Chemistry 6

UNIT OVERVIEW

Practical Skills in Chemistry II

Unit 6 develops and assesses practical chemistry skills using contexts drawn from the advanced chemistry content. The focus is not simply on carrying out an experiment, but on planning, analysing and evaluating experimental work.

TOPIC 01

Experimental Design

A good experiment must be designed so that the results can answer the question being investigated.

Variables

• Independent variable — the variable deliberately changed.
• Dependent variable — the quantity measured.
• Control variables — factors kept constant.

Planning an Investigation

• Identify the purpose of the investigation.
• Choose suitable apparatus and measurements.
• Control relevant variables.
• Consider safety and appropriate precautions.
• Collect enough data to identify a reliable trend.

TOPIC 02

Measurements and Apparatus

Practical chemistry requires choosing appropriate apparatus and recording measurements with suitable precision.

Common Apparatus

• Burette
• Volumetric flask
• Measuring cylinder
• Pipette
• Balance
• Thermometer
• Gas syringe
• Stopwatch

Choosing Apparatus

The apparatus should be appropriate for the required measurement and should provide sufficient precision.

TOPIC 03

Titration and Volumetric Analysis

Volumetric analysis uses accurately measured volumes to determine an unknown concentration or amount of substance.

Typical Procedure

1. Rinse and prepare the apparatus appropriately.
2. Measure a known volume of solution.
3. Add the titrant carefully from the burette.
4. Use a suitable indicator when required.
5. Identify the endpoint.
6. Repeat until concordant titres are obtained.

Processing Results

Mean titres should normally be calculated using appropriate concordant results rather than including an obvious anomalous result.

TOPIC 04

Rates of Reaction Experiments

Rate experiments investigate how factors such as concentration, temperature or catalysts affect reaction rate.

Common Measurements

• Time taken for a reaction to reach a specified point.
• Volume of gas produced over time.
• Change in concentration over time.

Graphs

Experimental data can be plotted to determine the relationship between variables.

TOPIC 05

Equilibrium Investigations

Practical investigations can be used to examine the effect of changing conditions on chemical equilibria.

Variables That May Be Investigated

• Concentration
• Temperature
• Pressure for suitable gaseous systems

TOPIC 06

Organic Chemistry Practical Skills

Practical questions may require knowledge of organic reactions, purification and identification techniques from the A2 chemistry content.

Important Techniques

• Reflux
• Distillation
• Filtration
• Crystallisation
• Separation and purification
• Testing organic compounds

Observations

Observations should be precise and distinguish between what is actually observed and the chemical explanation of that observation.

TOPIC 07

Inorganic Analysis

Practical chemistry may involve identifying ions or compounds using chemical tests and characteristic observations.

Types of Evidence

• Colour changes
• Precipitate formation
• Dissolution of precipitates
• Gas formation
• Changes following addition of reagents

TOPIC 08

Recording Experimental Data

Experimental results should be organised clearly so that trends, anomalies and relationships can be identified.

Good Data Tables

• Include appropriate headings.
• Include units in headings where appropriate.
• Use consistent decimal places when appropriate.
• Record raw measurements clearly.
• Do not hide anomalous results.

TOPIC 09

Uncertainty

Every experimental measurement has a degree of uncertainty. Understanding uncertainty helps determine how reliable a result is.

Absolute Uncertainty

Absolute uncertainty describes the uncertainty associated with an individual measurement.

Percentage Uncertainty

Reducing Uncertainty

• Use more precise measuring equipment.
• Take repeated measurements.
• Use a larger measurable quantity where appropriate.
• Calculate suitable mean values.

TOPIC 10

Errors and Reliability

TOPIC 11

Graphs and Data Analysis

Graphs are used to identify relationships between experimental variables and to allow quantitative analysis.

Good Graph Practice

• Label both axes.
• Include units where appropriate.
• Use sensible scales.
• Plot points accurately.
• Use a suitable line or curve of best fit.

Gradient

The gradient of a graph can provide useful information about a physical or chemical relationship.

TOPIC 12

Anomalous Results

An anomalous result does not follow the general pattern of the other experimental results.

Identifying an Anomaly

Look for a measurement that lies significantly away from the expected trend or from the pattern shown by repeated results.

Dealing With Anomalies

• Do not simply delete a result without justification.
• Repeat the measurement where possible.
• Consider possible experimental causes.
• Explain how the anomaly affects the conclusion.

TOPIC 13

Evaluation of Experimental Methods

Evaluation requires more than saying that an experiment could be improved. The weakness should be identified and linked to a realistic improvement.

Strong Evaluation

• Identify a specific limitation.
• Explain how it affects the result.
• Suggest a practical improvement.
• Explain why the improvement would help.

TOPIC 14

Risk Assessment and Safety

Safe experimental practice requires identifying hazards, assessing risks and taking suitable precautions.

Typical Safety Considerations

• Wear appropriate eye protection.
• Handle corrosive substances carefully.
• Use appropriate ventilation where required.
• Take care with heating equipment.
• Dispose of chemical waste appropriately.

PRACTICAL SKILLS

What You Should Be Able To Do

Use this checklist as a final Unit 6 readiness scan.

QUICK REVIEW

Unit 6 Key Ideas