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Leaving Cert Chemistry: Water treatment

How often Water treatment comes up on the Chemistry papers, every year it was asked, and questions to try.

HL Asked on 10 of the last 10 Higher Level papers, most recently in 2026. banker

OL Asked on 10 of the last 10 Ordinary Level papers, most recently in 2026. banker

June 2027 is the first exam on the new Chemistry course. These come from the 2016–2026 papers, so read them as a guide.

Water treatment, Higher Level(7 marks)

Quick ones on Water treatment.

(a)Chemical added at a water treatment plant to kill bacteria?
  1. Aluminium sulfate
  2. Chlorine
  3. Sodium carbonate
(b)Chemical added during water treatment to make fine suspended particles clump together?
  1. Sodium chloride
  2. Chlorine
  3. Aluminium sulfate
(c)Ions that cause hardness in water?
  1. Calcium and magnesium ions
  2. Chloride and nitrate ions
  3. Sodium and potassium ions
Show the answers

(a) Chlorine

(b) Aluminium sulfate

(c) Calcium and magnesium ions

Your turn: Higher Level questions on Water treatment.

Higher Level

Asked on 10 of the last 10 Higher Level papers, most recently in 2026. banker

Every paper, year by year

YearWhere it came up
2026Q3, Q4(h)
2025Q11(a)
2024Q4(h), Q11(c)
2023Q7
2022Q1, Q4(h)
2021Q11(b)
2019Q4(j)
2018Q1
2017Q4(j)
2016Q9

Links open the State Examinations Commission’s paper for that year.

More Water treatment questions

Water treatment, 2 marks

Why is chlorine still added to water that has had UV treatment?

  1. UV treatment makes the water acidic, and chlorine corrects it
  2. Chlorine removes any UV light left in the water
  3. It leaves a residual disinfectant that protects water in the pipes
Show the answer

It leaves a residual disinfectant that protects water in the pipes

UV kills micro-organisms only as the water passes the lamps. A small amount of chlorine keeps the water safe all the way to the tap.

Water treatment, 2 marks

Why is clean drinking water described as a finite resource?

  1. Sea water can be drunk without any treatment at all
  2. Supplies of clean fresh water are limited and treating it uses energy
  3. Rain constantly makes new water, so supplies can never run out
Show the answer

Supplies of clean fresh water are limited and treating it uses energy

Most of Earth's water is salty or frozen. Pumping and treating water has an energy cost and a carbon footprint.

Water treatment, 2 marks

Why is the pH of treated drinking water adjusted before it leaves the plant?

  1. To stop it corroding pipes
  2. To kill any remaining bacteria
  3. To add fluoride to the water
Show the answer

To stop it corroding pipes

Water that is too acidic attacks metal pipes and can dissolve lead or copper, so lime or acid is used to adjust the pH.

Ordinary Level

Asked on 10 of the last 10 Ordinary Level papers, most recently in 2026. banker

Every paper, year by year

YearWhere it came up
2026Q4(d), Q7, Q11(c)
2025Q4(i), Q10(a)
2024Q4(h), Q9
2023Q3, Q4(i), Q11(a)
2022Q4(e), Q11(a)
2021Q7
2019Q3, Q4(i), Q4(j), Q9
2018Q4(h), Q7
2017Q10(a)
2016Q4(i), Q9

Links open the State Examinations Commission’s paper for that year.

More Water treatment questions

Water treatment, 2 marks

Stage in which small suspended particles clump together into larger ones?

  1. Screening
  2. Fluoridation
  3. Flocculation
Show the answer

Flocculation

A flocculating agent is added. The particles clump into flocs that settle to the bottom of the tank.

Water treatment, 2 marks

What is water passed through during filtration at a treatment plant?

  1. Cotton wool pads
  2. Beds of sand and gravel
  3. Sheets of filter paper
Show the answer

Beds of sand and gravel

Sand and gravel beds trap the fine particles still left after settling. They are cleaned by pumping water back through them.

Water treatment, 2 marks

One disadvantage of hard water?

  1. It forms limescale in kettles and pipes
  2. It is poisonous to drink in large amounts
  3. It has a pH of about 2, so it is acidic
Show the answer

It forms limescale in kettles and pipes

Hard water also forms scum with soap, so more soap is needed. Its calcium is good for bones and teeth.

Other Chemistry topics

All of Leaving Cert Chemistry