reciprocal times can provide a simple measure of relative rate.
TOPIC 09
Qualitative Observations & Chemical Tests
Practical questions may require accurate descriptions of observations rather than explanations of what is happening at particle level.
Good Observations
- Colour changes.
- Formation of a precipitate.
- Dissolving of a solid.
- Gas production.
- Temperature changes.
- Changes in appearance.
An observation describes what is seen or measured. An inference explains what that observation suggests.
TOPIC 10
Recording & Processing Data
Tables
Tables should have clear headings and appropriate units.
Significant Figures
Calculated answers should normally be given to an appropriate number of significant figures based on the data provided.
Mean
Percentage Error
TOPIC 11
Graphs & Data Analysis
Graphs allow relationships between variables to be investigated visually.
Good Graph Practice
- Put the independent variable on the x-axis.
- Put the dependent variable on the y-axis.
- Label both axes with quantities and units.
- Use an appropriate scale.
- Plot points accurately.
- Draw an appropriate line or curve of best fit.
Gradient
The gradient can be calculated using two well-separated points on a line of best fit.
TOPIC 12
Errors, Limitations & Improvements
Identifying Problems
Experimental results may be affected by heat loss, incomplete reactions, inaccurate measurements, contamination or limitations of apparatus.
Suggesting Improvements
- Use more precise measuring equipment.
- Repeat measurements and calculate a mean.
- Use insulation to reduce heat loss.
- Increase the number of readings.
- Use a wider range of values where appropriate.
- Control variables more carefully.
An improvement should directly address the identified limitation. Do not give a generic improvement without explaining what problem it solves.
TOPIC 13
Applying Practical Skills to Unfamiliar Situations
Unit 3 can present experimental situations that are not identical to experiments you have previously studied.
Identify
Identify what is being measured.
Analyse
Identify independent and dependent variables.
Inspect
Look carefully at the apparatus provided.
Collect
Determine what data needs to be collected.
Evaluate
Identify possible sources of error.
Apply
Use the information given rather than assuming unnecessary details.
PRACTICE
Unit 3 Mixed Practice
Show answer
A burette allows a solution to be delivered and measured more precisely, making it suitable for determining an accurate titre.
Show answer
Random errors cause measurements to vary unpredictably, whereas systematic errors cause a consistent shift in measurements.
Show answer
c = n / V
c = 0.025 / 0.500
c = 0.050 mol dm⁻³
Show answer
Insulate the apparatus and use a lid where appropriate. These reduce energy transfer to the surroundings.
Show answer
Repeats can reveal anomalous results and allow a mean value to be calculated, reducing the effect of random variation.
Show answer
Temperature.
Show answer
Recording observations immediately reduces the risk of forgetting or inaccurately recalling what was observed.
Show answer
Clear headings and appropriate units should be included. Values should also be recorded consistently.
QUICK REVIEW
Unit 3 Key Ideas
Amount of substance from mass and molar mass.
Concentration from amount of substance and volume.
Energy transfer in calorimetry.
Temperature change measured during calorimetry.
Gradient of a graph.
Percentage uncertainty/error used to evaluate measurements.
Read the experimental situation carefully, identify what is being measured, use appropriate practical techniques and justify your decisions with reference to the experiment.