Introduction to Chemistry Required Practicals

In your AQA GCSE Combined Science: Trilogy course, you don’t just learn about chemistry from a book—you see it in action! There are six specific chemistry practicals (numbered 8 to 13) that you must know. The exam will ask you about the methods, the equipment, and how to stay safe. Don't worry if these seem like a lot to remember; we will break them down step-by-step.

Note: For help with the "Working Scientifically" skills like identifying variables or drawing graphs, please see the "Working scientifically: skills, evaluation and experimental design" chapter.

Required Practical 8: Making a Soluble Salt

The goal of this practical is to prepare a pure, dry sample of a soluble salt (like copper sulfate) starting from an insoluble base (like copper oxide).

The Step-by-Step Method:

1. Start with a fixed volume of dilute acid (e.g., sulfuric acid) in a beaker. Gently heat it over a Bunsen burner.
2. Add the insoluble base (e.g., copper oxide powder) a little bit at a time and stir.
3. Keep adding the base until it is in excess. You will know it is in excess when some unreacted powder settles at the bottom and no more dissolves.
4. Filtration: Filter the mixture using filter paper and a funnel to remove the excess (unreacted) base.
5. Crystallisation: Pour the clear solution into an evaporating dish. Heat it gently over a water bath until about half the water has evaporated.
6. Leave the remaining solution in a cool place for at least 24 hours to allow crystals to form. Pat them dry with filter paper.

Quick Tips:

Why add "excess" base? This ensures that all the acid has reacted, so your final salt isn't contaminated with leftover acid.
Why heat gently? If you heat the salt crystals too strongly at the end, they might decompose or "spit" out of the dish.

Required Practical 9: Electrolysis

This practical investigates what happens when you pass an electric current through an aqueous solution using inert electrodes (usually carbon/graphite).

What You Do:

1. Pour approximately \(50 \text{ cm}^3\) of the solution (e.g., copper chloride or sodium chloride) into a beaker.
2. Place two graphite rods (the electrodes) into the solution, making sure they do not touch each other.
3. Connect the rods to a low-voltage DC power supply.
4. Observe what happens at the anode (positive electrode) and the cathode (negative electrode).

Key Rules for Observations:

• At the Cathode (-): You will see a coating of metal if the metal is less reactive than hydrogen (like copper). If the metal is more reactive than hydrogen (like sodium), you will see bubbles of hydrogen gas \(H_2\).
• At the Anode (+): If the solution contains halide ions (chloride, bromide, iodide), you will see the halogen (e.g., bubbles of chlorine gas \(Cl_2\)). If no halide ions are present, you will see bubbles of oxygen gas \(O_2\).

(HT only): You may be asked to write half-equations, such as \(2Cl^- \rightarrow Cl_2 + 2e^-\) or \(Cu^{2+} + 2e^- \rightarrow Cu\).

Required Practical 10: Temperature Changes

In this experiment, you investigate the variables that affect temperature changes in reacting solutions, such as neutralisation (acid + alkali) or displacement reactions.

The Method (Neutralisation example):

1. Measure a set volume of dilute acid into a polystyrene cup.
2. Record the starting temperature of the acid.
3. Add a set volume of an alkali (e.g., sodium hydroxide).
4. Put a lid on the cup, stir through a hole in the lid, and record the maximum temperature reached.
5. Repeat the experiment, increasing the volume of alkali each time.

Why use a Polystyrene Cup?

Polystyrene is a great thermal insulator. This reduces the amount of heat lost to the surroundings, making your temperature readings more accurate. Adding a lid helps even more!

Required Practical 11: Rates of Reaction

There are two ways to investigate how concentration affects the rate of a reaction. You need to know both.

Method 1: Measuring Turbidity (Color Change)

Reaction: Sodium thiosulfate + Hydrochloric acid
1. Draw a black cross on a piece of paper and place a flask on top of it.
2. Add sodium thiosulfate solution to the flask.
3. Add HCl and start a stopwatch.
4. The solution will turn cloudy (turbid) as solid sulfur forms. Stop the watch when you can no longer see the cross.
5. Repeat with different concentrations of sodium thiosulfate.

Method 2: Measuring Gas Volume

Reaction: Magnesium + Hydrochloric acid
1. Add HCl to a conical flask.
2. Add a piece of magnesium ribbon and quickly attach a gas syringe or a delivery tube leading to an upturned measuring cylinder full of water.
3. Measure the volume of hydrogen gas produced at regular time intervals (e.g., every 10 seconds).
4. Repeat with different concentrations of HCl.

Key Takeaway: Increasing the concentration increases the rate of reaction because there are more particles in the same volume, leading to more frequent successful collisions.

Required Practical 12: Chromatography

Chromatography is used to separate mixtures of colored substances, like food dyes or inks.

The Setup:

1. Draw a pencil line near the bottom of the chromatography paper (pencil is insoluble, so it won't run).
2. Put small dots of known colors and an "unknown" mixture on the line.
3. Place the paper in a beaker with a small amount of solvent (e.g., water), ensuring the solvent level is below the pencil line.
4. Let the solvent travel up the paper, carrying the dyes with it.

Calculating \(R_f\) Values:

The \(R_f\) value is a ratio used to identify substances:
\(R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}\)

Memory Tip: "Substance over Solvent" (alphabetical order). The \(R_f\) value will always be less than 1!

Required Practical 13: Water Analysis and Purification

This practical involves checking if a water sample is pure and then purifying it using distillation.

Part 1: Analysis

• pH Test: Use universal indicator or a pH probe. Pure water has a pH of 7.
• Dissolved Solids: Weigh an empty evaporating dish. Pour in some water and evaporate it to dryness using a Bunsen burner. Re-weigh the dish. If the mass increased, the water contained dissolved solids (it wasn't pure).

Part 2: Purification (Distillation)

1. Heat the water sample in a conical flask until it boils.
2. The water vapor (steam) travels through a delivery tube into a test tube sitting in an ice-water bath.
3. The steam condenses back into liquid water in the cold test tube.
4. This "distilled water" is pure and contains no dissolved solids.

Quick Summary Table

RP 8: Making Salts -> Excess base, filter, crystallise.
RP 9: Electrolysis -> Cathode (-), Anode (+), observe gas/metal.
RP 10: Temp Change -> Polystyrene cup + lid for insulation.
RP 11: Rates -> Disappearing cross or gas volume.
RP 12: Chromatography -> \(R_f = \text{dist. substance} / \text{dist. solvent}\).
RP 13: Water -> Distillation (boil and condense).