Hardness in water Flashcards
Hard water defn
Water that does not form a lather easily with water.
What is the hardness in water caused by?
Ca²⁺ and Mg²⁺ ions
Temporary hardness
Caused by the presence of calcium hydrogencarbonate.
How is temporary hardness removed?
By boiling
Permanent hardness
Caused by the presence of calcium sulfate or Magnesium sulfate.
Why can permanent hardness not be removed by boiling?
As sulfates do not decompose on heating.
Advantages of hard water
- Tastes better
- Contains calcium, healthy
- Good for brewing industry
Disadvantages of hard water
- ‘Dirty’ looking scum produced with soap
- Wastes soap
- Produces ‘scale’ that can block pipes and made the heating elements useless
Deionised water
- No ions
- Has dissolved gases and organic material
Distilled water
- No ions
- No dissolved gases and organic material
Biochemical oxygen demand
The amount of dissolved oxygen consumed by biochemical action when a sample of water is kept in the dark at 20C, for five days.
Eutrophication
The enrichment of a body of water with nutrients resulting in deoxygenation of the water.
Instrumental methods of analysis
- pH meter
- Atomic absorption spectrometry
- Colorimetry
Suspended solids
Insoluble substances that are ‘floating’ in the water and can be removed by filtering.
Dissolved solids
Soluble substances in solution in the water and can be measured by evaporating off the water leaving the dissolved solids behind.
Principle of the colorimetric experiment to estimate free chlorine in pool water.
The intensity of the colour is proportional to concentration.
Free chlorine
The chlorine in chloric (I) acid and hypochlorite ions is known as ‘free chlorine’.
State 3 methods if removing both types of hardness
- Distillation
- Addition of washing soda crystals
- Ion exchange resins
Distillation
The water is boiled off through a Liebeg condensor and the hydrogencarbonates and sulfates remain in the flask. (too expensive on large scale)
Addition of washing soda crystals
Na₂CO₃.10H₂O. The carbonate ions in the washing soda crystals react with the calcium ions that cause the hardness and thus remove them.
Na₂CO₃ + Ca₂⁺ → CaCO₃ + 2Na⁺
Cation exchange resin
RH + cation → H⁺ + spent resin
Anion exchange resin
ROH + anion → OH⁻ + spent resin
RH + cation → H+ + spent resin
ROH + anion → OH- + spent resin
Then:
H⁺ + OH⁻ → H₂O
Ion exchange resin Na₂R
Na₂R + Ca²⁺ → CaR + 2Na⁺
Preparation of water for drinking
SFSFCFP
- Screening
- Flocculation
- Settlement (sedimentation)
- Filtration
- Chlorination
- Fluoridation
- pH adjustment
Preparation of water for drinking - Screening
The water is passed through graded screens to remove debris.
Preparation of water for drinking - Flocculation
A flocculating agent such as Al₂(SO₄)₂ is added to cause any suspended particles to coagulate together so that they can be removed.
Preparation of water for drinking - Settlement (sedimentation)
The water passes into settlement tanks so that the coagulated suspended particles can settle on the bottom of the tank.
Preparation of water for drinking - Filtration
The water passes through filter beds and any remaining suspended particles can be removed.
Preparation of water for drinking - Chlorination
Cl is added in small quantities, usually as NaClO to sterilise the water.
Preparation of water for drinking - Fluoridation
Hexafluorosilicic acid is added ( 1 ppm) to prevent tooth decay.
Preparation of water for drinking - pH adjustment
- pH too high - add dilute sulphuric acid
- pH too low - add calcium hydroxide
What would an acidic pH lead to?
Corrosion of pipes
At what stage of sewage treatment (i) are most solids removed, (ii) are phosphate ions removed?
2000
What term is usually used to describe the particles removed from water by filtration? Give the name or formula of a compound used to help remove these particles in the purification of water for drinking.
2000
These stages may include sedimentation, flocculation and filtration. Describe what happens at each of these.
Sed: Settling of suspended particles
Floc: Adding chemicals, causing coagulating of particles
Filt: Passing through sand to remove suspended matter
Various chemicals are often added in other stages of water treatment. Identify one other stage in water treatment, which involves the addition of a chemical to the water. Name one chemical added during this stage and state why this chemical is added.
Stage: Chlorination
Name and why: Chlorine, to kill germs
Distinguish between the primary and secondary stages of sewage treatment.
Primary: Removal of solids by screening and sedimentation (physical).
Secondary: Oxidation by micro-organisms
What is the purpose of tertiary treatment.
Removal of nitrates and phosphates
What property of water makes it very useful in the human body as a medium in which chemical reaction occur, and also allows it to become polluted or contaminated very easily in other situations?
Property : Good solvent and polar
What happens during secondary sewage treatment?
Oxidation by micro-organisms
What is meant by the biochemical oxygen demand (BOD) of a water sample?
p.p.m of oxygen consumed when sample kept in the dark for five days at 20°C
Describe clearly the processes involved in the primary and secondary stages of urban treatment. What substances are removed by tertiary treatment of sewage?
Primary: Removal of solids by screening and settlement (physical)
Secondary: Oxidation by micro-organisms
What: Nitrates and phosphates
In an experiment to measure the concentration of dissolved oxygen in a river sample, why was it necessary to analysis the sample from the river immediately?
So oxygen content doesn’t increase due to photosynthesis.
In an experiment to measure the concentration of dissolved oxygen in a river sample, in making the additions to the sample of river water, why should the solutions used to be concentrated?
To minimise the amount of the water sample that is displaced.
In an experiment to measure the concentration of dissolved oxygen in a river sample, describe how the additions of the concentrated solution of manganese sulfate and alkaline potassium iodide to the bottle of river water should be carried out. What essential precautions should be taken when replacing the stopper of the bottle after these additions are made?
- Remove a few cm³ of river water from the bottle.
- Make additions under the level of the water using a dropper.
- Do not bubble air into the water process
Precaution: Do not trap air bubbles
In an experiment to measure the concentration of dissolved oxygen in a river sample, describe clearly the procedure for using a pipette to measure exactly 50cm³ portions of the iodine solution into the titration flask.
- Rinse with water followed by iodine
- Fill pipette using a pipette filler and adjust to have bottom of meniscus on mark
- Remove droplets adhering to outside and drain under gravity into titration flask.
- Touch tip of pipette against side of flask to add droplet adhering to outside tip and do not blow out drop inside pipette.
In an experiment to measure the concentration of dissolved oxygen in a river sample, what indicator is used in this titration? State when the indicator should be added to the titration flask and describe the colour change observed at the end point.
Starch - when the solution is straw-yellow
Colour: Blue-black to colourless
A colorimetric experiment was used to estimate the concentration of free chlorine in the sample. What is the general principle of all colorimetric experiments?
Intensity of colour proportional to concentration.
Identify a suitable reagent to test for free chlorine in swimming pool water and state the colour which develops when this reagent reacts with free chlorine.
Acidified potassium iodide
Colour: Brown
Describe briefly how you would estimate the concentration of free chlorine in a sample using either a comparator or colorimeter.
Colorimeter:
- Prepare standard solutions
- Place in colorimeter and note readings
- Plot readings vs concentration
- Get concentration from graph
Give the name or formula of a free chlorine species in the swimming pool water. Give a reason why the concentration of free chlorine in treated drinking water is usually between 0.2 - 0.5 ppm whereas in swimming pool water it should be between 1 - 5 ppm.
Chlorine
Reason: To kill pathogens added by swimmers
Describe the processes occurring in the water leading to eutrophication.
- Enrichment with nutrients due to pollution.
- Rapid growth of plants, light blocked from plants below surface and oxygen level falls.
Explain how heavy metal ions are removed from large quantities of water.
Precipitation
Write the formula of (i) a substance which causes temporary hardness in water, (ii) a substance which causes permanent hardness in water.
(i) Ca(HCO₃)₂
(ii) CaSO₄
Explain clearly how suspended solids are removed in the treatment of water for drinking.
- Sedimentation
- Flocculation
- Allow water to overflow
- Filtration
Identify two chemicals added at the final stages of the treatment of water for drinking state the purpose of adding each chemical you have identified.
CO₂: Lower pH
Cl₂: Disinfect
A student is given a bucket of seawater. Describe how the student could determine by filtration the total suspended solids in the water.
- Filter known volume through weighed filter paper
- Dry filter paper
- Reweigh and find increase
- Express mass in mg l⁻1
How could the student determine the total dissolved solids in a sample of the filtered seawater?
- Evaporate known volume to dryness in weighed container
- Cool, reweigh and find increase
- Express mass in mg l⁻1
Describe a test to confirm the presence of the chlorine ion in aqueous solution.
Add silver nitrate, white precipitate formed
Give two methods for removing all of the hardness in a water sample.
- Distillation
- Ion exchange resin
What is meant by the biochemical oxygen demand of a water sample.
p.p.m of oxygen consumed when sample kept in the dark for five days at 20°C
Describe how the BOD was reduced by about 30% in primary sewage treatment.
Removal of solids by screening and sedimentation.
Explain the processes by which the BOD was further reduced in secondary sewage treatment.
Digestion by oxidation involving microorganisms.
What happens during the secondary treatment of sewage?
Biological oxidation of sewage.
Explain how hard water is caused and how it wastes soap.
Dissolution of calcium ions
Waste: Soap used up in react with Ca and Mg ions to give scum
How can hard water be softened by ion exchange so that it is suitable for use as deionised water in the laboratory.
Pass through resin to replace positive ions with hydrogen ions and negative ions with hydroxyl ions.
In water treatment, what is the purpose of adding a flocculating agent?
Clumping
In water treatment, what is the purpose of adding chlorine?
Kills pathogens
In water treatment, what is the purpose of adding a fluorine-containing compound?
Prevents tooth decay
In water treatment, what is the purpose of adding calcium hydroxide?
Raises pH
In water treatment, what is the purpose of adding sulfuric acid?
Lowers pH
Why is water pollution by heavy metal ions a cause of concern?
Danger to health (toxic)
Name an instrumental technique that could be used to detect and measure the concentration of a heavy metal ion in a water sample.
Atomic absorption spectroscopy
Explain how Hg²⁺ or Pb²⁺ ions can be removed from a water supply.
Precipitation
Describe a test for the presence of chlorine ion (Cl⁻) in water.
Add silver nitrate, white precipitate formed
In an experiment to measure the dissolved oxygen content of a river water sample, why was it important to avoid trapping air bubbles each time the stopper was inserted into the sample bottle and when using the dropper?
Increase oxygen concentration
In an experiment to measure the dissolved oxygen content of a river water sample, what was observed on addition of the concentrated sulfuric acid followed by the mixing of the contents of the bottle.
A brown solution
In an experiment to measure the dissolved oxygen content of a river water sample, describe how the burette was rinsed and filled for use in the titrations.
Rinse: Rinse with deionised water and rinse with a little of the thiosulfate solution.
Fill: Add solution to burette and clamp vertically, Mark at eye-level and open tap to fill jet.
In an experiment to measure the dissolved oxygen content of a river water sample, name the indicator used in the titrations.
Starch
In an experiment to measure the dissolved oxygen content of a river water sample, what conclusion should have been reached had a white precipitate been observed instead of the brown precipitate after the first two additions of reagents to the bottle filled with river water?
No oxygen in water
In an experiment to measure the dissolved oxygen content of a river water sample, why is the immediate determination of dissolved oxygen considered best practice?
Biochemical reactions occur
Explain how high nitrate levels can result in a reduction in the dissolved oxygen content of lakes and rivers.
Causes rapid growth of water plants, decay of these plants by aerobic microorganisms, respiration by decomposers of these plants uses up oxygen, surface plants block light preventing photosynthesis.
State two processes that are carried out during the primary treatment of sewage.
- Screening
- Sedimentation
State the problem that would arise when a flocculating agent, chlorine and calcium hydroxide is added in excessive quantity.
Flocculating agent: Slight danger to health
Chlorine: Toxic
Calcium hydroxide: pH too high
How does boiling remove any temporary hardness, caused by the presence of calcium hydrogen carbonate, in a water sample.
Calcium hydrogen carbonate decomposes
To determine the BOD of water in a lake, two samples were taken and one was tested immediately, under what conditions was the second sample stored?
20°C in the dark
To determine the BOD of water in a lake, two samples were taken and one was tested immediately, after what time interval was it tested?
5 days
Define eutrophication and state one of its harmful effects.
D: Over enrichment of water with nutrients
S: Excessive plant growth
Suggest a possible cause of eutrophication.
Fertilizer run-off
Why is there concern about lead in drinking water? Name an instrumental method used to analyse water for heavy metals. How can lead by removed from a contaminated water supply?
- Lead is toxic
- Atomic absorption spectroscopy
- Precipitation
How does temporary hardness arise in rainwater flowing over limestone?
Rain water acidic due to dissolved carbon dioxide.
Write a balanced equation to describe the formation of limescale when rainwater is boiled.
Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂
Explain in words or by balanced equations how this water with temporary hardness is deionised using a cation-exchange resin and an anion exchange resin.
- Two hydrogen ions from the cation-exchange resin replace a Ca²⁺ in the water.
- A hydroxide ion from the anion-exchange resin replaces a HCO₃⁻ in the water.
- Hydrogen ions and hydroxide ions combine to form water.
Identify a compound of calcium that is one of the main causes of permanent hardness in water.
xx
Describe the procedure used to measure out 50cm³ of the hard water from a beaker into a conical flask.
x
Temporary hardness
H₂CO₃ + CaCO₃ → Ca(HCO₃)₂
Identify a reagent used to detect free chlorine in a water sample.
DPD
Hardness in swimming-pool water can contribute to cloudiness. What solution was titrated against a known volume of a sample to measure its total hardness.
EDTA
Estimation of total hardness in a water sample using EDTA - indicator.
Eriochrome Black T
Describe Eriochrome Black T
Black powder
Hard water + Eriochrome Black T =
Wine/red colour
Hard water indicator colour change
Wine/red → Blue
