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Chemistry C3 > Hard water > Flashcards

Hard water Flashcards

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1
Q

Hard water makes what?

A

Scum and scale

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2
Q

What do you get with soft water but not hard, what do you get with hard?
Why do you not get with a lather with hard water?
So what needs to be done to get a decent lather, but…?

A

A nice lather with soap, scum.
The dissolved calcium and magnesium ions in the water react with the soap to form insoluble scum.
Use more soap, this is more expensive

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3
Q

What happens when hard water is heated and where?

A

It forms scale (calcium carbonate) on the inside of pipes, boilers and kettles.

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4
Q

What happens when pipes get badly scaled? why is this bad? What would happen eventually?

A

They reduce the efficiency of heating systems and may need replacing. The will block completely.

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5
Q

What property of scale reduces the efficiency of a kettle?

What does this mean for the kettle?

A

It’s a thermal insulator.

It takes longer for the heating element to biol the water than a clean kettle so it’s less efficient

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6
Q

What property of scale reduces the efficiency of a kettle?

What does this mean for the kettle?

A

It’s a thermal insulator.

It takes longer for the heating element to biol the water than a clean kettle so it’s less efficient

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7
Q

What is hardness caused by?

A

Ca2+ and Mg2+ ions

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8
Q

How does water become hard?

A

Rain falling on some types of rocks (limestone chalk, gypsum) can dissolve compounds like magnesium sulphate and calcium sulphate, both only a bit soluble.

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9
Q

Why isn’t hard water bad for health? (2)

A

Ca2+ ions are good for healthy teeth and bones
It has been found that people living in hard water areas are at less risk of developing heart disease than people in soft water areas. This could be due to the minerals in the water.

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10
Q

What needs removed to make hard water soft?

A

The dissolved Ca2+ and Mg2+ ions

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11
Q

What are the two types of hardness?

A

Temporary and permanent

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12
Q

What is temporary hardness caused by?

A

Hydrogencarbonate ion, HCO₃− in Ca(HCO₃)₂

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13
Q

What is permanent hardness caused by?

A

Dissolved calcium sulphate (and others)

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14
Q

How is temporary hardness removed?

A

By boiling water

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15
Q

What happens when temporary hard water is heated?

A

The calcium hydrogencarbonate decomposes to form Calcium carbonate which is insoluble, water and CO2. The solid is the limescale in the kettle.

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16
Q

What is the equation for the removal of temporary hardness? Word and symbol

A

Calcium hydrogen carbonate = Calcium carbonate + water + CO2

Ca(HCO₃)₂ (aq) = CaCO3 (s) + H2O (l) + CO2 (g)

17
Q

Why doesn’t the removal of temporary hardness work for permanent hardness?

A

Heating sulphate ions does nothing, they don’t decompose.

18
Q

How are both types of hardness softened?

How (why) does this soften the water?

A

By adding washing soda which is Na2CO3, sodium carbonate.
The added carbonate ions react with Mg2+ and Ca2+ ions to make an insoluble precipitate of calcium carbonate or magnesium carbonate. As the ions are no longer dissolved in the water it is not hard.

19
Q

How are both types of hardness removed?

A

By running the water through ion exchange columns

20
Q

What do ion exchange columns contain?

What do they do with these?

A

Lots of sodium ions or hydrogen ions

Exchange them for the calcium or magnesium ions in the water running through them.

21
Q

What is the equation for what happens in an ion exchange column?

A

Na2Resin (s) + Ca(2+) (aq) = CaResin (s) + 2Na+ (aq)

So the calcium ions dissolved in the water (aq) have been replaced with sodium ions, 2Na+ (aq)

22
Q

what do you use a titration reaction for?

A

Comparing the hardness of water.

23
Q

What is the method for a titration reaction? (8)

A

Fill a burette with 50 cm3 of soap solution
Add 50 cm3 of the first water sample into a flask
Use the burette to add 1cm3 of soap solution to the flask
Put a bun gin the flask and shake for 10 seconds
Repeat the previous 2 steps until a GOOD LASTING LATHER is formed, this is where bubbles cover the surface for at least 30 seconds
Record how much soap was needed to create a lasting lather
Repeat the steps with other water samples
Boil fresh samples of each type of water for ten minutes then repeat the experiment to see if it is permanent or temporary.

24
Q

If the volume of soap needed went down from 7 to 1 after boiling, what does that tell you about its hardness?
If it went down from 17 to 7 what would it mean?

A

It’s temporary

It contains half temporary hardness and half permanent

25
Q

What does water contain that we must remove? Why is one of them removed, what does it cause?

A 
Poisonous salts (phosphates and nitrates)
Harmful microbes - cause diseases such as cholera and dysentery.
26
Q

Where does most of our water come from?
Where does this get its water from?
What do government agencies do?

A

Reservoirs - companies build these were there is a good
supply of clean water
Water flows in from rivers and ground water
Keep an eye on the pollution of reservoirs, rivers and ground water.

27
Q

Where does reservoir water go for treatment?

A

The treatment works

28
Q

What are the four stages of water treatment?

A
  1. The water passes through a mesh screen to remove big bits like twigs
  2. Chemicals are added to the water to make the microbes stick together and fall to the bottom
  3. The water is filtered through gravel beds to remove all the solids
  4. Water is chlorinated to kill and harmful microbes left
29
Q

What do people who aren’t satisfied with water treatment buy?
What do these do?
What else do we buy to treat our water?

A

Filters containing carbon or silver to remove substances from their tap water.
Carbon in the filters removes the chlorine taste and silver is supposed to kill bugs
Water softeners which contain the ions exchange columns with ions exchange resins.

30
Q

How can totally pure uncontaminated water be produced? … which is
Why is this process not used for tap water?
But where is this water used anyway?

A

Distillation - the boiling of water to make steam and then condensing the steam.
It’s too expensive as it requires a lot of energy to boil the water.
In chemistry labs.

31
Q

Why is fluoride added to drinking water in some parts of the country?
Why is chlorine added?

A

It helps to reduce tooth decay

To prevent disease

32
Q

Yet why has the use of chlorine in water been disputed?

A

It can react with other natural substances in water to produce toxic or poisonous by-products
No consumer choice on whether it should be added

33
Q

What is the problem with fluoride in water?

What is the other concern?

A

In too higher doses it can cause cancer and bone problems so some people think it shouldn’t be added.
Whether it’s right to mass medicate people if they can just chose fluoride tooth paste, but they can’t chose if it’s in their tap water

34
Q

What needs to be monitored then?

A

The levels of chemicals added to drinking water as too much fluoride (for example) could be harmful.

35
Q

Explain how scale forms on heating elements: (2 marks)

A

When heated calcium hydrogencarbonate decomposes

forming a scale of insoluble calcium carbonate

Chemistry C3 (7 decks)

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