Crystal size and cooling rate: fast and slow cooling of lead iodide

Igneous rock is rock that forms when molten magma or lava cools. It often contains crystals. The size of these crystals is related to the rate of cooling of the molten rock.  This activity actually investigates the formation of crystals from a saturated solution rather from a melt, although the principles are the same.

What you will need

Apparatus

• eye protection
• 2 boiling tubes
• boiling-tube rack
• Bunsen burner
• heatproof mat
• boiling-tube holder
• spatula
• thermometer (0-100 °C)

Chemicals

• a spatula measure of lead iodide (harmful by ingestion and inhalation of dust)
• samples of granite and of rhyolite (or photographs if samples are not available)

Safety notes

• Wear eye protection.
• Lead iodide is harmful.

What to do

Look at samples (or photographs, if samples are not available) of the following two types of rock – granite and rhyolite. Both these types of rock are igneous, that is they formed from molten rock as it cooled down. What do you notice about the sizes of the crystals in these two types of rock?

Half fill a boiling-tube with water. Add a small spatula measure of lead iodide. Heat over a Bunsen flame until the mixture starts to boil, taking care, as the mixture can ‘bump’ very easily, spraying hot liquid out of the tube.  Continue to boil for a further minute, then quickly tip half of the contents into another clean boiling tube. Cool this second tube and contents immediately under a stream of cold water from the tap.  Place the original tube in a rack to cool down slowly.

Leave both boiling tubes and contents for about 15 minutes until both tubes are at about the same temperature. Look carefully the contents.