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.