C14 : 14.2 -Water safe to drink

Question 1

uses of water

Answer 1

• agriculture
• industry
• important raw material as a solvent and coolant
• washing
• cleaning
• drinking

Question 2

what is potable water

Answer 2

water that has been processed and is safe for human consumption and daily use

Question 3

for humans, Potable water should have the following characteristics:

Answer 3

• pH between 6.5 and 8.5
• have sufficiently low levels of dissolved substances (eg salt)
• be free of bacteria or potentially harmful microbes

Question 4

is potable water pure water

Answer 4

• NO in chemical terms
• Pure water will only contain H20 molecules
• whereas potable water may contain different substances, usually dissolved minerals and salts

Question 5

in the UK, how are fresh water supplies replenished

Answer 5

• when rainwater falls to the ground as precipitation during natural water cycle

Question 6

when is water considered “fresh”

Answer 6

when it is relatively free from dissolves substances e.g. rainwater

Question 7

what is surface water

Answer 7

• Water collected in reservoirs, lakes, rivers, ponds, streams etc

Question 8

what is groundwater

Answer 8

• when water seeps down through soil and rocks to underground sources called aquifers (porous rocks that store water)
• freshwater can be obtained by drilling a pipe down to form a water well

Question 9

what do The methods used to produce potable water depend on

Answer 9

available supplies of water and local conditions - eg climate in the region

Question 10

In the United Kingdom (UK), rain provides water with

Answer 10

• rainwater dissolves some gases as it falls
• ## will dissolve soluble substances as it passes over them

Question 11

main steps of producing potable water are

Answer 11

• choosing an appropriate source of fresh water
• passing the water through filter beds
• sterilising.

Question 12

describe and explain the steps in producing potable water as it enters a water treatment facility

Answer 12

• water passed through wire mesh screen : to remove debris e.g pieces of soil and dirt, small pebbles, twigs, etc
• water goes to settlement tank : sand and soil settle out
• lime + AlSO4 added : small particles of dirt clump together => sinks => sludge dumped in landfill
• water filtered through sand beds and gravel : removes any remaining dirt/grit particles
• water sterilised using chlorine gas/ozone/UV light: to remove harmful bacteria + microbes
• pH of water checked and corrected to neutral

Question 13

Sterilising agents used for potable water include

Answer 13

chlorine, ozone or
ultraviolet light.

Question 14

why would desalination of salty water or sea water be required

Answer 14

• If supplies of fresh water are limited
• Where aquifers are not present and/or the collection of surface water is limited => due to drier climates

Question 15

how can desalination be done

Answer 15

• distillation
• reverse osmosis

Question 16

briefly how does reverse osmosis work

Answer 16

• salt water is forced and pressurised through a semi-permeable membrane
• only water molecules can pass through it => as the membrane stops larger molecules and ions passing through

Question 17

disadv. of desalination

Answer 17

• expensive process as it consumes large amounts of energy
• not ideal when producing large quantities of fresh water
• used in regions w/ very hot climate (eg Saudi Arabia)

Question 18

materials required for Method for Required practical 8: analysis and purification of water samples from different sources, including pH, dissolved solids and distillation. ONLY ANALYSIS

Answer 18

Question 19

Method for Required practical 8: analysis and purification of water samples from different sources, including pH, dissolved solids and distillation. ONLY ANALYSIS

Answer 19

• use universal indicator/pH probe to determine pH of water sample
• Accurately weigh and record mass of an empty evaporating basin to 2 dp
• Add 25 cm3 of water sample into evaporating basin and heat over a TRIPOD AND GAUZE OVER A bunsen burner until the solids start to form and majority of water has evaporated
• Don’t overheat during above step as you run the risk of thermally decomposing some of the solids, leading to erroneous results
• leave to cool and evaporate remaining water
• reweigh the evaporating basin and calculate change in mass - this is the mass of the solids dissolved
• repeat above steps for all water samples
• results could be compared to the national water safety levels, and by analysis, the regions of each sample could be deducted, e.g. region of high acid rain, water from a saltwater supply etc.

Question 20

materials required for Method for Required practical 8: analysis and purification of water samples from different sources, including pH, dissolved solids and distillation. ONLY PURIFICATION

Answer 20

Question 21

Method for Required practical 8: analysis and purification of water samples from different sources, including pH, dissolved solids and distillation. ONLY PURIFICATION

Answer 21

• add water sample to concial flask and set up apparatus in image ( conical stand over a bunsen burner, delivery tube from conical flask into a test tube clamped and submerged in ice bath)
• heat water w/ bunsen burner until it boils
• Reduce the heat so that the water boils gently for some time
• The distilled water will collect in the cooled test tube
• Collect about 2 cm depth of water in this way, then stop heating
• Analyse the water you have distilled by determining its boiling point (as pure water boils at 100c) or pH
• can use blue cobalt (II) chloride or white anhydrous copper(II) sulfate to test - but they only show presence of water , not if its pure or not

Question 22

+ve test for water w/ white anhydrous copper (II ) sulfate

Answer 22

• turns blue

Question 23

+ve test for water w/ blue cobalt (II) chloride paper

Answer 23

• turns pink