Precipitation

Question 0

When does a precipitation reaction occur?

Answer 0

When two or more solutions mix to form precipitate or solids in the solution. One can use the solubility table to predict which substances are insoluble.

Question 1

What is precipitate the opposite of?

Answer 1

It’s the opposite of a substance becoming soluble.

Question 2

What are ionic equations used to represent?

Answer 2

They are used to represent the ‘actual’ chemicals that react within the substance.
The point of these equations is to identify the ‘what’ in the reactant whist eliminating those that do no react - merely switch partners

Question 3

What reactions play an important role in the treatment of water?

Answer 3

Precipitation

Question 4

In Australia we treat water using the following steps:

Answer 4

• Flocculation
• Settling of the ‘Floc’
• Filtering
• Chlorination

Question 5

What is flocculation?

Answer 5

Flocculation is the process by which particles suspended in the water join together to form larger, combined particles. The heavier particles then sink under their own weight and settle in the water sample.

Question 6

What can flocculation be achieved by?

Answer 6

By adding Alum (Aluminium sulfate) and if necessary, lime.

Question 7

What is Floc and what does it do?

Answer 7

Aluminum hydroxide is produced in the form of a gelatinous (jelly like) precipitate called the “Floc.” This traps other fine particles and removes color and some micro organisms from the water.

Question 8

What are the two key equations in flocculation?

Answer 8

Ca(OH)2 (s) —(H2O)—-> Ca2+ (aq) + 2OH - (aq)

Al3+ (aq) + 3OH- (aq) —-> Al(OH)3 (s)

Question 9

Settling

Answer 9

The water is left for a period of time to allow the Floc to settle to the bottom and a sludge forms. The sludge accumulates at the bottom of the settling tank and is removed. The rest of the water passes onto the filtering stage.

Question 10

Filtering stage

Answer 10

Water from the settling stage is now passed through the filter made up of sand over gravel. This removes any remaining suspended matter.

Question 11

Chlorination

Answer 11

The clear water is treated with chlorine gas. The main purpose of chlorination is to remove any biological contaminants. It is the hypochlorous acid (HOCL) that kills the bacteria.

Question 12

What is the hardness in water caused by?

Answer 12

Hardness in water is caused by the presence of some metal ions, mainly calcium, magnesium and iron.

Question 13

What’s desalination?

Answer 13

Involves the removal of salts from seawater to obtain fresh water.
OR obtaining fresh water from salt water

Question 14

What are the types of desalination?

Answer 14

• Distillation
• Reverse Osmosis
• Ion exchange

Question 15

Distillation

Answer 15

Is an effective but uses alot of energy to remove water from salts

• is expensive and generally not used to desalinate large quantities of water
• salt solution (eg. NaCl) is boiled, water vapour passes into condenser, then water vapour is cooled and liquified and recovered.
• the water and salt are separated from one another

Question 16

Reverse osmosis

Answer 16

• requires high energy to push water from a high concentration gradient to a low concentration gradient through FILTERS
• most commonly used desalination method
• requires no heat energy

Question 17

Ion exchange

Answer 17

• provides high quality drinking water but expensive to run
• the dissolved Na+ ions are captured by a resin and exchanged for H+ ions. The dissolved Cl- ions are then exchanged for OH-. These replacement ions then combine to form water.