22 Water Chemistry & Balance Practice Questions & Answers

Every Water Chemistry & Balance practice question from the Pool Operator (CPO) Practice Test, with the correct answer and a short explanation.

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1. Pool pH is reported on a logarithmic scale. If a pool's pH falls from 7.5 to 6.5, what has happened to the water?
- A.The hydrogen ion concentration is unchanged; only total alkalinity moved
- B.The water is exactly 10 percent more acidic than before
- C.The hydrogen ion concentration is now ten times greater than before✓ Answer
- D.The hydrogen ion concentration is now one tenth of what it was
Each whole pH unit is a tenfold change in hydrogen ion concentration, so a one-unit drop multiplies acidity by ten. That is why a pH that looks only slightly low is actually a large chemical change, and why corrosivity and base demand climb so fast at the low end.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: the pH scaleReport a problem with this question
2. An operator adds a small amount of acid and the pH plunges far past the intended value, then drifts back up over the next day. Which condition best explains this behavior?
- A.Calcium hardness is too high, so the water resists chemical change
- B.Total alkalinity is low, so the water has little buffering capacity against pH change✓ Answer
- C.The water is too warm for the acid to react properly
- D.Total dissolved solids are too low to carry the acid through the pool
Total alkalinity is the reserve of bicarbonate and carbonate that neutralizes added acid or base. When it is low there is almost nothing to absorb the addition, so pH swings wildly on small doses and then bounces as carbon dioxide off-gasses.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: total alkalinity as bufferReport a problem with this question
3. Every time acid brings a pool's pH down it climbs again within a day, and acid demand is constant. Total alkalinity tests far above the operator's working target. What is the most likely explanation?
- A.The pH reagent reads low because of dissolved solids
- B.Excess carbonate alkalinity holds a large carbon dioxide reserve that off-gasses and pushes pH back up, so total alkalinity has to be brought down first✓ Answer
- C.Calcium hardness must be raised before pH will hold
- D.High alkalinity has no effect on pH; the pool must be losing acid through the filter
Water carrying excessive carbonate alkalinity releases carbon dioxide readily, and losing carbon dioxide raises pH. Until total alkalinity itself is lowered, each acid addition only buys a day, which is why alkalinity is corrected before pH.
Source: PHTA Recreational Water Quality Committee guidance on total alkalinity in pool and spa waterReport a problem with this question
4. An operator needs to raise total alkalinity while moving pH as little as possible. Which product fits that job, and why?
- A.Muriatic acid, because it adds carbonate to the water
- B.Soda ash, because the carbonate ion has almost no effect on pH
- C.Sodium bicarbonate, because it adds bicarbonate alkalinity while sitting close to the pool's own pH✓ Answer
- D.Calcium chloride, because it buffers the water
Sodium bicarbonate dissolves to bicarbonate, the main component of total alkalinity, and its own solution pH is close to normal pool pH, so it lifts the buffer with only a slight pH nudge. Soda ash, by contrast, drives pH up strongly.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: alkalinity adjustment chemicalsReport a problem with this question
5. Compared with sodium bicarbonate, how does sodium carbonate (soda ash) behave when added to pool water?
- A.It raises total alkalinity but always lowers pH
- B.It raises pH strongly and raises total alkalinity as well✓ Answer
- C.It lowers both pH and total alkalinity
- D.It changes pH only and cannot affect total alkalinity
Carbonate is a stronger base than bicarbonate, so soda ash lifts pH sharply and also counts toward the alkalinity titration. This is the classic case of one chemical moving two parameters, so the operator must re-check alkalinity after using it.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: pH adjustment chemicalsReport a problem with this question
6. Muriatic acid is added to bring a high pH down. What else does that addition do to the water?
- A.It raises total alkalinity while lowering pH
- B.It lowers total alkalinity along with pH✓ Answer
- C.It affects pH only and leaves every other parameter untouched
- D.It raises calcium hardness
Acid neutralizes the bicarbonate and carbonate that make up total alkalinity, so pH and alkalinity fall together; dry acid (sodium bisulfate) does the same. The operator therefore plans acid additions knowing the buffer will shrink too.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: effect of acids on pH and alkalinityReport a problem with this question
7. A pool's total alkalinity is exactly where the operator wants it, but pH is low. Which action raises pH without adding alkalinity?
- A.Add calcium chloride
- B.Aerate the water with features or a fountain to drive off carbon dioxide✓ Answer
- C.Add sodium bicarbonate
- D.Lower the water temperature
Dissolved carbon dioxide forms carbonic acid, so stripping it out by aeration raises pH while the bicarbonate that the alkalinity test measures stays in the water. That makes aeration the standard tool when pH is low but alkalinity is already correct.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: aeration and carbon dioxideReport a problem with this question
8. A facility feeds carbon dioxide for pH control. What long-term side effect should the operator expect?
- A.Calcium hardness falls steadily
- B.Cyanuric acid accumulates in the water
- C.Total alkalinity climbs over time as the carbon dioxide converts to bicarbonate, eventually calling for acid or dilution✓ Answer
- D.Total alkalinity falls with every feed
Carbon dioxide lowers pH by forming carbonic acid, but that acid ends up as bicarbonate, which is measured as total alkalinity. Unlike a mineral acid, carbon dioxide feed therefore builds the buffer up instead of tearing it down.
Source: PHTA Pool & Spa Operator Handbook, chemical feed chapter: carbon dioxide pH controlReport a problem with this question
9. A stabilized outdoor pool tests 110 ppm total alkalinity and 100 ppm cyanuric acid. The cyanurate correction factor for this water's pH is 0.31. What carbonate alkalinity value should be carried into the saturation index?
- A.110 ppm
- B.10 ppm
- C.141 ppm
- D.79 ppm✓ Answer
Multiply cyanuric acid by the correction factor and subtract it from total alkalinity: 100 x 0.31 = 31, and 110 - 31 = 79 ppm. Cyanurate titrates as alkalinity but does not participate in calcium carbonate saturation, so leaving it in makes the index look more scale-forming than the water really is.
Source: PHTA Recreational Water Quality Committee guidance on carbonate alkalinity correction for cyanuric acidReport a problem with this question
10. Water tests show total alkalinity low, pH low, and calcium hardness low. Which adjustment does the operator make first, and why?
- A.Calcium hardness, because it is the anchor of the index
- B.Water temperature, because it drives all the other factors
- C.pH, because it is the only parameter swimmers can feel
- D.Total alkalinity, because correcting it also moves pH and makes every later adjustment hold✓ Answer
Alkalinity is the buffer, so setting it first stops pH from bouncing and often carries pH part of the way on its own. The standard sequence is total alkalinity, then pH, then calcium hardness, with temperature treated as a given rather than something adjusted.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: order of adjustmentReport a problem with this question
11. A plaster pool filled from a very soft water supply shows a rough, etched finish and grout washing out of the tile line. What is happening chemically?
- A.Calcium hardness is too high and is depositing on the plaster, roughening it
- B.Calcium hardness is too low, so the water dissolves calcium out of the plaster and grout to satisfy its demand✓ Answer
- C.Dissolved solids are physically pitting the surface
- D.The water is over-buffered, and buffering erodes plaster
Water low in calcium is undersaturated with calcium carbonate and will take calcium from whatever it touches, including plaster, grout and tile mortar. Raising calcium hardness with calcium chloride, along with alkalinity and pH, removes the aggressive driving force.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: calcium hardness and aggressive waterReport a problem with this question
12. Calcium hardness has climbed far above the operator's working target and scale is forming at the waterline. What is the practical way to bring calcium hardness down?
- A.Add sodium bicarbonate to precipitate the calcium
- B.Drain part of the water and refill with lower-hardness water✓ Answer
- C.Hold pH below 7 until the calcium reading drops
- D.Run the filter continuously to strain the calcium out
No routine chemical removes dissolved calcium and a filter cannot strain out a dissolved ion, so dilution with softer water is the practical correction. Until water can be replaced, the operator offsets the index by holding pH and alkalinity toward the lower end of their working ranges.
Source: PHTA Recreational Water Quality Committee guidance on calcium hardness and water replacementReport a problem with this question
13. A facility's fill water is already very hard. Which chlorine choice keeps calcium hardness from rising further, and why?
- A.Sodium hypochlorite or trichlor, because neither one carries calcium into the water✓ Answer
- B.It makes no difference; every chlorine product adds the same calcium
- C.Calcium hypochlorite blended with calcium chloride, to keep hardness steady
- D.Calcium hypochlorite, because the calcium it carries precipitates out immediately
Calcium hypochlorite delivers calcium along with every ppm of chlorine, so in hard fill water it steadily pushes hardness and the saturation index upward. Sodium hypochlorite and trichlor contain no calcium, which is why they are preferred where hardness is already high.
Source: PHTA Recreational Water Quality Committee fact sheet on calcium hypochloriteReport a problem with this question
14. An operator computes the saturation index as SI = pH + temperature factor + calcium factor + alkalinity factor - dissolved solids factor. The readings convert to pH 7.2, temperature factor 0.7, calcium factor 1.9, carbonate alkalinity factor 2.0, and dissolved solids factor 12.3. What is the index and what tendency does it describe?
- A.-0.5; the water tends to be corrosive and aggressive to plaster and metal✓ Answer
- B.-0.5; the water tends to deposit scale on the heater
- C.0.0; the water has no tendency in either direction
- D.+0.5; the water tends to deposit scale
Adding the four positive terms gives 7.2 + 0.7 + 1.9 + 2.0 = 11.8, and 11.8 - 12.3 = -0.5. A negative index means the water is undersaturated with calcium carbonate, so it dissolves calcium from plaster, grout and equipment rather than depositing it.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: saturation index calculationReport a problem with this question
15. Using SI = pH + temperature factor + calcium factor + alkalinity factor - dissolved solids factor, a heated pool converts to pH 7.7, temperature factor 0.9, calcium factor 2.2, alkalinity factor 2.0, and dissolved solids factor 12.4. What is the index?
- A.-0.4
- B.+1.2
- C.+0.4✓ Answer
- D.+0.8
The positive terms total 7.7 + 0.9 + 2.2 + 2.0 = 12.8, and 12.8 - 12.4 = +0.4. A positive result means the water leans toward depositing calcium carbonate rather than dissolving it, and each factor is simply looked up and added, never averaged or weighted.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: saturation index worked examplesReport a problem with this question
16. A spa measures 104 degrees F. The saturation-index temperature factors are published only at set intervals; the listed value just below the reading is 0.7 and the listed value just above it is 0.9. Which value does the operator use?
- A.0.9, because when a measured value is not listed the next greatest listed value is used✓ Answer
- B.0.8, by averaging the two listed values
- C.Neither; temperature is left out of the index when the reading is not listed
- D.0.7, because the reading is closer to the lower listed temperature
The published rule is to use the next greatest listed value when a measurement does not appear, which keeps the calculation conservative and consistent between operators. Interpolating or rounding down produces a different index than the method intends.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: using the index factor valuesReport a problem with this question
17. An operator uses the lowest possible dissolved-solids factor when calculating the saturation index for a salt-chlorinated pool carrying several thousand ppm of dissolved solids. What is wrong with that?
- A.The dissolved-solids factor is a fixed constant, so nothing is wrong
- B.Salt pools are exempt from the saturation index
- C.The dissolved-solids factor increases as TDS rises, so using the lowest value subtracts too little and reports the index as higher, and less aggressive, than the water really is✓ Answer
- D.In salt water the dissolved-solids factor should be added instead of subtracted
The dissolved-solids term is a lookup that steps upward as TDS rises, and it is the value subtracted from the sum, so a too-small factor inflates the result. Salt-generated pools run high dissolved solids by design, which is exactly where treating the factor as a constant misleads the operator.
Source: PHTA Water Balance Fact Check on saturation index errorsReport a problem with this question
18. Over a long season a pool's total dissolved solids rise far above the level measured in the source water at start-up. What should the operator expect, and what actually lowers TDS?
- A.TDS does not affect water balance and falls on its own as water evaporates
- B.Higher TDS raises total alkalinity; it is lowered with acid
- C.Higher conductivity and water that is more corrosive and harder to keep clear; only dilution, replacing part of the water, lowers TDS✓ Answer
- D.Higher TDS makes water less corrosive; it is lowered by adding sequestrant
Evaporation removes water and leaves the solids behind, so dissolved solids only concentrate; no chemical destroys them. High TDS raises conductivity and corrosivity and feeds the dissolved-solids term of the saturation index, so replacing water is the only real correction.
Source: PHTA Recreational Water Quality Committee guidance on total dissolved solids and water replacementReport a problem with this question
19. A test kit reports total hardness. What does the operator actually need for the saturation index?
- A.Either reading; the two are interchangeable
- B.Total hardness, since magnesium behaves the same as calcium
- C.Total hardness minus total alkalinity
- D.Calcium hardness alone, because the index predicts calcium carbonate saturation and magnesium does not form that scale✓ Answer
The saturation index describes calcium carbonate solubility, so only the calcium share of hardness belongs in it. Substituting total hardness inflates the calcium term and makes water look more scale-forming, and therefore safer, than it actually is.
Source: PHTA Water Balance Fact Check on saturation index errorsReport a problem with this question
20. An unheated outdoor pool sits at cold water temperature in the off-season with pH, alkalinity and calcium hardness all mid-range, yet the calculated index is clearly negative. What is going on and what can the operator do?
- A.Cold water lowers the index because calcium carbonate is more soluble; since temperature is not an adjustable parameter, the operator offsets it by nudging pH, alkalinity or calcium hardness upward✓ Answer
- B.Cold water raises the index, so scale is the real risk here
- C.The calculation must be wrong; mid-range parameters always give a balanced index
- D.Temperature is not part of the saturation index at all
Temperature is one of the terms in the index, and cold water holds calcium carbonate in solution more readily, pushing the result negative even when every other reading looks fine. Because an operator does not heat a pool to fix chemistry, the offset has to come from the parameters that can be dosed.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: temperature as an index factorReport a problem with this question
21. Scale has been forming inside a pool heater. Repeat tests now show calcium hardness and alkalinity lower than a week earlier, and the index closer to zero. Why?
- A.The heater consumes alkalinity and releases calcium
- B.Scale formation raises calcium hardness in the water
- C.Test reagents lose strength whenever water is scaling
- D.Precipitating calcium carbonate pulls calcium and carbonate out of solution, which lowers the index; the water moves back toward saturation on its own, but the deposit on the heater stays✓ Answer
Scale is calcium carbonate leaving the water, so both the calcium and the carbonate alkalinity readings drop and the index falls toward zero. The chemistry self-corrects, but the operator still has to remove the deposit and fix the conditions that caused it.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: precipitation and the indexReport a problem with this question
22. A pool tests pH 7.0, total alkalinity 50 ppm, calcium hardness 120 ppm, with cool water. The plaster is roughening and metal fittings are pitting. Which corrective step comes first, and why?
- A.Do nothing; each reading is acceptable on its own
- B.Raise calcium hardness first, since it is the largest term in the index
- C.Raise pH first with soda ash and re-test everything the next day
- D.Raise total alkalinity first, because a restored buffer keeps the pH correction from bouncing and moves the index upward at the same time✓ Answer
All four terms here push the index negative, and the symptoms are classic aggressive-water damage. Correcting total alkalinity first restores the buffer so the pH adjustment holds, after which pH and then calcium hardness are set and the index is recalculated.
Source: PHTA Pool & Spa Operator Handbook, water balance chapter: corrective sequence for aggressive waterReport a problem with this question

Practice questions based on the Pool & Hot Tub Alliance (PHTA) Pool & Spa Operator Handbook and public CDC healthy-swimming guidance. CPO and Certified Pool/Spa Operator are marks of the Pool & Hot Tub Alliance; this site is not affiliated with or endorsed by PHTA. Water-quality limits, turnover requirements and bather loads are set by the health code where your facility operates — always follow that code and your local health department, and confirm current course requirements before testing. About the CPO certification →

1. Pool pH is reported on a logarithmic scale. If a pool's pH falls from 7.5 to 6.5, what has happened to the water?

2. An operator adds a small amount of acid and the pH plunges far past the intended value, then drifts back up over the next day. Which condition best explains this behavior?

3. Every time acid brings a pool's pH down it climbs again within a day, and acid demand is constant. Total alkalinity tests far above the operator's working target. What is the most likely explanation?

4. An operator needs to raise total alkalinity while moving pH as little as possible. Which product fits that job, and why?

5. Compared with sodium bicarbonate, how does sodium carbonate (soda ash) behave when added to pool water?

6. Muriatic acid is added to bring a high pH down. What else does that addition do to the water?

7. A pool's total alkalinity is exactly where the operator wants it, but pH is low. Which action raises pH without adding alkalinity?

8. A facility feeds carbon dioxide for pH control. What long-term side effect should the operator expect?

9. A stabilized outdoor pool tests 110 ppm total alkalinity and 100 ppm cyanuric acid. The cyanurate correction factor for this water's pH is 0.31. What carbonate alkalinity value should be carried into the saturation index?

10. Water tests show total alkalinity low, pH low, and calcium hardness low. Which adjustment does the operator make first, and why?

11. A plaster pool filled from a very soft water supply shows a rough, etched finish and grout washing out of the tile line. What is happening chemically?

12. Calcium hardness has climbed far above the operator's working target and scale is forming at the waterline. What is the practical way to bring calcium hardness down?

13. A facility's fill water is already very hard. Which chlorine choice keeps calcium hardness from rising further, and why?

15. Using SI = pH + temperature factor + calcium factor + alkalinity factor - dissolved solids factor, a heated pool converts to pH 7.7, temperature factor 0.9, calcium factor 2.2, alkalinity factor 2.0, and dissolved solids factor 12.4. What is the index?

16. A spa measures 104 degrees F. The saturation-index temperature factors are published only at set intervals; the listed value just below the reading is 0.7 and the listed value just above it is 0.9. Which value does the operator use?

17. An operator uses the lowest possible dissolved-solids factor when calculating the saturation index for a salt-chlorinated pool carrying several thousand ppm of dissolved solids. What is wrong with that?

18. Over a long season a pool's total dissolved solids rise far above the level measured in the source water at start-up. What should the operator expect, and what actually lowers TDS?

19. A test kit reports total hardness. What does the operator actually need for the saturation index?

20. An unheated outdoor pool sits at cold water temperature in the off-season with pH, alkalinity and calcium hardness all mid-range, yet the calculated index is clearly negative. What is going on and what can the operator do?

21. Scale has been forming inside a pool heater. Repeat tests now show calcium hardness and alkalinity lower than a week earlier, and the index closer to zero. Why?

22. A pool tests pH 7.0, total alkalinity 50 ppm, calcium hardness 120 ppm, with cool water. The plaster is roughening and metal fittings are pitting. Which corrective step comes first, and why?