🎯 Objective
To determine the surface tension of a given liquid (e.g., water, soap solution) by the drop-weight method using a stalagmometer.
📖 Principle / Theory
Surface tension (γ) is the force per unit length acting at the surface of a liquid due to intermolecular cohesive forces. The drop-weight method uses a stalagmometer to count the number of drops of liquid that fall from a capillary tip for a fixed volume. Using water as reference:
γ_liquid / γ_water = (m_liquid × ρ_water) / (m_water × ρ_liquid)
where m is mass per drop (proportional to 1/n, where n = number of drops) and ρ is density.
Surface tension of water at 25°C: 72.8 × 10⁻³ N/m
🧰 Apparatus Required
Stalagmometer, beakers, thermometer, burette, rubber tubing, weighing balance.
🧪 Chemicals Required
Distilled water, given liquid (e.g., ethanol, soap solution), acetone (for cleaning).
⚗️ Procedure
- Clean the stalagmometer thoroughly with acetone and then with distilled water.
- Fill the stalagmometer with distilled water and count the number of drops (n_w) for a fixed volume (usually the volume between the two marks).
- Repeat 3 times for concordant readings.
- Clean again and fill with the given liquid. Count the number of drops (n_l) for the same volume.
- Measure the density of the liquid using a density bottle or pycnometer.
- Calculate surface tension using the formula.
📊 Observations & Calculations
| Observation | Trial 1 | Trial 2 | Trial 3 |
|---|---|---|---|
| No. of drops for distilled water (n_w) | ______ | ______ | ______ |
| No. of drops for test liquid (n_l) | ______ | ______ | ______ |
| Temperature (°C) | ______ | ______ | ______ |
| Density of test liquid, ρ_l (g/mL) | ______ | ||
| Density of water, ρ_w (g/mL) | 1.00 | ||
| Mean n_w / n_l | ______ | ||
γ_liquid = γ_water × (n_water / n_liquid) × (ρ_liquid / ρ_water)
γ_water at 25°C = 72.8 × 10⁻³ N/m
γ_liquid = 72.8 × (______ / ______) × (______ / 1.00) = ______ × 10⁻³ N/m
✅ Result
Surface tension of the given liquid at room temperature = ______ × 10⁻³ N/m (or mN/m).
⚠️ Precautions
- Ensure the stalagmometer is scrupulously clean and free from grease.
- Count drops carefully and ensure the rate of drop formation is slow (1 drop per 2–3 seconds).
- Maintain constant temperature throughout the experiment.
- Avoid parallax error when reading burette/stalagmometer marks.
❓ Viva-Voce Questions
📚 References
- Atkins, P.W. – Physical Chemistry
- Castellan, G. W. – Physical Chemistry, 3rd Ed.