Exam Questions Stories

Purifying Water

Read about the Singapore water story, as told through the lens of a chemist, and learn the separation techniques and chemicals involved in water treatment

In Singapore, water is sacred. We describe the 1962 Water Agreement between Singapore and Malaysia as “sacrosanct”. We build expensive monuments — NEWater plants, desalination facilities, and waterworks — purifying water for drinking and industrial manufacturing. At home, we are in turn purified by water, as we see it as a cure-all. Our elders remind us to “drink more water” to get rid of our fever, sore throat, COVID-19… Just about anything.

© BiblioAsia, 2018

Purifying “Water” to Get Water

In Chemistry, water refers to one thing and one thing only: a pure compound with the chemical formula of H2O. However, we use the term much more loosely in everyday lives. We say that we import “water” from Malaysia and collect “water” in reservoirs. But what we are really referring to is an impure mixture of water and other substances, like sediments, micro-organisms, and dissolved minerals. That’s why we have to treat “water” to remove most of these impurities.

The water treatment process in Singapore, which involves many separation techniques and biological disinfection. (©PUB 2020)

Water treatment is a complex, multi-step process. Many of the steps shown in the diagram above make use of separation techniques to remove unwanted impurities:

  • Sedimentation (step 3), to remove larger particles that sink to the bottom of the basin
  • Filtration (step 4), to remove particles that are suspended in water
  • Biologically activated carbon (step 6), to remove dissolved organic compounds that are very small

Pure Water?

However, the goal of water treatment is not to produce pure water, but clean drinking water in which chemicals are present within safety limits. In fact, some useful substances are added after the separation steps. For example, PUB adds calcium oxide to neutralise any acid, such that the overall pH of drinking water is between 6.5 and 9. They also add chlorine to destroy any bacteria and viruses that are not removed during the separation steps.

Surprisingly, our drinking water plays a part in our dental health. That’s because PUB adds sodium silicofluoride, a compound which releases fluoride ions in drinking water. Fluoride ions stick to the calcium ions in the teeth to strengthen them. This prevents tooth decay.

Pure Water, I Demand!

© 2012 The Oregonian/AP

Unfortunately, the idea of adding substances to drinking water can be scary to some. There have even been protests in the US against the addition of fluoride.

Indeed, there are legitimate concerns that too much fluoride may have adverse health effects. Some are backed by rigorous scientific studies. A 1989 study conducted in Singapore showed that a third of young children had dental fluorosis, a tooth developmental problem due to an overexposure to fluoride. This prompted the Ministry of Health to eventually lower the fluoride concentration in drinking water from 0.7 ppm to 0.5 ppm.

However, other concerns are exaggerated in protest banners like “fluoride is a rat poison”. This extreme reaction betrays our irrational attraction towards “natural” substances, untarnished by anyone. Like how we prefer #unfiltered Instagram posts and unadulterated organic food, we want our drinking water to be left untouched. Some Singaporean families spend up to $200 a week on bottled water that does not have added chlorine.

Pure, Purer, Purest

Many brands of bottled water market themselves as “pure”, “natural”, or “healthier” (© NTUC, 2020)

Many brands of bottled water claim that they are “pure”. Yet, by definition, drinking water is never pure due to its dissolved minerals. This makes “pure drinking water” an awkward oxymoron in the eyes of an awkward chemist.

On the other hand, deionised water that we use in the laboratory does not contain dissolved minerals. However, they too are not totally free of impurities. Once they have been exposed to the air, atmospheric carbon dioxide would have dissolved in it. Water is not called the universal solvent without a reason.

Ultra-Pure Water is perhaps the closest we can get to complete purity. A process called reverse osmosis is used to form water virtually devoid of even dissolved gases. The semiconductor industry uses Ultra-Pure Water for their precise manufacturing of electronic parts that are sensitive to impurities.

Yet, even if technology were to allow us to make pure water without even a modicum of dissolved substance, we can never get pure H2O. This is because in a sample of water, there will always be some molecules that undergo self-ionisation to form hydrogen ions and hydroxide ions:

H2O(l) H+(aq) + OH(aq)

This production of hydrogen ions explains why water has a pH of 7. Its pH is greater than that of acids because self-ionisation is a partial reaction, whereby only a handful of molecules ionise to form a low concentration of hydrogen ions.

Perhaps water does not need to be pure to be sacred.


Data-based Questions à la Paper 2 Section B

QUESTION 1: Separation Techniques
Water treatment involves sedimentation in step 3 and filtration in step 4. To facilitate these steps, aluminium sulfate is first added as a coagulant in step 2.

Aluminium sulfate causes particles to clump together. This process is called coagulation.

Suggest, in terms of particle size, why coagulation facilitates sedimentation and filtration. [2 marks]

Coagulation causes clumping, which increases particle size.

During sedimentation, coagulated particles are heavier, hence they settle to the bottom of the basin more easily for removal.

Furthermore, coagulated particles are trapped by the filter as they are too big to pass through the small pores, hence facilitating filtration.

QUESTION 2: Separation Techniques
In step 6 of water treatment, biologically activated carbon is used to separate dissolved organic compounds. It is termed biologically activated because the carbon is coated with bacteria.

Firstly, the carbon preferentially attracts dissolved organic compounds. The bacteria coating then digests and breaks down the attracted compounds into simpler products that are harmless.

Compare filtration and step 6 that uses biologically activated carbon. [2 marks]

Filtration removes undissolved, insoluble particles as the solid residue. However, biologically activated carbon removes dissolved, soluble particles.

Filtration separates by particle size, as water molecules and solutes are sufficiently small to pass through the pores of the filter. However, biologically activated carbon removes by attraction. It attracts dissolved organic compounds more strongly than water molecules.

QUESTION 3: Acids, Bases & Salts
After the separation steps, the water may still be too acidic. To increase the pH, calcium oxide is added.

Explain how and why calcium oxide increases the pH. [2 marks]

Calcium oxide is a basic oxide. It undergoes neutralisation with any acid present to form salt and water only, hence increasing the pH to 7.

QUESTION 4: Stoichiometry and the Mole Concept
To introduce fluoride ions into drinking water, different fluoride-containing compounds can be added. Some of these compounds are:

1. Sodium fluorosilicate, Na2SiF6
2. Hydrofluoric acid, HF

The fluoride content of the substances is important because we can add less of them to achieve the same fluoride concentration in drinking water.

Show by calculation that hydrofluoric acid has a higher percentage by mass of fluoride than sodium fluorosilicate. [2 marks]

Relative mass of fluoride in a formula unit of Na2SiF6 = 19 × 6 = 114
Relative formula mass of Na2SiF6 = 23×2 + 28 + 19×6 = 188
Percentage by mass of fluoride in Na2SiF6 = (114/188) × 100% = 60.6%

Relative mass of fluoride in a formula unit of HF = 19
Relative formula mass of HF = 1 + 19 = 20
Percentage by mass of fluoride in HF = (19/20) × 100% = 95%

HF has a higher percentage by mass of fluoride of 95% than that of Na2SiF6.

QUESTION 5: Stoichiometry and the Mole Concept
In Singapore, the fluoride concentration in drinking water is kept at 0.5 ppm.

The unit ppm stands for parts per million. 1 ppm refers to 1 milligram of something dissolved in 1 dm3 of water.

(a) State the mass in grams of fluoride ions dissolved in 1 dm3 of drinking water. [1 mark]
(b) The percentage purity of an impure sample of sodium fluorosilicate is 97%. Using this information and your answer in question 4, calculate the mass of the impure sample needed to give a fluoride concentration of 0.5 ppm. [2 marks]

Fluoride concentration in ppm = 0.5 ppm
Mass of fluoride in 1 dm3 of water = 0.5 mg

Mass of pure sodium fluorosilicate needed = 0.5 ÷ 60.6% = 0.8251 g
Mass of the impure sample of sodium fluorosilicate needed = 0.8251 ÷ 97% = 0.851 g

QUESTION 6: Acids, Bases & Salts
While deionised water does not contain dissolved ionic compounds, it is not pure as carbon dioxide in the air can dissolve in it.

Explain, using equations, what solute particles will be present when carbon dioxide dissolves in deionised water. [2 marks]

CO2 (g) + H2O (l) H2CO3 (aq)

Carbon dioxide is an acidic oxide that dissolves in and reacts with water to form aqueous carbonic acid.

H2CO3 (aq) H+(aq) + HCO3(aq)

As the carbonic acid formed is a weak acid, some molecules ionise partially to form hydrogen ions and bicarbonate ions.

QUESTION 7: Acids, Bases & Salts
Water undergoes partial self-ionisation, whereby some molecules ionise to form both hydrogen ions and hydroxide ions.

H2O(l) H+(aq) + OH(aq)

Explain how water acts as both acids and alkalis during self-ionisation. [2 marks]

As an acid, self-ionisation produces aqueous hydrogen ions. As an alkali, self-ionisation produces aqueous hydroxide ions.

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