Water Flashcards
What is a solvent
A thing that does the dissolving
What is a solute
Something that gets dissolved
What is a solution
Mixture of a solute dissolved in a solvent
What is the composition of water
Dissolved gases, mineral ions, bacteria and pollutants
Potable water
suitable for human consumption
What are the processes of water treatment
- sedimentation
- filtration
- chlorination
What is sedimentation
Large insoluble particles fall to the bottom of water due to gravity
What is filtration
Removal of smaller, insoluble particles with a filter.
What is chlorination
Bubbled through water to kill all bacteria. Chlorine is toxic to all living things.
Why is fluoridation good (4)
- decreases tooth decay
- makes teeth stronger
- some people are too poor to buy good toothpaste
- many people don’t follow dentist’s advice on brushing
Why is fluoridation bad? (4)
- fluorosis - brown mottling of teeth
- infertility, stomach and bone cancers
- toxic in high concentrations
- unethical (mass medication)
How can pure water be collected from salty
distillation
What is osmosis
Movement of water through a semi permeable membrane from a low solute concentration to a high solute concentration.
Hypertonic
High concentration of solute
Hypotonic
Low concentration of solute
Reverse Osmosis
Forcing water across the semipermeable membrane in the opposite direction it would normally flow.
Name for hard water
Calcium sulfate
How can temporary hard water be made soft
Boil it
How does boiling calcium hydrogen carbonate affect it
It decomposes to form (insoluble) calcium carbonate, water and carbon dioxide
What is the effect of boiling calcium sulfate
There is none. It does not decompose
What is limescale
Insoluble calcium carbonate
How can hard water be softened
Adding sodium carbonate (washing powder) or using an ion exchange column.
What are the benefits of hard water
- Calcium strengthens teeth and bones
- Helps to reduce heart disease
What are the disadvantages of hard water
- Hot water pipes ‘fur up’ becoming blocked.
- Limescale form (e.g. kettles become more inefficient)
- soap scum wastes soap
How does washing powder react with hard water
Adding sodium carbonate (washing powder) reacts with calcium from hard water forming calcium carbonate precipitate (limescale)
Where is an ion exchange column used
Dishwashers
If water lathers easily before and after being boiled what is it
Soft
If water takes along time to lather before and after boiling what is it
Permanent hard water
If water takes a long time to lather before boiling but lathers quickly after what is it
Temporary hard water
How do you test water hardness
Tests how much soap is needs to make it lather before and after boiling.
Sources for public water
- reservoirs
- lakes
- rivers
What does rainwater contain
Dissolved CO2 and O2
What are the effects of dissolved CO2 in rainwater
Lowers the pH
Why is water in increasing demand (2)
- population increase
- industrialisation
How else can water be desalinated other than by distillation
Use of membrane systems
Boiling point of water
100°C
Why is it not possible to obtain pure ethanol (bp 78°C)
Some water evaporates at temperature below its boiling point
Solubility
Amount of solute able to dissolve in a solvent
Units for solubility
g/100cm3
Saturated solution
No more solute is able to dissolve
What methods can be used to measure solubility (2)
- add mass of solute to a volume of water that dissolves some of it. Filter, dry and weigh the excess solute. Determine solubility.
- add weighed solute that dissolves in volume of water at room temperature. Heat solution so all solute dissolves. Allow to cool and record when crystals first appear. Repeat with increasing volume of water, determine solubility at each temperature and plot solubility curve
Why does boiling remove temporary water hardness
It thermally decomposes hydrogen carbonate ions to precipitate insoluble calcium carbonate (limescale)
Disadvantage of ion exchange column
Expensive
What causes water hardness
Dissolved calcium and magnesium ions
Natural water pollutants (2)
- bacteria
- viruses
Man- made water pollutants
- fertilisers
- pesticides
- household and industrial waste (sewage)
Can fluoride ions naturally be found in water
yes
Why is desalination by distillation not viable across many parts of the world
It uses large amounts of costly energy to boil water
Advantages of softening water using a kettle
No need for expensive equipment
Disadvantages of using a kettle to soften water (2)
- Does not remove permanent hardness
- Only useful for small volumes
Advantages of softening water using sodium carbonate (washing powder)
- removes temporary and permanent hardness
Disadvantages of softening water using sodium carbonate (washing powder)
- limescale is formed which can block washing machine pipes
Advantages of using an ion exchange column to soften water (2)
- uses concentrated sodium chloride that is cheap and easily available
- removes temporary and permanent hardness and is a continuous process
What is the ratio of calcium ions to sodium ions in an ion exchange column
1:2
How can the resin be regenerated in ion exchange
by being rinsed in concentrated sodium chloride solution
How does an ion exchange column remove water hardness
Calcium ( and magnesium) ions are exchanged for sodium ions as the water passes through an ion exchange resin
