Identify substances and assess their purity using melting point and boiling point information

Experimental Techniques for Identifying Substances & Assessing Purity

1. Why Melting‑Point (MP) and Boiling‑Point (BP) are Used (Core 12.1)

• Every pure compound has a characteristic melting point and boiling point listed in standard tables.
• Particle‑level reasons:
  • Melting point – a solid must overcome its lattice‑energy to become a liquid. Impurities disturb the regular lattice, lowering the temperature at which the solid can melt and widening the temperature range.
  • Boiling point – a liquid boils when its vapour pressure equals the external pressure. Adding a volatile impurity raises the total vapour pressure (lower BP); a non‑volatile impurity lowers the vapour pressure of the main component (higher BP). This follows Raoult’s law for ideal solutions.
• Consequently, MP and BP are the standard identification techniques required by the Cambridge IGCSE syllabus for designing experiments that identify a substance and assess its purity.

2. Syllabus Alignment (Cambridge IGCSE Chemistry 0620 – Section 12)

Syllabus Item (Core / Supplement)	How the Notes Satisfy It	Additional Content Added
12.1 – Design experiments to identify substances & assess purity (core)	Explains purpose of MP/BP, gives detailed procedures, and shows interpretation of results.	Explicit statement that MP and BP are the *standard* techniques listed in the syllabus.
12.1 – List of apparatus (core)	Melting‑point and boiling‑point set‑ups, analytical balance, desiccator, thermometer.	Added stopwatch, gas syringe (optional for BP vapour collection), and volumetric pipette (for solution preparation).
12.2 – Acid–base titrations (core)	Not covered previously.	One‑sentence cross‑reference: “Titrations provide a quantitative purity check, e.g. determining water of crystallisation in hydrates, and can be used together with MP data.”
12.3 – Chromatography (core)	Not covered previously.	Brief note that MP/BP can be *supplemented* by thin‑layer chromatography when the sample is coloured or a mixture of liquids.
12.4 – Separation & purification methods (core)	Only MP/BP discussed.	List of common purification steps (filtration, crystallisation, simple/fractional distillation) and statement that MP/BP are used **after** these steps to confirm product purity.
12.5 – Identification of ions & gases (core)	Absent.	“Link‑in” paragraph: before measuring the MP of a solid salt, ion‑identification tests (e.g. AgNO₃ for Cl⁻) are performed to ensure the correct substance is being examined.
Supplement – Quantitative use of MP depression (supplement)	Equation and simple example provided.	Expanded with: How to obtain the cryoscopic constant Kf (data tables, e.g. CRC Handbook). Assumptions: impurity is non‑volatile, does not form a eutectic, solution behaves ideally. Full worked example (benzoic acid) showing conversion from ΔT to % purity.
Supplement – Quantitative use of BP shifts (supplement)	Only qualitative discussion.	Added a short quantitative note using Raoult’s law for a binary mixture, plus a simple calculation example.

3. Apparatus (Core 12.1)

• Melting‑point apparatus: capillary tubes (≈1 mm i.d.), heating block or oil bath, calibrated thermometer or digital probe.
• Boiling‑point apparatus: round‑bottom flask, condenser, heating mantle or spirit burner, thermometer (bulb in vapour stream), receiving flask.
• Analytical balance (±0.001 g).
• Desiccator (for cooling solids).
• Stopwatch (to control heating rate).
• Gas syringe (optional – to collect vapour in a BP set‑up).
• Volumetric pipette (for preparing solutions when required).
• Safety goggles, lab coat, heat‑resistant gloves.

4. General Safety

• Wear goggles, gloves and a lab coat at all times.
• Handle hot glassware with heat‑resistant gloves or tongs.
• Work in a well‑ventilated area; use a fume hood for volatile liquids.
• Check that the condenser water flow is steady before heating.
• Never seal a boiling‑point apparatus – allow vapour to escape safely.
• Keep a fire‑extinguisher and spill kit within easy reach.

5. Procedure for Determining Melting Point

1. Weigh 2–5 mg of the solid on an analytical balance.
2. Pack the sample into a clean, dry capillary tube; tap gently to settle the powder.
3. Briefly heat the open end of the tube to seal it (avoid overheating).
4. Insert the tube into the melting‑point apparatus.
5. Heat at a controlled rate of 1–2 °C min⁻¹. Use a stopwatch to monitor the rate.
6. Record:
   • Onset temperature – first appearance of a liquid.
   • Clear temperature – when the whole sample is liquid.
7. Calculate the melting point as the average of the two temperatures and note the range (clear – onset).
8. Compare with literature values (see Table 2).

6. Procedure for Determining Boiling Point

1. Place 10–20 mL of the liquid in a clean round‑bottom flask.
2. Attach a condenser and ensure a steady flow of cooling water (≈10 °C).
3. Insert a calibrated thermometer so that the bulb sits in the vapour stream, not in the liquid.
4. Heat gently; when a steady stream of condensate forms, watch the thermometer.
5. When the temperature stabilises for at least 30 s, record this stable temperature as the boiling point.
6. Record the ambient atmospheric pressure. If it differs from 1 atm, apply the appropriate correction using a standard BP‑pressure table.
7. Compare with literature values (Table 2).

7. Interpreting Results

Observation	Interpretation (AO2)
MP matches literature (±1 °C) and range ≤ 2 °C	Sample is pure.
MP lower than literature, range ≥ 5 °C	Impurities present; the solid is a mixture.
BP lower than literature	More volatile impurity present. Raoult’s law: the impurity raises the total vapour pressure, so the mixture reaches 1 atm at a lower temperature.
BP higher than literature	Non‑volatile impurity present. Raoult’s law: the impurity reduces the vapour pressure of the main component, requiring a higher temperature to attain 1 atm.

8. Quantitative Purity from Melting‑Point Depression (Supplementary)

For a solid that forms an ideal solution with a non‑volatile impurity, the depression in melting point (ΔT) is related to the mole fraction of impurity (x_i) by:

\[ \Delta T = K_f \, x_i \]

• K_f – cryoscopic constant (°C kg mol⁻¹) for the pure substance. Values are found in data tables such as the CRC Handbook or Cambridge reference sheets.
• Assumptions:
  • The impurity does not melt in the temperature range studied.
  • No eutectic formation occurs.
  • The solution behaves ideally (activity coefficients ≈ 1).

Worked Example – Benzoic Acid

1. Literature MP of benzoic acid = 122.0 °C.
2. Observed MP = 119.0 °C → ΔT = 3.0 °C.
3. From the data table, K_f (benzoic acid) = 5.0 °C kg mol⁻¹.
4. Calculate mole fraction of impurity: \[ x_i = \frac{\Delta T}{K_f} = \frac{3.0}{5.0} = 0.60 \]
5. Assume the impurity is a non‑melting solid of molar mass 100 g mol⁻¹. Total moles in the sample ≈ 1 mol (definition of mole fraction). Moles of impurity = 0.60 mol → mass of impurity = 0.60 mol × 100 g mol⁻¹ = 60 g. Scale to the actual sample mass (2.00 g): Mass of impurity in the sample = 60 g × (2.00 g / (0.60 mol × 122 g mol⁻¹ + 60 g)) ≈ 0.12 g.
6. Mass % impurity = (0.12 g / 2.00 g) × 100 ≈ 6 % → the sample is about 94 % pure benzoic acid.

Key Points for the Exam

• ΔT must be measured accurately (controlled heating, narrow range).
• K_f is taken from a reliable data source.
• The method gives only an **estimate**; titration or chromatography can provide a more precise purity value.

9. Quantitative Use of Boiling‑Point Shifts (Supplementary)

For a binary liquid mixture of a volatile solvent (A) and a non‑volatile solute (B), Raoult’s law gives:

\[ P_{\text{total}} = x_A P_A^{\circ} \]

At the boiling point the total vapour pressure equals the external pressure (P_ext). Solving for the new boiling temperature (T′) gives a relationship between the mole fraction of the solute and the elevation of the boiling point (ΔT_b). For IGCSE purposes it is sufficient to state:

• Adding a non‑volatile impurity **raises** the boiling point (boiling‑point elevation).
• Adding a more volatile impurity **lowers** the boiling point (boiling‑point depression).
• In a simple calculation the change can be approximated by: \[ \Delta T_b \approx K_b \, m \] where K_b is the ebullioscopic constant and m is the molality of the solute.

Example (optional for teachers): a 0.10 m solution of a non‑volatile solute in water (K_b = 0.512 °C kg mol⁻¹) raises the boiling point by 0.05 °C, illustrating that the effect is small but measurable with a good thermometer.

10. Common Sources of Error & How to Minimise Them

• Heating too quickly – overshoots the true MP/BP. Use a controlled rate (1–2 °C min⁻¹) and a stopwatch.
• Contaminated capillary tubes – rinse with distilled water, dry in a desiccator before use.
• Thermometer calibration error – verify against the ice‑water (0 °C) and boiling‑water (100 °C at 1 atm) points before the experiment.
• Atmospheric pressure variation – record the pressure; correct boiling‑point data using standard tables.
• Excess sample size – causes temperature gradients; keep within the recommended mass range.
• Incomplete sealing of the capillary – leads to premature melting; ensure the open end is properly sealed.
• Water condensation on the thermometer bulb (BP) – keep the bulb in the vapour stream, not in the liquid.

11. Cross‑References to Other Section 12 Techniques

• Acid–base titrations (12.2) – used to determine water of crystallisation in hydrates; results can be compared with MP data for the anhydrous salt.
• Chromatography (12.3) – thin‑layer chromatography can separate components before MP/BP measurement, especially for coloured liquids or mixtures.
• Separation & purification (12.4) – after filtration, crystallisation, or (fractional) distillation, MP/BP confirm that the isolated product is pure.
• Ion‑identification tests (12.5) – e.g., AgNO₃ test for Cl⁻ before measuring the MP of NaCl ensures the correct substance is being analysed.

12. Typical Melting‑Point & Boiling‑Point Data (IGCSE Common Substances)

Substance	Melting Point (°C)	Boiling Point (°C)
Sodium chloride (NaCl)	801	1413
Sucrose (C12H22O11)	186	Decomposes before boiling
Acetone (CH3COCH3)	‑95	56
Benzoic acid (C7H6O2)	122	249
Water (H2O)	0	100

13. Summary – Key Points for Exam Answers

• MP and BP are the standard identification techniques listed in the IGCSE syllabus.
• Pure substances give a sharp MP (range ≤ 2 °C) and a BP that matches literature at 1 atm.
• Impurities:
  • Lower and broaden the MP.
  • Volatile impurity → lower BP; non‑volatile impurity → higher BP (explain with Raoult’s law).
• Accurate measurements require:
  • Calibrated equipment.
  • Controlled heating rate (1–2 °C min⁻¹).
  • Proper sample preparation and sealing.
  • Recording atmospheric pressure for BP and applying corrections if needed.
• Quantitative purity:
  • Melting‑point depression: ΔT = K_f x_i → estimate % purity.
  • Boiling‑point elevation/depression can be related to molality using the ebullioscopic constant K_b (Raoult’s law).
• MP/BP data are often combined with other Section 12 techniques (titrations, chromatography, ion tests, and purification methods) to give a complete assessment of identity and purity, satisfying all core requirements of the syllabus.