Environmental Chemistry- Water

hard water

water that will not easily form a lather with soap

what causes hard water?

presence of calcium ions (Ca2+) or magnesium ions (Mg2+) i.e. group II ions

Q: Why does hard water not form a lather with soap/how does hard water waste soap/why does hard water form an insoluble grey scum with soap?

- Soap is the salt of a fatty acid e.g. sodium stearate (C17H25COONa)

- The stearate ion (C17H25COO-) reacts with Ca2+ forming an insoluble grey scum.

- Soap does not form a lather until all Ca2+ used up

- Soap is wasted

CHEM EQ: 2C17H25COO- + Ca2+ ————> (C17H25COO)2Ca↓ (precipitate: calcium stearate i.e. insoluble grey scum)

temporary hardness

can be removed by boiling

- caused by calcium hydrogencarbonate Ca(HCO3)2

permanent hardness

can't be removed by boiling

- cause by calcium sulfate CaSO4

Q: How does temporary hardness arise in water?

- Rainwater is acidic due to carbon dioxide dissolving in it, forming carbonic acid.

CHEM EQ: H2O + CO2 <——half arrows——> H2CO3 (rainwater: carbonic acid)

- Rainwater (carbonic acid) flows over limestone rocks (calcium carbonate) forming calcium hydrogencarbonate.

CHEM EQ: H2CO3 + CaCO3 <——half arrows——> Ca(HCO3)2

Q: How does boiling remove temporary hardness?

Calcium hydrogencarbonate breaks down into calcium carbonate which is insoluble and precipitates.

CHEM EQ: Ca(HCO3)2 (aq) ——Heat/Boil——> CaCO3↓ + H2O + CO2

What is this insoluble calcium carbonate precipitate known as?

Limescale (CaCO3↓)

Disadvantages of hard water

(1) causes limescale which damages machines

(2) wastes soap

How can the total hardness in a water sample be removed?

(a) distillation

(b) washing soda tablets

N.B!! (c) ion-exchange resin

N.B!!! Q: how can water be softened by an ion-exchange resin so that is it suitable for use in home? i.e. remove Ca2+ and Mg2+ cations

- A cation exchange resin containing Na+ ions is used (RNa).

- Each Ca2+ in water replaced by two Na+ ions.

CHEM EQ: Ca2+ + 2RNa ————> R2Ca + 2Na+

- The Ca2+ ions kept in resin and the Na+ ions pass out in water.

N.B!!! Q: how can water be softened by an ion-exchange resin so that it is suitable for use as deionised water in the laboratory? i.e. removing ALL ions

- Mixed resin used:

Cation-exchange resin containing H+ ions (RH) and an

Anion-exchange resin containing OH- ions (R'OH).

- Any cation in water replaced by two H+ ions.

CHEM EQ: Ca2+ + 2RH ————> R2Ca + 2H+

- Any anions in water replaced by OH- ions.

CHEM EQ: HCO3- + R'OH ————> R'HCO3 + OH-

- The H+ ions and OH- ions form water.

CHEM EQ: H+ + OH- ————> H2O

Q: compare DI water with distilled water

- DI water contains no ions, but can contain suspended solids, dissolved organic material and dissolved gases. Not as pure.

- Distilled water is the most pure form.

Stages in water treatment to provide suitable water to houses and factories:

(1) Screening

(2) Flocculation

(3) Sedimentation

(4) Filtration

(5) Chlorination

(6) Fluoridation

(7) pH adjustment

What is flocculation/coagulation?

Clumping together of small, suspended particles into larger particles.

Q: what is a flocculating agent/coagulant?

A chemical added to water to clump together small, suspended particles into larger particles, aiding sedimentation e.g. aluminium sulfate

Q: why must the amount of aluminium sulfate added be carefully controlled?

(1) causes corrosion of pipes

(2) affects taste

Q: what happens in sedimentation?

water passed into settling tanks and suspended solids settle to bottom

Q: what happens in (4) filtration?

water is passed through beds of sand and gravel. At this stage all suspended solids have been removed.

Q: what happens in chlorination?

chlorine (Cl2) added to kill harmful microorganisms

Q: why must amount of chlorine added be carefully controlled?

(1) toxic

(2) unpleasant taste and smell

Q: what does fluoridation do?

helps prevent tooth decay e.g. fluoride compound is sodium fluoride.

Q: why must amount of fluoride compounds be carefully controlled?

causes fluorosis which stains teeth

Q: what happens in pH adjustment?

pH of water to be distributed to homes and factories is 7.2

- if water too acidic, use calcium hydroxide to raise ph.

- if water too hard, use sodium carbonate to soften.

- if water too alkaline, use sulfuric acid to lower pH.

Q: why must the amount of calcium hydroxide be carefully controlled?

Can cause water to become hard

Q: why must the amount of sodium carbonate be carefully controlled?

Affects taste

Q: why must the amount of sulfuric acid be carefully controlled / problems that can result from water being too acidic?

Corrosion of pipes

Q: eutrophication?

over enrichment of water with nutrients

Q: why is eutrophication problematic/harmful effect of eutrophication?

leads to excessive algal blooms and plant growth, depleting oxygen, killing aquatic life

Q: name 2 nutrients whose over enrichment in water can lead to eutrophication and give a major source of these nutrients entering waterways?

1. nitrates

2. phosphates

sources: fertilisers, sewage, or slurry

Q: describe the processes that occur in the water leading to eutrophication?

- nitrates and phosphates are absorbed, causing algal blooms.

- algal blooms block sunlight, causing underwater plants to die.

- as these dead plants are decayed, the dissolved oxygen in the water is used up by microorganisms

- decreased dissolved oxygen can cause aquatic life to die

Q: bc of water pollution and eutrophication, the BOD in water/sewage is measured. what is meant by BOD?

Biochemical Oxygen Demand.

it's the amount of dissolved oxygen consumed by biological action when a sample of water is kept at 20 degrees Celsius in the dark for 5 days

(measured in ppm)

Q: how is the BOD of a water/sewage sample measured?

- 2 samples are taken.

- first sample has the amount of dissolved oxygen immediately.

- second sample is kept at 20 degrees Celsius in the dark for 5 days then the dissolved oxygen is measured by titration

BOD level = first [O2] - second [O2]

note for measuring the BOD of effluents/slurry (highly polluted liquids)

often diluted with well aerated pure water before analysis. remember diluted concentration x dilution factor = original concentration!

2021 Q: explain why samples of effluents/slurry's are often diluted and well aerated before analysis

- effluents have a high BOD level

- if not diluted, all of the oxygen would be used up before the 5 days have elapsed meaning there'd be no oxygen left.

- diluting and aerating ensures a high enough level of oxygen

Q: heavy metal ions are another source of water pollution, name 3 such heavy metal ions

1. lead (Pb2+)

2. mercury (Hg2+)

3. cadmium (Cd2+)

Q: give 3 ways in which these heavy metals can enter water supplies?

1. industrial effluents

2. batteries

3. lead plumbing

Q: what health concerns are caused by heavy metal ions in drinking water?

toxic

Q: instrumental method used to analyse water for the presence and concentration of heavy metals

atomic absorption spectrometry (AAS)

Q: how are heavy metal ions removed from water?

precipitation e.g. Pb2+ + 2HCl ----> PbCl2↓ + 2H+

Q: outline the stages involved in a sewage/wastewater treatment plant, converting discharged sewage from houses, factories and businesses to water that can be added back to rivers

1. primary treatment

2. secondary treatment

3. tertiary treatment

Q: primary sewage/wastewater treatment?

mechanical process in which large solids are physically removed by screening and some suspended solids are removed by settlement

Q: what does primary sewage/wastewater treatment involve?

1. screening

2. grit channels

3. settlement

Q: secondary sewage/wastewater treatment?

biological process involving aerobic oxidation by microorganisms to breakdown material

Q: why is secondary treatment known as a biological process?

bc sewage is broken down by microorganisms

Q: what does secondary sewage/wastewater treatment involve?

activated sludge process.

- sewage pumped into aeration tank.

- microorganisms break sewage down

- sludge is removed

Q: tertiary sewage/wastewater treatment?

removal of nitrates and phosphates

Q: why is the tertiary sewage/wastewater treatment stage carried out?

to remove nitrates and phosphates to avoid eutrophication

Q: why is tertiary sewage/wastewater treatment not carried out in all sewage/wastewater treatment plants?

expensive

Q: how are nitrates removed from water in tertiary treatment?

1. biologically by adding denitrifying bacteria

2. ion-exchange

Q: how are phosphates removed from water in tertiary treatment?

precipitation by adding aluminium sulfate

EQ: 2Na3PO4(aq) + Al2(SO4)3 ----> 2AlPO4↓ + 3Na2SO4

Q: in which sewage/wastewater treatment stage is BOD decreased the most?

@ secondary treatment stage BOD reduced by ~95%!

Q: what property of water makes it very useful in the human body as a medium in which chemical rxns occur, and also allows it to become polluted/contaminated very easily in other situations?

water is polar, making it an excellent solvent as it will readily dissolve ionic compounds and other polar molecules

Q: name 3 instrumental methods that are used when analysing water

1. pH measurement

2. atomic absorption spectrometry (AAS)

3. colorimetry

water vs wastewater treatment rule of thumb

water treatment: 7 steps so water is clean enough to drink

wastewater/sewage treatment: 3 steps so water can be added back to rivers