Quiz & test questions Grade 10 Science
Lake Everhart Raw Water Chemistry and Testing Assessment
A grade 10 chemistry assessment mixing multiple-choice, short-answer, and computation items on pH, dissolved oxygen, and water's solvent properties.
Objectives
- Explain, using molecular polarity, why water dissolves both ionic salts and atmospheric gases such as oxygen.
- Interpret raw-water pH and dissolved-oxygen readings against real ecological and regulatory benchmarks.
- Convert a dissolved-oxygen concentration between mg/L and parts per million and calculate a percent change.
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.
- pH
- pH is a number that tells whether water is acidic, like lemon juice, or basic, like baking soda mixed in water. Pure water sits in the middle of the scale at a pH of 7.
- Surface tension
- Surface tension is the way water molecules pull together at the surface, making the surface act a little like a stretchy skin. It is why some insects can walk on water and why water forms rounded drops instead of flat puddles.
- 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.
- Parts per million (ppm)
- Parts per million tells how many tiny bits of a substance are mixed into a million bits of water. Scientists use it to measure very small amounts of chemicals in water.
- Raw water
- Raw water is water taken from a river, lake, or well before it has been treated. It may look clean, but it is not ready to drink yet.
Teacher overview
This 40-minute assessment (24 points total) checks whether students can connect water's molecular structure to its behavior as a solvent and can interpret raw-water chemistry readings against real benchmarks. It mixes three multiple-choice items, three short-answer items, and two computation items, all built around a fictional reservoir's water-testing records.
Assessment
Lake Everhart is a fictional reservoir invented for this assessment; it supplies the intake for a regional treatment plant. Technicians recorded the readings below across one week in early summer.
(2 points) Which property of water best explains why it can dissolve both table salt, an ionic compound, and atmospheric oxygen gas?
- A) Water molecules are nonpolar and repel charged particles.
- B) Water molecules are polar, so their partially charged ends attract and surround charged or polar particles and pull them apart.
- C) Water has a very low boiling point compared with other liquids of similar molecular mass.
- D) Water is a metal and conducts electricity directly.
(3 points) Water's strong hydrogen bonding also produces surface tension, the bulk property that lets some insects walk on still water and lets water bead on a waxed surface. In two or three sentences, explain how the strength of the hydrogen bonds between individual water molecules produces this bulk-scale behavior.
(3 points) A technician at Lake Everhart's intake reports a dissolved-oxygen reading of 7.2 mg/L. Convert this reading to parts per million (ppm), using the equivalence that in fresh water, 1 mg/L is approximately equal to 1 ppm. Show your reasoning.
(3 points) Most fish need dissolved oxygen above 5 mg/L to survive, and cold water holds more dissolved oxygen than warm water. A summer heat wave raises Lake Everhart's temperature and lowers its dissolved oxygen from 7.2 mg/L to 3.8 mg/L. By what percent did the dissolved-oxygen concentration decrease? Round to the nearest whole percent.
(2 points) Based on your answer to item 4, is Lake Everhart's water still safe for most fish species after the heat wave? Use the 5 mg/L benchmark in your answer.
(3 points) A raw-water sample from Lake Everhart has a pH of 8.9. Compared with the secondary drinking-water standard range of 6.5 to 8.5, this sample's pH is
- A) within the standard range
- B) slightly below the standard range
- C) slightly above the standard range
- D) at neutral pH, matching the standard exactly
(4 points) Fertilizer runoff after a storm raised nutrient levels in Lake Everhart, and technicians linked this event to the drop in dissolved oxygen recorded in item 4. In two to three sentences, explain the chain of events connecting nutrient runoff to lower dissolved oxygen.
(4 points) Which single change would most directly help raise the lake's dissolved-oxygen concentration back toward its pre-heat-wave level?
- A) Adding more fertilizer to encourage plant growth
- B) Mechanically aerating the water to increase oxygen exchange with the atmosphere
- C) Raising the water's temperature further
- D) Filtering out all dissolved minerals
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: B) Water molecules are polar, so their partially charged ends attract and surround charged or polar particles and pull them apart. Polarity lets water surround and separate the sodium and chloride ions in salt. Small nonpolar gas molecules such as oxygen dissolve only at low concentrations, through weaker attractions to the polar water molecules.
Answer: Accept any explanation that connects strong particle-level attraction to the bulk-scale result: because hydrogen bonds pull neighboring water molecules toward one another, molecules at the surface are pulled inward and sideways more than outward, creating a taut, film-like surface. The stronger those individual bonds are, the stronger the resulting surface tension.
Answer: 7.2 ppm. Because 1 mg/L is approximately equal to 1 ppm in fresh water such as a lake, a reading of 7.2 mg/L converts directly to 7.2 ppm with no additional math required.
Answer: 47%. (7.2 − 3.8) ÷ 7.2 = 3.4 ÷ 7.2 ≈ 0.472, which rounds to about 47%.
Answer: No. At 3.8 mg/L, the lake's dissolved oxygen is below the 5 mg/L benchmark most fish need, so the water is likely to stress or kill fish that cannot move to better-oxygenated water.
Answer: C) slightly above the standard range. The standard range runs from 6.5 to 8.5, and 8.9 sits just above the upper end of that range.
Answer: Accept any answer describing the chain: nutrients from fertilizer runoff fuel rapid growth of algae (a bloom); when the algae die, bacteria decompose the dead algae and consume dissolved oxygen in the process, which is why the oxygen reading dropped alongside the nutrient increase.
Answer: B) Mechanically aerating the water to increase oxygen exchange with the atmosphere. Aeration directly increases the contact between water and air, raising dissolved oxygen, while the other options either add more nutrients, raise temperature further, or address minerals rather than oxygen.
Differentiation
Support (below grade level)
Provide a labeled diagram showing a water molecule's polar ends before item 1, and give students the mg/L-to-ppm equivalence written out as a rule ("in fresh water, 1 mg/L = about 1 ppm") before item 3. Allow item 4's percent calculation to be checked with a calculator, and accept a two-word answer ("no, unsafe") for item 5 if the reasoning is discussed aloud.
Extension (above grade level)
Ask students to decide, using only the readings given in this assessment, what dissolved-oxygen level Lake Everhart would need to recover to have a comfortable safety margin above the 5 mg/L benchmark, and to propose a second cause, besides temperature, that can lower dissolved oxygen in a lake.
ELL supports
Pre-teach "dissolved oxygen," "surface tension," and "nutrient" with simple diagrams before the assessment begins. Allow items 2 and 7, which ask for explanations, to be answered with a labeled sketch plus a short phrase rather than full sentences.