Chemistry- Water Hardness

water is a source of life and conflict

Hydrated salts

are salts that contain water molecules in their structure. This water is referred to as water of crystallisation. When hydrated salts are heated the water of crystallisation is removed to form an anhydrous salt.

Homogenous mixtures

are mixtures where the parts of the mixture are all in the same phase or state of matter.

Heterogenous mixtures

are mixtures where its parts are in different states of matter.

Production of salt from seawater in Malta

1. salt pans are built next to the sea.

2. they are filled with seawater.

3. the water evaporates due to the sun heating up the rocks.

4. as the water evaporates, crystals of salt form in the salt pans [crystallisation] .

Filtration

used to separate an insoluble solid from a liquid or aqueous solution.

Evaporation to dryness

used to obtain a soluble solid from a liquid solvent.

• the slower the rate of evaporation, the larger the crystals formed because they have more time to form properly.

Crystallisation

used to obtain a soluble solid from the liquid solvent.

• a water bath should be used

• a hydrated salt is required

• the salt required is not thermally stable [ do not decompose on heating]

• large crystals are required.

• remaining solution becomes super saturated

Salt

a chemical substance that is made of metal and nonmetal ions which are chemically combined together and are solid at RTP. The bond that exists between the metal and nonmetal ions is very strong and is called an ionic bond.

Sodium chloride [NaCl] / common salt

• ionic solid.

• salts form giant ionic lattices which are large structures of cations[positive ions] and anions[negative ions] held together by strong electrostatic forces of attraction

Ionic [electrovalent] bonding

• metals can combine chemically with nonmetals to form compounds

• this means that metals lose one or more electrons while the nonmetals gain these electrons

• during the reaction between metals and nonmetals, metals form positively charged metal ions[cations] while the nonmetal atoms form negatively charged ions[anions[

Ions are charged particles since the number of protons is not equal to the number of electrons.

The ionic bond that forms between these ions is an electrostatic force of attraction between oppositely charged ions.

polyatomic ions

a group of covalently chemically combined atoms that is charged and exists in several compounds

Endings

-ide presence of a nonmetal ion #

-ate a polyatomic ion containing oxygen

-ite a polyatomic ion that also has oxygen in it but when there are two polyatomic ions of the same nonmetal that have different number of oxygen atoms, the one that has the lowest amount gets its name ending in -ite and the higher amount of oxygen gets the -ate.

SOLVENT ACTION OF WATER, SOLUTIONS AND SOLUBILITY

The solubility of a solute is the max quantity of solute that can dissolve in a certain quantity of solvent at a a specific temp.

Main factors that influence solubility:

• the nature of the solute and solvent

• temperature

• pressure

Factors determining how fast a substance dissolves in a solvent:

• size of particles

• stirring

• amount of solute already dissolved

• temperature

Hard and soft water

in areas where sedimentary rock is present, the rocks contains limestone will react with naturally slightly acidic rainwater resulting in dissolved calcium hyrogencarbonate[hard water]. Ground water also contains any other soluble salts that may be present in the bedrock.

Water Hardness

Hard water contains dissolved calcium/magnesium ions

Soft water does not have calcium/magne4sium ions dissolved in it

Temporary Hardness

• caused by the presence of dissolved calcium/magnesium hydrogen carbonate

• calcium/magnesium hydrogen carbonate is formed by the reaction of acidified rainwater with limestone/dolomite.

• removed by boiling since calcium/magnesium hydrogen carbonate thermally decompose.

Permanent Hardness

• caused by the presence of dissolved calcium/magnesium ions

• contains gypsum that dissolves calcium/magnesium sulfate.

• cannot be removed by boiling but can be removed by other methods.

Boiling

only temporary hardness can be removed by boiling

Ca(HCO₃)_2(aq) → CaCO_3(s) + H₂O_(l) + CO_2(g)

Distillation

separates the solvent from any dissolved solutes, removes all soluble ions to produce pure water

100^oC

Washing soda

both temporary and permanent hardness can be removed by adding washing soda to hard water

the addition of sodium carbonate would remove such hardness because the carbonate ions from the washing soda react with the calcium/magnesium ions to form a precipitate.

Testing for water hardness

One method to show the difference between hard and soft water is too add soap to it.

Soft water readily forms lather with soap

hard water does not form lather readily but forms scum.

Advantages of hard water

• the calcium salts in the water help form healthy teeth and bones

• calcium carbonate is important for shell and egg formation in many animals

Disadvantages of hard water

• soap wastage increases since all dissolved salts have to be precipitated before soap can function.

• the scum produced is difficult to remove from fabrics.

• formation of limescale in pipes