Water Purification Practice Questions
20 free Water Purification practice questions for the General Science. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which chemical is commonly used in the process of 'Coagulation' during municipal water treatment to help suspended particles clump together?
- A Sodium chloride
- B Aluminium sulfate
- C Calcium carbonate
- D Potassium permanganate
Correct answer: Aluminium sulfate
Aluminium sulfate, commonly called alum, acts as a coagulant by neutralizing the negative charge on light, suspended particles. This allows them to stick together and form larger 'flocs' that are easier to remove by sedimentation.
The process of 'Reverse Osmosis' (RO) primarily relies on which component to remove dissolved salts and impurities from water?
- A Activated carbon filter
- B Semi-permeable membrane
- C Ultraviolet lamp
- D Ion-exchange resin
Correct answer: Semi-permeable membrane
In Reverse Osmosis, pressure is applied to push water through a semi-permeable membrane that blocks larger molecules and ions. This technology is highly effective for desalination and removing chemical contaminants.
Which gas is often bubbled through water in the final stages of treatment to kill bacteria and viruses without leaving a chemical taste, though it provides no residual protection?
- A Chlorine
- B Chlorine dioxide
- C Ozone
- D Chloramine
Correct answer: Ozone
Ozone ($O_3$) is a powerful oxidizing agent that destroys pathogens more effectively than chlorine. However, because it breaks down quickly, it does not keep the water protected as it travels through the pipe network.
What is the primary function of 'Activated Carbon' in household water purifiers?
- A To kill viruses and bacteria using ultraviolet radiation
- B To remove organic compounds, odors, and chlorine by adsorption
- C To increase the mineral content of the water for a better taste
- D To soften hard water by removing calcium and magnesium ions
Correct answer: To remove organic compounds, odors, and chlorine by adsorption
Activated carbon has a massive surface area with microscopic pores that trap organic pollutants and chemicals. It is particularly effective at removing the 'chlorine' smell and taste from municipal water.
Which of the following methods of water purification is most effective at removing 'Permanent Hardness' caused by magnesium and calcium sulfates?
- A Boiling
- B Simple cloth filtration
- C Ion-exchange method
- D Sedimentation
Correct answer: Ion-exchange method
Boiling only removes temporary hardness (bicarbonates). Ion-exchange resins replace calcium and magnesium ions with sodium or hydrogen ions, effectively softening the water regardless of the type of hardness.
In water treatment, 'Chlorination' is used primarily for disinfection. What is the active germicidal substance formed when chlorine gas dissolves in water?
- A Hydrochloric acid
- B Hypochlorous acid
- C Sodium chlorite
- D Perchloric acid
Correct answer: Hypochlorous acid
When chlorine reacts with water, it forms hypochlorous acid ($HOCl$), which can penetrate the cell walls of microorganisms. This disrupts their internal enzymes and effectively kills them.
Which process involves the conversion of water into vapor and then back into liquid to remove non-volatile impurities and minerals?
- A Chlorination
- B Flocculation
- C Aeration
- D Distillation
Correct answer: Distillation
Distillation separates water from contaminants based on boiling points. Since most minerals and heavy metals do not turn into vapor at $100^\circ C$, they remain in the boiling flask while pure water is collected as condensate.
Why is 'Fluoridation' sometimes practiced in municipal water treatment?
- A To clarify the water and make it transparent
- B To kill the eggs of aquatic parasites
- C To neutralize the pH of acidic rain water
- D To prevent dental cavities in the population
Correct answer: To prevent dental cavities in the population
Controlled amounts of fluoride are added to help strengthen tooth enamel. While controversial in some regions, it is a public health measure aimed at reducing the prevalence of tooth decay.
Which metals are used in 'KDF' (Kinetic Degradation Fluxion) filters to remove heavy metals and chlorine through a redox reaction?
- A Iron and Lead
- B Gold and Silver
- C Aluminium and Nickel
- D Copper and Zinc
Correct answer: Copper and Zinc
KDF filters use high-purity copper-zinc granules. These create an electrochemical reaction that changes harmful contaminants into harmless components, such as turning free chlorine into chloride.
The presence of 'Coliform' bacteria in a water sample is generally used as an indicator of:
- A High mineral content
- B Contamination by industrial chemicals
- C Potential fecal contamination
- D Lack of dissolved oxygen
Correct answer: Potential fecal contamination
Coliforms themselves may not always cause disease, but they are found in the intestines of warm-blooded animals. Their presence suggests that disease-causing pathogens (like E. coli) might also be present in the water.
Which purification method uses electromagnetic radiation to deactivate the DNA of bacteria, rendering them unable to reproduce?
- A Gamma ray irradiation
- B Infrared heat treatment
- C X-ray sterilization
- D Ultraviolet treatment
Correct answer: Ultraviolet treatment
UV-C light at specific wavelengths ($254\text{ nm}$) penetrates the cells of microorganisms and damages their genetic material. This stops them from replicating, effectively neutralizing the threat without adding chemicals.
What is the purpose of the 'Aeration' step in water treatment?
- A To add dissolved salts back into the water
- B To cool the water down before it is piped to homes and businesses
- C To increase the density of the water
- D To release volatile gases and oxidize iron and manganese
Correct answer: To release volatile gases and oxidize iron and manganese
Aeration exposes water to air, which helps release trapped gases like hydrogen sulfide (which smells like rotten eggs). It also helps iron and manganese form insoluble solids that can be filtered out.
In the context of water hardness, 'Temporary Hardness' is caused by the presence of:
- A Calcium and magnesium chloride salts
- B Sodium and potassium sulfates
- C Iron and copper nitrates
- D Calcium and magnesium bicarbonates
Correct answer: Calcium and magnesium bicarbonates
Bicarbonate hardness is called 'temporary' because it can be removed by simply boiling the water. Heat decomposes the bicarbonates into insoluble carbonates, which precipitate as scale.
Which chemical is often used as a 'disinfectant residual' to ensure water remains safe as it travels through long pipelines?
- A Chloramine
- B Chlorine dioxide
- C Ozone
- D Iodine
Correct answer: Chloramine
Chloramines (chlorine combined with ammonia) are more stable and last longer in water distribution systems than free chlorine. They provide a long-lasting residual that prevents bacterial regrowth.
What is the primary disadvantage of 'Boiling' as a sole method of water purification in an urban setting?
- A It makes the water noticeably softer
- B It removes all dissolved oxygen and makes water taste flat
- C It does not remove chemicals like lead or pesticides
- D It destroys the natural minerals in the water
Correct answer: It does not remove chemicals like lead or pesticides
While boiling is excellent for killing biological pathogens (bacteria, viruses, cysts), it can actually increase the concentration of non-volatile chemicals like nitrates or lead as the water evaporates.
The 'SODIS' method is a low-cost water purification technique used in developing countries. What does it stand for?
- A Sodium Disinfection
- B Solar Distillation System
- C Solar Water Disinfection
- D Soil-based Filtration
Correct answer: Solar Water Disinfection
SODIS involves placing contaminated water in clear plastic bottles and exposing them to direct sunlight for several hours. The combination of UV radiation and increased temperature kills most pathogens.
Why is potassium permanganate ($KMnO_4$) added when treating well water?
- A To give the water a pleasant pink tint
- B To replace the dissolved calcium ions with potassium ions
- C To oxidize iron and manganese and remove sulfur odors
- D To lower the boiling point of water
Correct answer: To oxidize iron and manganese and remove sulfur odors
Potassium permanganate is a strong oxidant. It reacts with dissolved iron and manganese to form solids that can be filtered, and it effectively neutralizes hydrogen sulfide gas.
A 'Slow Sand Filter' works through a biological layer that forms on the surface of the sand. What is this layer called?
- A Schmutzdecke
- B Silt layer
- C Algal bloom
- D Coagulant mat
Correct answer: Schmutzdecke
The schmutzdecke is a gelatinous biofilm containing bacteria, fungi, and protozoa. This biological layer is what actually does the majority of the filtration and pathogen removal in slow sand systems.
Which unit is commonly used to measure the turbidity (cloudiness) of water?
- A NTU (Nephelometric Turbidity Units)
- B ppm (parts per million of dissolved solids)
- C mS/cm (milliSiemens per centimetre)
- D pH (acidity scale)
Correct answer: NTU (Nephelometric Turbidity Units)
NTU measures the amount of light scattered by suspended particles in a water sample. Low turbidity is an indicator of efficient filtration and is essential for effective disinfection.
Which of the following is a byproduct of chlorination that can be harmful to human health over long-term exposure?
- A Trihalomethanes (THMs)
- B Dissolved oxygen
- C Sodium bicarbonate
- D Calcium sulfate crystals
Correct answer: Trihalomethanes (THMs)
When chlorine reacts with naturally occurring organic matter in water, it can form THMs. These disinfection byproducts are regulated because they are considered potential carcinogens.