🧪 Practical Chemistry

1. Introduction

Practical Chemistry involves using laboratory equipment and chemical substances to investigate chemical ideas. Students should be able to follow procedures, make accurate observations, record results and draw suitable conclusions.

Learning Objectives

• Use common laboratory apparatus correctly.
• Follow basic laboratory safety rules.
• Measure mass, volume and temperature.
• Separate mixtures using appropriate methods.
• Carry out simple titrations.
• Prepare substances safely.
• Record observations and results correctly.
• Identify sources of experimental error.

2. Laboratory Safety

• Wear appropriate protective clothing and eye protection.
• Never taste chemicals.
• Do not smell chemicals directly; use the wafting method when instructed.
• Keep chemical containers closed when not in use.
• Never point a heated test tube towards another person.
• Use tongs when handling hot apparatus.
• Report spills and accidents immediately.
• Wash hands after practical work.

3. Common Laboratory Apparatus

4. Measuring Volume

Liquid volume is commonly measured in cm³ or dm³.

Remember:

When using a measuring cylinder, read the liquid level at eye level. For a concave meniscus, read the bottom of the meniscus.

5. Measuring Mass

A balance is used to measure mass.

Good technique

• Place the balance on a stable surface.
• Check that it is zeroed before use.
• Do not place chemicals directly on the balance pan.
• Use a suitable container.
• Record the correct number of decimal places shown by the balance.

6. Filtration

Filtration separates an insoluble solid from a liquid.

The liquid that passes through the filter paper is called the filtrate. The solid left on the filter paper is the residue.

7. Evaporation and Crystallisation

Evaporation can be used to recover a dissolved solid from a solution.

Crystallisation is used when a soluble solid is required as crystals. The solution is concentrated and then allowed to cool so crystals form.

8. Simple Distillation

Distillation can be used to obtain a solvent from a solution.

During distillation:

1. The solution is heated.
2. The substance with the lower boiling point vaporises.
3. The vapour enters the condenser.
4. The vapour cools and condenses into a liquid.

Distillation is useful for obtaining pure water from a solution containing dissolved salts.

9. Separating Immiscible Liquids

Two liquids that do not mix can be separated using a separating funnel.

The lower layer is released first through the tap.

10. Titration

Titration is used to determine the concentration of a solution by reacting it with a solution of known concentration.

Basic procedure

1. Rinse and fill the burette with the solution to be delivered.
2. Record the initial burette reading.
3. Use a pipette to transfer a measured volume of the other solution into a conical flask.
4. Add a suitable indicator.
5. Add solution from the burette while swirling the flask.
6. Near the end-point, add the solution drop by drop.
7. Record the final burette reading.
8. Calculate the titre.

11. Reading a Burette

Burette readings are normally recorded to two decimal places when the apparatus allows this precision.

Always read the burette at eye level to reduce parallax error.

12. Preparing a Soluble Salt

A soluble salt can often be prepared by reacting an acid with an appropriate insoluble base or carbonate.

General method

1. Warm the acid carefully.
2. Add the insoluble solid in small portions.
3. Stir after each addition.
4. Continue until some solid remains unreacted.
5. Filter off excess solid.
6. Concentrate the filtrate.
7. Allow it to cool and crystallise.
8. Filter and dry the crystals.

13. Observations and Results

Results should be recorded clearly in a table when appropriate.

Do not write conclusions as observations.

14. Accuracy, Precision and Error

Accuracy

Accuracy describes how close a measurement is to the true value.

Precision

Precision describes how closely repeated measurements agree with one another.

Parallax Error

Parallax error can occur when a scale is viewed from the wrong angle. Read scales at eye level.

Random Error

Random errors cause measurements to vary unpredictably. Repeating measurements and calculating a mean can reduce their effect.

Systematic Error

A systematic error shifts measurements consistently in one direction. It cannot normally be removed simply by repeating the experiment.

15. Improving an Experiment

When asked how to improve an experiment, give a specific method rather than simply saying “be more careful”.

16. MANEB-style Practical Questions

17. Practice Exercises

1. State two laboratory safety rules.
2. Name the apparatus used to measure a fixed accurate volume of solution.
3. What is the purpose of filtration?
4. Distinguish between residue and filtrate.
5. What is the purpose of a condenser in distillation?
6. Calculate the titre when the initial reading is 0.35 cm³ and the final reading is 23.75 cm³.
7. Why should a burette be read at eye level?
8. What is the difference between accuracy and precision?

Answers

18. Common Mistakes

• Reading the burette from the wrong direction.
• Forgetting to subtract the initial reading from the final reading.
• Confusing residue with filtrate.
• Using a measuring cylinder when a pipette or burette is required for greater accuracy.
• Giving conclusions instead of observations.
• Failing to include units in numerical answers.
• Giving vague suggestions for improving experiments.

19. Exam Tips

• Always include units in measurements and calculations.
• Record readings consistently to the required decimal places.
• Use tables for repeated experimental results.
• Show calculations clearly.
• When asked for an observation, describe exactly what happened.
• When asked for a conclusion, explain what the observation means.
• For practical-method questions, write the steps in a logical order.