Background

Covalent compounds are molecular and in the liquid state the molecules are held together by forces of intermolecular attraction. These forces may be:

• • London dispersion forces (all particles)
• • Permanent dipole-dipole interactions (polar molecules only)
• • Hydrogen bonding (molecules that have hydrogen atoms attached to nitrogen or oxygen atoms)

The students will investigate the following properties of a selection of covalent liquids.

• 1. Solubility in water and non-polar solvents
• 2. Density
• 3. Boiling point

Note: The boiling point of a liquid is the temperature at which the vapor pressure of the liquid is equal to atmospheric pressure.

The ambient pressure can be found using the Phyphox app on mobile phones

---

^ top

Requirements

Chemicals

• methanol
• ethanol
• propanone
• hexane

Apparatus

• electronic balance
• burette, 50ml
• dropping pipette
• beaker, 100ml
• analogue thermometer
• capillary tubes
• rubber ring
• beaker, 400ml
• test-tube
• Small Bunsen burner, tripod and gauze
• Organic residues disposal bottle

---

^ top

Procedure

Density determination

• A known volume of the covalent liquid is delivered from a burette into a dry pre-weighed beaker.
• The mass of the liquid + dry beaker is then recorded.
• The covalent liquid can then be returned to the appropriate burette.

Solubility in water and a non-polar solvent

• A few drops of the covalent liquid is added to about 5 ml of water in a test tube and shaken gently
• A few drops of the covalent liquid is added to about 5 ml of methylbenzene in a test tube and shaken gently (to be disposed of in the organic residues container)

Boiling point determination (Siwolobov)

The boiling point will be determined using Siwolobov’s method

• A 10ml sample of the covalent liquid is poured into a clean, dry test-tube – the volume is not critical as the vapour pressure (and hence, the boiling point at this atmospheric pressure) is independent of everything apart from temperature.
• The capillary tube can be sealed at one end by rotating carefully in a Bunsen burner flame. Allow it to cool before continuing.
• Ensure that the rubber fixing ring is not beneath the surface of the covalent liquid.
• The water bath is pre-heated and then the test tube with sample is lowered into the hot water. It remains there until a steady stream of bubbles is observed coming out of the capillary tube. The test-tube is then removed from the water bath until the steam of bubbles stops. The temperature is recorded at this point.
• This procedure is then repeated several times until the temperature reading is steady. This is the boiling point.
• Do not forget to record the ambient pressure.

---

^ top

Safety

• The organic liquids are flammable and their vapours are toxic.
• Care must be taken to not inhale the vapours.
• The liquids must not be in the vicinity of a naked flame.
• Residues must be collected in an organic residues bottle, for subsequent disposal by the lab technicians.

---

^ top