Desalination & Required Practical 8 - Water Purification

What is desalination?

The removal of dissolved salts from seawater or other salty water.

Why is desalination useful?

It produces fresh water from seawater, especially where natural fresh water supplies are limited.

Name two methods of desalination.

Distillation and reverse osmosis.

How does distillation produce fresh water from seawater?

Seawater is heated so that water evaporates. -- Dissolved salts remain behind. -- Water vapour is cooled and condensed to form liquid water.

Why do salts remain behind during distillation?

They are not volatile under the conditions used, so they do not evaporate with the water.

What is reverse osmosis?

A process in which water is forced through a partially permeable membrane under high pressure, leaving most dissolved salts behind.

Why is high pressure required for reverse osmosis?

To force water through the membrane against the natural direction of osmosis.

Why is desalination expensive?

It requires large amounts of energy, especially for heating water or producing high pressure.

Why might desalination be used despite its high cost?

It provides potable water in regions with limited fresh water supplies.

What is Required Practical 8 (Water Purification)?

Analysis and purification of water samples from different sources, including testing pH, dissolved solids and purification by distillation.

How can the pH of a water sample be measured (RP - Water Purification)?

Using universal indicator or a pH probe.

Why is a pH probe often preferable to universal indicator (RP - Water Purification)?

It provides a numerical pH reading, which can be more precise than estimating a colour.

How can the amount of dissolved solids in water be estimated (RP - Water Purification)?

Measure a known volume of water. -- Evaporate the water. -- Measure the mass of solid residue remaining.

Why is the mass of residue measured after evaporation (RP - Water Purification)?

The residue represents the dissolved solids originally present in the water sample.

Why should equal volumes of water be used when comparing samples (RP - Water Purification)?

To allow a fair comparison of dissolved solids.

How can a water sample be purified by distillation (RP - Water Purification)?

Heat the water to evaporate it. -- Pass the vapour through a condenser. -- Collect the condensed liquid water. -- Dissolved salts remain in the original container.

Why is the condenser used (RP - Water Purification)?

To cool water vapour so that it condenses into liquid water.

Why does distillation separate water from dissolved salts (RP - Water Purification)?

Water evaporates and condenses, while dissolved salts remain behind.

Why might distillation be preferred over filtration for salty water (RP - Water Purification)?

Filtration cannot remove dissolved salts, but distillation can.

How could you compare the purity of two water samples (RP - Water Purification)?

Compare their dissolved solid residue after evaporating equal volumes.

Why should a sample be allowed to cool before weighing the residue (RP - Water Purification)?

To obtain a more reliable mass measurement and avoid weighing hot apparatus.

Why might the residue be heated, cooled and reweighed (RP - Water Purification)?

To check that a constant mass has been reached, indicating that the water has been removed.

What is a potential limitation of distillation (RP - Water Purification)?

It requires energy for heating and cooling, making it relatively expensive.