Topic 2 - Water Flashcards
Describe the two chemical tests for water, and the observations for a positive test
Cobalt chloride paper, turns from blue to pale pink in the presence of water
Anhydrous Copper Sulfate, White powder to blue powder in presence of water, temperature increase
Describe the effects of detergents on both hard and soft water
Soapless detergents will lather well with hard and soft water
Describe the effect of soap on both hard and soft water
Soft water readily forms a lather with soap. Hard water is water that does not lather easily with soap and therefore requires more soap to form a lather. Hard water reacts with soap to form scum.
How can temporary hardness be removed in water?
Boiling, thermal decomposition
Ion exchange resin
Washing soda crystals (hydrated sodium carbonate)
How can permanent hardness be removed in water?
Ion exchange resin
Washing soda crystals (hydrated sodium carbonate)
How does temporary hardness arise in water?
Water vapour in the air reacts with carbon dioxide in the air to form carbonic acid (pH 5.5). When this weak acid falls, it reacts with the Calcium Carbonate in the limestone rock to form a soluble Calcium Hydrogen Carbonate.
Give an equation to show how temporary hardness arises in water
(H2O + CO2) + CaCO3 = Ca(HCO3)2
What causes permanent hardness in water?
Calcium Sulfate
Magnesium Sulfate
Calcium chloride
Magnesium chloride
Ca2+
Mg2+
What are the disadvantages of hard water?
Hard water produces a scum with soap, which wastes soap.
Causes fur or limescale on hot water pipes, kettles and in hot water tanks; blocks fine pipes in dish washers and the tiny holes in shower heads (temporary hard water.
Dishwasher salt is needed to soften water in a dishwasher, which adds to the cost.
What are the advantages of hard water?
- Hard water tastes better.
- It is better for brewing beer.
- It is good for tanning lather.
- It provides calcium ions (Ca2+) for healthy teeth and bones.
What is a deliquescent?
A deliquescent substance is one that absorbs moisture from the air - eventually absorbing enough to dissolve in it, forming a solution.
What is a desiccant?
Desiccants are drying agents.
A desiccant is a chemical that is hydroscopic (absorbs moisture from the air).
Name two desiccants (drying agents).
Silica gel
Anhydrous Calcium chloride
What are the two types of silica gel?
Describe the colour changes as they absorb water.
Self-indicating silica gel changes from blue to pink as it absorbs moisture from the air.
Normal silica gel is white and remains white when it absorbs moisture from the air.
What are desiccants used for?
Desiccants are useful in the packaging of optical equipment and leather items where moisture would be a problem. Small packets containing silica gel are often found in the packaging of these items.
What are the two types of hardness in water?
Temporary hardness
Permanent hardness
What is hard water?
Hard water is water that does not readily produce a lather with soap.
What is soft water?
Water that lathers easily with soap is called soft water.
Describe a general test for hardness in water.
The general test for hardness is:
- Add some soap solution to a sample of the water.
- Shake the mixture well.
- If scum forms, or there is no immediate lather, the water is hard.
If the water is hard, a second test must be carried out to determine whether it is temporary or permanent hardness.
- Temporary hardness
1. Take a sample of the water and add soap solution.
2. Shake the mixture - there should be no lather.
3. Take another sample of the water and boil it.
4. Add soap solution.
5. Shake the mixture - there should be a lather. - Permanent hardness
1. Take a sample of the water and add soap solution.
2. Shake the mixture - there should no lather.
3. Take another sample of the water and boil it.
4. Add soap solution.
5. Shake the mixture - there should be no lather.
What is soap?
Soap is sodium stearate.
What is scum?
Hard water reacts with soap to form a white insoluble scum/solid called calcium/magnesium stearate.
What is a precipitation reaction?
A precipitation reaction involves the ions in two solutions reacting together to make an insoluble substance.
Give an example of a precipitation reaction.
Washing soda (hydrated sodium carbonate)
Washing soda removes both permanent and temporary hardness in water. Dissolved Ca2+ ions (or Mg2+ ions) are removed from the hard water in a reaction with carbonate ions CO2-3 from the washing soda to form insoluble CaCO3 (or MgCO3). The ionic equation for this reaction is: Ca2+(aq) + CO2-3(aq) —> CaCO3(s).
What changes occur to anhydrous calcium chloride (a drying agent) as it absorbs water from the air?
The white solid calcium chloride absorbs water from the air until, eventually, a colourless solution forms.
What are the arguments for and against the fluoridation of public water supplies?
For:
• In some areas, fluoride ions are added to drinking water because they have been shown to help prevent tooth decay. This is why ‘fluoride’ is added to toothpaste.
Against:
• Some people object to fluoridation of drinking water because it is considered to be ‘mass medication’ with no choice for the consumer.
• Also, fluoridation has been linked to various diseases including stomach cancer.
What is barium sulfate?
Barium sulfate is a toxic, insoluble salt used for X-rays.
Barium sulfate does not allow X-rays to pass through it.
Medical experts use barium sulfate on patients with stomach and intestinal problems to X-ray the stomach or bowel.
Barium sulfate is chosen as a safe barium compound because it has a very low solubility in water so it cannot act as a poison in the human body.
What are the main physical properties of water?
- It is a colourless liquid at room temperature and pressure.
- Its melting point is 0°C.
- Its boiling point is 100°C.
