Evaporating Basin

Description

An evaporating basin is a wide, shallow dish typically made of heat-resistant porcelain (white) or borosilicate glass. Its shape maximises the surface area of the liquid exposed to the air, promoting rapid evaporation.

Key features:

• Wide, shallow shape for maximum surface area
• Heat-resistant material (porcelain withstands direct flame heating)
• Available in various sizes (50 cm3 to 250 cm3 capacity)
• Pouring lip on some designs

Uses in IGCSE Chemistry

Evaporation to dryness

Used to obtain a solid salt from a solution by evaporating all the water:

1. Pour the solution into the evaporating basin.
2. Place on a tripod and gauze over a Bunsen burner.
3. Heat gently until the liquid has fully evaporated, leaving the dry solid behind.
4. Suitable only for thermally stable, non-hydrated salts (e.g., NaCl).

Crystallisation

Used to obtain hydrated crystals from a solution:

1. Pour the solution into the evaporating basin.
2. Heat gently (ideally over a water bath) to evaporate some of the solvent.
3. Test for saturation: dip a glass rod into the hot solution; if crystals form on the rod as it cools, the solution is saturated.
4. Remove from heat and allow to cool slowly. Crystals form as the temperature drops.
5. Filter the crystals, wash with a small amount of cold distilled water, and dry between filter papers or in a warm oven.

Use with a water bath

For temperature-sensitive substances, place the evaporating basin on top of a beaker of boiling water (the water bath). This provides indirect heating limited to 100 degrees C, preventing decomposition of hydrated salts.

Common errors

Heating too strongly: Rapid boiling causes spattering (liquid spitting out), which loses product and is a safety hazard. Heat gently.

Evaporating a hydrated salt to dryness: Heating CuSO4.5H2O too strongly converts it from blue hydrated crystals to white anhydrous powder. Use the crystallisation method instead.

Not testing for saturation: Without the glass rod test, you either evaporate too much (small, impure crystals) or too little (the solution does not crystallise properly).

Leaving crystals in a hot basin: The residual heat can decompose the crystals. Remove from the heat source once crystals start to appear and allow cooling.