Core Practicals Overview — Chemistry

This lesson covers the key Chemistry core practicals for Edexcel GCSE Combined Science (1SC0). As with Biology, around 15% of marks assess practical knowledge. You must know the method, variables, results and safety precautions for each practical.

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Core Practical 4: Investigating Paper Chromatography

Aim

To use paper chromatography to separate and identify substances in a mixture (e.g. food colourings or inks).

Method

1. Draw a pencil line (the origin or baseline) about 2 cm from the bottom of a piece of chromatography paper. Use pencil because it is insoluble and will not dissolve in the solvent.
2. Place small spots of the samples and known reference substances on the pencil line, equally spaced.
3. Pour a shallow layer of solvent (e.g. water or ethanol) into a beaker — the solvent level must be below the pencil line.
4. Place the paper in the beaker and cover with a lid (to prevent evaporation of the solvent).
5. Allow the solvent to rise up the paper by capillary action.
6. Remove the paper when the solvent front is near the top and immediately mark the solvent front with a pencil line.
7. Allow the paper to dry.
8. Calculate Rf values for each spot.

Calculating Rf Values

$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$Rf​=distance moved by solvent frontdistance moved by substance​

• Distances are measured from the pencil baseline to the centre of the spot (substance) or to the solvent front line.
• R_f values are always between 0 and 1 (they have no units).
• Each substance has a characteristic R_f value in a given solvent.

Variables

Variable type	Variable
Independent variable (IV)	Different substances / samples being tested
Dependent variable (DV)	R_f value (distance moved by substance / distance moved by solvent)
Control variables	Same solvent, same paper, same temperature, same volume of sample applied

Key Results and Conclusions

• A pure substance produces one spot.
• A mixture produces multiple spots (one for each component).
• If a spot from an unknown sample has the same Rf value as a known reference, they are likely the same substance.

Exam Tip: If a question asks why we use pencil (not pen) for the baseline, the answer is: pencil is insoluble in the solvent and will not interfere with the results.

Safety

• If using ethanol as the solvent, keep it away from naked flames (flammable).
• Work in a well-ventilated area.

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Core Practical 5: Preparing a Pure, Dry Sample of a Soluble Salt

Aim

To prepare a pure, dry sample of a soluble salt (e.g. copper sulfate) by reacting an insoluble base (e.g. copper oxide) with a dilute acid (e.g. sulfuric acid).

Method

1. Pour about 25 cm³ of dilute sulfuric acid into a beaker and warm gently on a tripod and gauze.
2. Add copper oxide (the insoluble base) a spatula at a time, stirring after each addition.
3. Continue adding copper oxide until it is in excess — you will see unreacted black powder at the bottom of the beaker.
4. Filter the mixture to remove the excess copper oxide (the filtrate contains the dissolved salt).
5. Pour the filtrate into an evaporating basin.
6. Heat the solution gently until about half the water has evaporated (you should see crystals starting to form at the edges).
7. Leave the solution to cool and crystallise slowly (this gives larger, purer crystals).
8. Filter off the crystals, wash with a small amount of cold distilled water and pat dry with filter paper.

Why Add Excess Base?

Adding excess ensures that all the acid has reacted. This is important because:

• The remaining solution should contain only the salt and water (no unreacted acid).
• Excess base is removed by filtration.

Variables

Feature	Detail
Acid used	Dilute sulfuric acid
Base used	Copper oxide (insoluble)
Salt produced	Copper sulfate
Key technique	Filtration, evaporation, crystallisation

Key Equation

$\text{copper oxide} + \text{sulfuric acid} \rightarrow \text{copper sulfate} + \text{water}$copper oxide+sulfuric acid→copper sulfate+water

$\text{CuO} + \text{H}_2\text{SO}_4 \rightarrow \text{CuSO}_4 + \text{H}_2\text{O}$CuO+H2​SO4​→CuSO4​+H2​O

Exam Tip: The question might ask you to adapt this method for a different acid-base pair (e.g. zinc oxide + hydrochloric acid → zinc chloride). The method is the same — just change the reagents.

Safety

• Wear eye protection (goggles) throughout.
• Handle hot apparatus with care (use tongs or heatproof mat).
• Dilute acid is an irritant — wash skin immediately if splashed.

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Core Practical 6: Investigating Electrolysis

Aim

To investigate the electrolysis of an aqueous solution (e.g. copper sulfate solution) using inert electrodes.

Method

1. Fill a beaker with copper sulfate solution.
2. Place two carbon (graphite) electrodes into the solution, connected to a DC power supply.
3. Turn on the power supply and observe what happens at each electrode.
4. After a set time, examine the electrodes.

Key Observations

Electrode	Observation	Explanation
Cathode (negative)	Copper metal deposited (pinkish-brown coating)	Cu²⁺ ions gain electrons (reduction): Cu²⁺ + 2e⁻ → Cu
Anode (positive)	Bubbles of gas (oxygen)	Water molecules lose electrons (oxidation): 2H₂O → O₂ + 4H⁺ + 4e⁻

Variables

Variable type	Variable
Independent variable (IV)	Depends on the investigation (e.g. concentration of solution, current)
Dependent variable (DV)	Mass of metal deposited or volume of gas produced
Control variables	Volume of solution, size of electrodes, time, temperature

Key Rules for Electrolysis of Aqueous Solutions

• At the cathode: the least reactive metal ion is discharged (metal deposited). If the metal is more reactive than hydrogen, hydrogen gas is produced instead.
• At the anode: if the solution contains a halide (Cl⁻, Br⁻, I⁻), the halogen is produced. Otherwise, oxygen is produced from water.

Exam Tip: Learn the cathode/anode rules carefully. A common question is: "Predict what is produced at each electrode during the electrolysis of sodium chloride solution." Answer: hydrogen at cathode (sodium is more reactive), chlorine at anode (halide present).

Safety

• Wear eye protection.
• Good ventilation (chlorine or oxygen may be produced).
• Do not touch electrodes while the circuit is on.

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Core Practical 7: Investigating Rates of Reaction

Aim

To investigate how changing the concentration of a reactant affects the rate of reaction, for example the reaction between sodium thiosulfate and hydrochloric acid.

Method