Practical skills Revision Notes

Principles of Chromatography

Chromatography separates components based on their different affinities for a stationary phase and a mobile phase. The stationary phase is typically a solid (like paper in paper chromatography) or a liquid supported on a solid. The mobile phase is a solvent (liquid) or a gas that moves through the stationary phase, carrying the components of the mixture. Components that are more soluble in the mobile phase or less strongly adsorbed by the stationary phase will move further and faster, leading to separation. This differential movement results in distinct bands or spots on the chromatogram.

Paper Chromatography Method

To perform paper chromatography: 1. Draw a baseline in pencil (not ink, as ink contains soluble dyes) near the bottom of a chromatography paper strip. 2. Apply a small spot of the mixture to be separated onto the baseline. 3. Place the paper in a beaker with a suitable solvent, ensuring the solvent level is below the baseline. 4. Allow the solvent to move up the paper by capillary action. As it moves, it separates the components of the mixture. 5. Once the solvent front is near the top, remove the paper and mark the solvent front in pencil. 6. Allow the paper to dry. If components are colourless, use a locating agent. This method requires careful handling of solvents and glassware.

Rf Value Calculation and Interpretation

The retardation factor (Rf value) is a ratio used to identify separated substances. It is calculated as: Rf = (distance travelled by substance spot from baseline) / (distance travelled by solvent front from baseline). The Rf value is always between 0 and 1. A unique Rf value helps identify a substance under controlled conditions (same stationary phase, mobile phase, and temperature). If two substances have the same Rf value, they are likely the same compound. Comparing Rf values to known standards can confirm identity. Note that Rf values are conditional and vary with changes in solvent or temperature.

Safety Precautions

When carrying out chromatographic separations, safety is crucial. Many solvents used are flammable, volatile, and potentially toxic. Always perform chromatography in a well-ventilated area, ideally a fume cupboard, to avoid inhaling solvent vapours. Wear appropriate personal protective equipment (PPE), including eye protection (safety goggles) to shield against splashes and gloves to prevent skin contact. Handle glassware carefully to prevent breakages. Ensure proper disposal of all chemical waste according to school or laboratory safety protocols to minimise environmental impact and risk to others.

Setting up an Electrochemical Cell Safely

To carry out electrolysis, you need an electrolytic cell. This involves a power supply, two electrodes (which can be inert like graphite or reactive), and an electrolyte. The electrolyte is typically a molten ionic compound or an ionic compound dissolved in water. Safety is paramount: always wear eye protection (safety goggles) to protect against splashes of corrosive electrolytes or sudden gas release. Ensure the work area is well-ventilated, especially if toxic gases like chlorine are expected to be produced. Handle all chemicals with care and avoid contact with skin. Connect the apparatus correctly to prevent short circuits – the anode connects to the positive terminal of the power supply, and the cathode to the negative terminal. Use appropriate voltages to avoid overheating or excessive gas production. Secure electrodes to prevent them from touching and causing a short circuit, or from falling into the electrolyte during the experiment.

Electrolysis Process: Cathode and Anode Reactions

In an electrochemical cell used for electrolysis, the direct current from the power supply forces non-spontaneous redox reactions to occur. Cations (positive ions) are attracted to the negatively charged cathode. At the cathode, they gain electrons (reduction). For example, if copper ions (Cu²⁺) are present, they will gain two electrons to form copper metal (Cu). Anions (negative ions) are attracted to the positively charged anode. At the anode, they lose electrons (oxidation). For example, if chloride ions (Cl⁻) are present, they will lose an electron each to form chlorine gas (Cl₂). The specific products formed depend on the electrolyte and, if aqueous, the relative ease of discharge of the ions present and water itself.

Qualitative Tests for Electrolysis Products

After conducting electrolysis, it's essential to identify the substances formed at each electrode using specific chemical tests. These are known as qualitative tests. For gases: Hydrogen gas, often produced at the cathode from aqueous solutions, creates a distinctive 'squeaky pop' sound when tested with a lit splint. Oxygen gas, also often from aqueous solutions at the anode, relights a glowing splint. Chlorine gas, frequently produced at the anode from solutions containing chloride ions, is identified by its ability to turn damp blue litmus paper red, and then bleach it white. For solids deposited, such as metals at the cathode, visual inspection can often give an indication (e.g., reddish-brown for copper, silvery for lead). Further chemical tests could confirm the identity of a deposited metal, if required.

Planning, safety and evaluation

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Controlled Reaction for Salt Formation

To prepare a pure salt, a controlled chemical reaction is essential. For example, to make a soluble salt from an insoluble base or metal, add the solid reactant to the acid gradually until no more reacts (the solid remains undissolved even after stirring, or effervescence stops). This ensures the acid is fully consumed, preventing contamination of the final salt with unreacted acid. Alternatively, if using a soluble alkali, a titration is performed to determine the exact volume of alkali needed to neutralise a specific volume of acid, ensuring a precise stoichiometric reaction.

Separation Techniques for Purity

After the reaction, separation techniques are used to remove impurities. If an excess of an insoluble base or metal was used, filtration is necessary to remove any unreacted solid from the salt solution. For reactions between an acid and an alkali (titration), there typically isn't an unreacted solid to filter. The pure salt solution must then be separated from the solvent. This often involves gently heating the solution to evaporate most of the water, concentrating the salt. Crystallisation is then used to obtain the solid salt.

Crystallisation and Drying

Crystallisation is the process of forming solid crystals from a solution. After concentrating the salt solution by evaporating most of the water, the hot, saturated solution is allowed to cool slowly. Slow cooling encourages the formation of larger, purer crystals as impurities are less likely to be incorporated into the crystal lattice. Once crystals have formed, they are separated from the remaining solution (mother liquor) by filtration. The crystals are then washed with a small amount of cold distilled water to remove any surface impurities and dried, often by placing them in a desiccator, on filter paper in a warm oven, or between sheets of filter paper.

Safety Precautions and Variables

Appropriate safety precautions are crucial when preparing salts. This includes wearing eye protection (safety goggles) at all times, handling acids and bases with care as they can be corrosive or irritant, and using a fume cupboard if volatile chemicals are involved. When heating, use a tripod and gauze, and never heat to dryness too quickly, as some salts may decompose or 'spit'. Key variables in this practical include the initial concentrations and volumes of reactants (independent), the mass and purity of the salt produced (dependent), and conditions like temperature or stirring rate (control).

Flame Tests for Metal Ions (Cations)

Flame tests are used to identify particular metal ions (cations) based on the characteristic colour they produce when heated in a Bunsen flame. Method:

1. Clean a platinum or nichrome wire by dipping it in concentrated hydrochloric acid and then heating it in a blue Bunsen flame until no colour is seen.
2. Dip the clean wire into the unknown solid sample.
3. Heat the sample in the hottest part of a blue Bunsen flame.
4. Observe the colour of the flame.

Characteristic colours:

• Lithium (Li⁺): Crimson
• Sodium (Na⁺): Orange-yellow
• Potassium (K⁺): Lilac
• Calcium (Ca²⁺): Orange-red
• Copper (Cu²⁺): Blue-green

Safety: Wear eye protection. Hydrochloric acid is corrosive. Handle the Bunsen burner with care.

Qualitative Tests for Anions (Negative Ions)

Qualitative chemical tests are used to identify specific negative ions (anions) by observing characteristic reactions, often the formation of a precipitate or a gas.

1. Carbonate (CO₃²⁻) test: Add dilute hydrochloric acid to the sample. If carbonates are present, effervescence (fizzing) will occur as carbon dioxide gas is produced. Test the gas by bubbling it through limewater; if carbon dioxide is present, the limewater will turn cloudy.
2. Sulfate (SO₄²⁻) test: Add dilute hydrochloric acid followed by barium chloride solution. If sulfates are present, a white precipitate of barium sulfate will form.
3. Halide (Cl⁻, Br⁻, I⁻) test: Add dilute nitric acid followed by silver nitrate solution.

• Chloride (Cl⁻): White precipitate (silver chloride)
• Bromide (Br⁻): Cream precipitate (silver bromide)
• I⁻: Yellow precipitate (silver iodide)

Safety: Wear eye protection. Acids and silver nitrate solution are corrosive/irritant.

Qualitative Tests for Other Metal Ions (Cations) with Sodium Hydroxide

Some metal ions (cations) can be identified by their characteristic precipitates formed when sodium hydroxide solution is added.

1. Copper (Cu²⁺): Forms a blue precipitate of copper(II) hydroxide. The precipitate does not redissolve in excess sodium hydroxide.
2. Iron(II) (Fe²⁺): Forms a green precipitate of iron(II) hydroxide. The precipitate does not redissolve in excess sodium hydroxide. On standing in air, it may turn brown as it oxidises.
3. Iron(III) (Fe³⁺): Forms a reddish-brown precipitate of iron(III) hydroxide. The precipitate does not redissolve in excess sodium hydroxide.
4. Aluminium (Al³⁺): Forms a white precipitate of aluminium hydroxide. In excess sodium hydroxide, the white precipitate redissolves to form a colourless solution.

Safety: Wear eye protection. Sodium hydroxide is corrosive.

Safety and Best Practice in Qualitative Analysis

When carrying out qualitative tests, safety is paramount. Always wear appropriate eye protection (safety goggles) to protect against chemical splashes and heating hazards. Handle all chemical reagents with care; many acids, alkalis, and silver nitrate are corrosive or irritant. Use small quantities of reagents to minimise waste and risk. When heating samples, ensure the Bunsen burner is stable and away from flammable materials. Always point test tubes away from yourself and others when heating. Dispose of chemical waste according to school or laboratory guidelines, as some substances (e.g., heavy metal salts) are harmful to the environment if not handled correctly. Proper ventilation is also important when producing gases.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.

Practical planning must identify the independent variable that is changed, the dependent variable that is measured and important control variables kept constant. Follow a repeatable method and record units. Complete a risk assessment: identify each hazard, explain the risk and use a suitable safety control. Repeat measurements, calculate a mean, plot an appropriate graph and evaluate anomalies, uncertainty and limitations before suggesting justified improvements.