Chapter 19 - Water Flashcards
Hard water
Water that will not easily form a lather with soap. Hardness in water is caused by Ca²⁺ and Mg²⁺ ions dissolved in water
What happens when the chemicals that cause hardness in water react with soap?
The Ca²⁺ and Mg²⁺ ions react with a soap chemical/ion called sodium stearate (C₁₇H₃₅COONa) to form a grey insoluble compound called scum which floats on water
Give the chemical equation for the reaction between the chemicals that cause hardness in water and soap
Ca²⁺ + 2C₁₇H₃₅COO⁻ → (C₁₇H₃₅COO)₂Ca↓
Temporary hardness
Hardness that can be removed by boiling water
Permanent hardness
Hardness which cannot be removed by boiling
What causes permanent hardness?
Presence of calcium/magnesium salts that don’t precipitate out of solution upon boiling e.g CaSO₄ and MgSO₄
What are the methods of removing hardness in water?
- Distillation
- Using washing soda
- Ion exchange resin
Describe distillation
Water is boiled and it’s vapour is cooled to condense back into liquid. All impurities are removed. Very expensive so not used on small scale
Describe removing hardness with washing soda
Hydrated sodium carbonate (Na₂CO₃.H₂O) reacts with calcium ions in the water and remove them as insoluble calcium carbonate
Ca²⁺ + CO₃²⁻ → CaCO₃↓
Can’t cook with it
Describe ion exchange resin
These resins swap ions that cause hardness for ions that don’t. A.k.a cation exchange resin (+). Usually swaps Na⁺ ions for Ca²⁺ or Mg²⁺
Ca²⁺ + 2RNa → R₂Ca + Na+
Eventually the resin will have to be replaced or washed out by NaCl
How do you deionise water?
Water goes through a mixed-bed resin that has both cation (+) and anion (-) exchange resin. This removes all positive and negative ions from the water. All positive ions (Ca²⁺ or Mg²⁺) are swapped for H⁺ ions:
RH + Ca²⁺ → R₂Ca + H⁺
All negative ions are swapped for OH⁻ ions:
ROH + Cl⁻ → RCl + OH⁻
The H⁺ and OH⁻ combine to form water
Advantages of hard water
Provides calcium for teeth and bones
Nicer taste
Good for brewing and tanning
Disadvantages of hard water
Blocks pipes and leaves limescale on kettles and boilers
Wastes soap
Produces scum
What does EDTA stand for?
ethylenediaminetetraacetic acid
What experiment is EDTA used in?
To determine the total hardness in a water sample
Complexometric titration
Titration involving the formation of a complex between metal ions and a reagent such as edta. In this type of titration the end point is marked by a sharp decrease in the concentration of free metal ions
What indicator is used in the titration of hard water and edta? What colour change is observed?
Erichrome Black T or Solochrome Black
wine → blue
Why is pH buffer added to the water sample in the test to determine the amount of water hardness?
To ensure the solution is kept around pH 10
►The reaction between Ca²⁺ and Mg²⁺ ions and the edta is pH dependent and (if not at pH 10) the edta may not fully complex with the ions
►Also Erichrome Black T requires a pH of 8-10 for the colour change to work
Why is edta stored in a plastic container?
Edta reacts with the glass ions if stored in a glass container for a long time
What are the stages of water treatment?
- Screening
- Flocculation
- Sedimentation/settlement
- Filtration
- Chlorination
- Fluoridation
- pH adjustment
Flocculation
The coming together (coagulation) of small suspended particles in water
What flocculating agent is added in the treatment of water?
Al₂(SO₄)₃
aluminium sulfate
Flocculating agent
Chemical added to the water to coagulate suspended particles and thus help sedimentation to take place
Give examples of fluorine compounds added to water
NaF
H₂SiF₆
What is the impact of adding too much fluorine to drinking water
fluorosis → staining of the teeth
Total suspended solids
Total amount of solids floating in a sample of water
Total dissolved solids
Amount of dissolved particles which cannot be seen dissolved up in a water sample
Eutrification
Enrichment of water with nutrients, leading to excessive growth of algae and other plants
Where do the nutrients that cause eutrification come from?
nitrates (NO₃⁻) and phosphates (PO₄³⁻) from fertilisers that have been washed into rivers and lakes
What is the problem with the excessive growth of algae in a lake or pond?
It is called an algae bloom and it covers the surface of the water. This blocks sunlight and stops plants beneath from photosynthesising and this can reduce oxygen levels
How are heavy metals removed from water?
By precipitation by adding HCl
Pb²⁺ + 2HCl → PbCl₂↓ + 2H⁺
How is the concentration of heavy metals in water measured?
Atomic absorption spectrometry
What is the test used to see how polluted a water sample is called?
Biochemical oxygen demand test - it shows the extent to which pollution reduces the amount of dissolved oxygen in a sample
Define Biochemical oxygen demand
- amount of dissolved oxygen
- consumed by biological (biochemical) action
- when a sample of water is kept at 20°C
- in the dark
- for 5 days
Why is the sample of water kept at 20°C in the BOD test?
the rate at which microorganisms use oxygen depends on temperature
Why is the sample of water kept in the dark in the BOD test?
to prevent photosynthesis from replacing the oxygen used up by microorganisms
Why is the sample of water left for five days in the BOD test?
This allows enough time for the microorganisms to adjust to the climate. Any longer and all the oxygen might be used up and the measurement of dissolved oxygen could not be taken
BOD is the difference in _______
the two dissolved oxygen levels
What is the name of the method we used to measure the amount of dissolved oxygen in a water sample? Briefly outline the steps of this experiment
Winkler method
- Rinse bottle with water sample
- Completely submerge bottle and fill to top
- Add small volume of manganese (II) sulfate solution (MnSO₄) under the surface of the water with a dropper
- Add small volume of alkaline potassium iodide solution under the surface of the water with a dropper
►brown precipitate - Stopper carefully and shake
- Allow precipitate to settle
- Add a small volume of concentrated sulfuric acid with a dropper below the surface of the water
►brown precipitate dissolves and a clear red-brown solution is formed - Titrate against standard sodium thiosulfate solution (Na₂S₂O₃)
- Titrate until it turns straw yellow
- Add starch indicator (now blue-black)
- Titrate until colourless
What are the three stages of sewage treatment?
Primary (mechanical/physical)
Secondary (biological)
Tertiary (chemical)
What happens in the primary treatment of sewage?
- Large solids are removed by screening and some suspended solids are removed by settlement.
- Grit channels remove pebbles and grit periodically.
What happens in the secondary treatment of sewage?
- Biological oxidation process
- The liquid is passed into aeration tanks containing microorganisms → called activated sludge
- Mechanical rotor introduces oxygen into the tank and the microorganisms break sewage into CO₂ and H₂O
- Then flows into settlement tanks where the sludge is removed and reused
What happens in the tertiary treatment of sewage?
Removal of phosphorus compounds by precipitation (using aluminium sulfate Al₂(SO₄)₃ ) and the removal of nitrogenous compounds by biological and ion-exchange methods (denitrifying bacteria)
Why are nitrates and phosphates sometimes removed in sewage treatment?
They might cause eutrification
Colorimetry
Can be used to identify the amount of chlorine in water
Principal of colorimetry
The amount of absorbance of light by a coloured sample is proportional to the concentration of the solution
Chlorine reacts with water to form..
hypochlorous acid
Cl₂ + H₂O → HOCL + HCl
What is free chlorine?
Chlorine available as an oxidising agent (HOCl, OCl⁻)
What is combined chlorine?
Chlorine that has combined with nitrogen compounds in sweat or urine (NH₂Cl or NHCl₂)
In what form in chlorine added to pool water?
calcium hypochlorite Ca(OCl)₂
How much chlorine should be in pool water?
1-5ppm
What two methods can be used to detect the amount of chlorine?
Comparator
Colorimeter
What chemical is added to a sample of water to detect/show the amount of chlorine?
DPD (diethyl-phenylenediamine)
reacts with chlorine to form a pink colour
