Hardness of Water
What causes *temporary* hardness?
*Ca(HCO₃)₂* calcium hydrogencarbonate also Mg(HCO₃)₂
Show the formation of *temporary* hardness in a single balanced symbol equation (including state symbols.
*CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)*
What is *limescale* (scale) mostly?
*Calcium carbonate*
What solid product is formed when *temporary* hard water is heated?
*Calcium carbonate* (insoluble limescale)
What is another name for *scum*?
*Calcium sterate*
The table below shows the number of cm³ of soap solution required to produce a lather with different water samples before and after boiling. *Which sample has no permanent hardness?* Sample -- Before -- After (boiling) Distilled ----- 1 ----- 1 Town A ------ 4 ------ 1 Town B ------ 6 ------ 6 Town C ------ 5 ------ 3
*Town A.* Boiling removed all of the hardness. Clearly the deionised water doesn't have any kind of hardness.
The table below shows the number of cm³ of soap solution required to produce a lather with different water samples before and after boiling. *Which sample has no temporary hardness?* Sample -- Before -- After (boiling) Distilled ----- 1 ----- 1 Town A ------ 4 ------ 1 Town B ------ 6 ------ 6 Town C ------ 5 ------ 3
*Town B* This sample has hardness, but boiling made no difference. Clearly the deionised water doesn't have any kind of hardness.
The table below shows the number of cm³ of soap solution required to produce a lather with different water samples before and after boiling. Which sample has both temporary and permanent hardness? Sample -- Before -- After (boiling) Distilled ----- 1 ----- 1 Town A ------ 4 ------ 1 Town B ------ 6 ------ 6 Town C ------ 5 ------ 3
*Town C.* Boiling removed some of the hardness; not all of the hardness.
What is *sodium sterate*?
*soap*
How can *BOTH* temporary and permanent hard water be softened?
1. By adding sodium carbonate (called *precipitation*). 2. Ionic exchange.
Write a balanced symbol equation to explain how *washing soda* removes hardness from water. Use *CaCl₂*(aq) as an example of hardness. Washing soda crystals comprise of hydrated sodium carbonate crystals. *Na₂CO₃∙10H₂O(s)*
CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaCl(aq) This is a '*precipitation reaction*'. By converting the calcium ions in solution into insoluble calcium carbonate they removed from the water. Although they have been replaced by sodium ion, they do not cause hardness.
How is *scum* formed?
Calcium ions react with the sodium sterate (soap) to produce insoluble calcium sterate (scum)
What causes hardness?
Dissolved *calcium* and *magnesium* salts. Mostly the chlorides, sulfates and hydrogencarbonates of these metals.
Why does *scum* stop the lathering of soap?
Have to react all the calcium and magnesium ions with the water before soap can lather.
How do you soften *temporary* hard water?
Heat or boil to remove calcium ions through thermal decomposition
Explain how *temporary* hardness forms in water.
In 2 stages: Rain water dissolves some CO₂ gas present in the air. This forms a very, very weak solution of carbonic acid. *H₂O(l) + CO₂(g) → H₂CO₃(aq)* While limestone (CaCO₃) doesn't dissolve in pure water, it reacts with the carbonic acid in the water to form a soluble salt called calcium hydrogencarbonate; the cause of temporary hardness. *CaCO₃(s) + H₂CO₃(aq) → Ca(HCO₃)₂(aq)*
With the help of a balanced symbol equation, explain how *limescale* is formed.
Limescale (CaCO₃) is formed from the thermal decomposition of temporary hardness [Ca(HCO₃)₂] in the water. The following happen when the water is boiled: Ca(HCO₃)₂(aq) → CaCO₃(s) + CO₂(g) + H₂O(l) The *insoluble CaCO₃* produces the scale.
What causes *permanent* hardness?
MgCl₂, CaCl₂, MgSO₄ and CaSO₄
Explain how *permanent* hardness forms in water.
Minerals in the soil, composed of MgCl₂, CaCl₂, MgSO₄ and CaSO₄ simply dissolve (sparingly) in the water.
Can you soften *permanent* hard water by heating/boiling?
NO
What does *scale* do to things? E.g. a pipe.
Scale is an insoluble solid and is a poor conductor of heat. It reduces the efficiency of radiators and means that the pipe carries a lower volume of water. Some electrical appliance costs will rise as a result.
How can you identify the *difference* between hard and soft water?
Soft water produces a *lather*, hard water produces *scum* or scale.
Name two benefits of hard water?
Supplies calcium *minerals* for bone and teeth development and reduce heart disease. Tastes better and better for brewing.
What happens in an *Ion exchange column* to the hard water?
The calcium/ magnesium ions *swap* with sodium ions in the resin to soften the water. Although the calcium and magnesium ions have been replaced by sodium ion, they do not cause hardness.
What can be done to avoid a *scum* forming with hard water? Scum can damage special fibres in some fabrics.
Use a *soapless detergent*. Unlike soap-based cleansers, the chemical composition of a soapless detergent prevents the formation of soap scum when combined with hard water.
Name 2 *disadvantages* of hard water.
Wastes soap. Forms limescale. Produces scum.
Define what is meant by *'hardness'* in reference to water.
Water that does not lather readily with soap.
Give the *word equation* for the thermal decomposition of temporary hard water?
calcium hydrogencarbonate produces calcium carbonate + water + carbon dioxide
