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Quiz & test questions Grade 11 Science

Westmere Water Chemistry Unit Test: Quality and Treatment

A 30-point grade 11 unit test on a fictional utility's water chemistry, mixing multiple-choice, short-answer, and computation items on quality parameters and treatment.

Objectives

  1. Students will identify key water-quality benchmarks for dissolved oxygen, pH, and turbidity.
  2. Students will use the pounds formula and the chlorine-demand relationship in computations.
  3. Students will explain hardness, alkalinity, and nutrient pollution in their own words.

Vocabulary

Dissolved oxygen
Dissolved oxygen is the oxygen gas mixed into water that fish and other underwater animals breathe. Cold, moving water holds more oxygen than warm, still water.
Alkalinity
Alkalinity is water's ability to resist becoming acidic, acting like a buffer that keeps pH from swinging wildly when acids are added. Water with good alkalinity helps protect fish and other aquatic life from sudden pH changes.
Eutrophication
Eutrophication is what happens when a lake or bay gets overfed with nutrients: algae grows out of control, and when it dies and rots, the water loses the oxygen that fish need.
Turbidity
Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.

Also: chlorine demand, hardness

Teacher overview

This unit test is worth 30 points and checks whether students can recall key water-quality benchmarks, apply treatment math, and explain the chemistry behind common parameters. It mixes three multiple-choice items, three short-answer items, and three computation items, all set at the fictional Westmere utility. The test draws on regulatory benchmarks that students should know: the dissolved-oxygen level most fish need, the secondary pH range, the chlorine residual limit, and the finished-water turbidity standard. Point values are marked on each item, and the answer key shows full reasoning for the computation items and the reason behind each correct multiple-choice answer.

Westmere Water Chemistry Unit Test: Quality and Treatment — Grade 11 Science
Name: ____________________________ Date: ______________

Assessment

The Westmere utility is a fictional water system. Use the pounds formula, loading (lbs/day) = concentration (mg/L) x flow (MGD) x 8.34, and chlorine demand = dose minus residual, where needed.

  1. (2 points) Most fish need dissolved oxygen above which level?
    • A) 0.3 mg/L
    • B) 5 mg/L
    • C) 15 mg/L
    • D) 40 mg/L
  2. (2 points) The secondary (aesthetic) standard sets drinking-water pH in which range?
    • A) 0 to 7
    • B) 6.5 to 8.5
    • C) 8.5 to 10
    • D) exactly 7
  3. (4 points) Westmere's plant treats 2.5 MGD and applies a chlorine dose of 3.0 mg/L. Using the pounds formula, how many pounds of chlorine per day are fed?
  4. (4 points) The chlorine residual leaving Westmere's clearwell is 1.1 mg/L after the 3.0 mg/L dose. What is the chlorine demand of the water?
  5. (2 points) The maximum residual disinfectant level for chlorine is 4.0 mg/L. Westmere's 1.1 mg/L residual is:
    • A) above the limit, a violation
    • B) well within the limit
    • C) exactly at the limit
    • D) below the minimum allowed
  6. (4 points) Westmere adds a coagulant to lower turbidity before disinfection. Explain why high raw-water turbidity makes disinfection less effective.
  7. (4 points) Explain the difference between hardness and alkalinity, and why a water can be high in one but low in the other.
  8. (4 points) Nitrogen and phosphorus are nutrients that plants need, yet Westmere's lake managers treat them as pollutants. Using the term eutrophication, explain how a nutrient can be both.
  9. (4 points) Last month, 60 of Westmere's 62 finished-water samples were at or below 0.3 NTU. What percent of samples complied, and did the plant meet the requirement that at least 95 percent of samples stay at or below 0.3 NTU?

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: B) 5 mg/L.
    Most fish need dissolved oxygen above 5 mg/L; below that, many species become stressed.
  2. Answer: B) 6.5 to 8.5.
    The secondary drinking-water standard for pH is 6.5 to 8.5, an aesthetic range rather than a health limit.
  3. Answer: 62.55 lbs/day.
    3.0 mg/L x 2.5 MGD x 8.34 = 7.5 x 8.34 = 62.55 pounds of chlorine per day.
  4. Answer: 1.9 mg/L.
    Chlorine demand = dose minus residual = 3.0 - 1.1 = 1.9 mg/L.
  5. Answer: B) well within the limit.
    A residual of 1.1 mg/L is far below the 4.0 mg/L maximum residual disinfectant level, so it is well within the limit, not a violation.
  6. Answer: Particles shield microbes from the disinfectant.
    High turbidity means many suspended particles, and microbes can hide on or inside those particles where chlorine or another disinfectant cannot reach them, so cloudy water must be clarified first for disinfection to work reliably.
  7. Answer: Hardness is dissolved calcium and magnesium; alkalinity is acid-neutralizing capacity.
    Hardness measures dissolved calcium and magnesium ions, while alkalinity measures the carbonate and bicarbonate that neutralize acid; a water can be high in one but low in the other because the ions that cause hardness are not the same ions that provide alkalinity.
  8. Answer: Nutrients feed excess algae, which is eutrophication.
    Nitrogen and phosphorus are needed for plant and algae growth, but in excess they trigger eutrophication, an overgrowth of algae whose later decay removes oxygen from the water and harms fish, so the same nutrients become pollutants when there are too many.
  9. Answer: 96.8%, and yes.
    60 divided by 62 = 0.968, or about 96.8 percent compliant; since 96.8 percent is above the 95 percent requirement, the plant met the standard.

Differentiation

Support

Provide a one-page reference sheet with the pounds formula, the chlorine-demand relationship, and the four benchmark values (dissolved oxygen, pH range, chlorine limit, turbidity standard) so students can focus on applying them, and allow a calculator on all computation items.

Extension

Suppose the water's chlorine demand stayed at 1.9 mg/L. Ask students to calculate what chlorine dose Westmere would need to leave a 2.0 mg/L residual, and to state whether that 2.0 mg/L residual would be within the maximum residual disinfectant level and why the limit applies to the residual rather than the dose.

ELL supports

Pre-teach "dissolved oxygen," "residual," "hardness," "alkalinity," and "eutrophication" with a picture glossary before the test. Allow the short-answer items (6, 7, 8) to be answered with a labeled diagram or short phrases, and provide sentence frames where the standard permits.