Lab / experiment Grade 9 Science

Verrin River Thermal Mixing: Heat, Equilibrium, and Oxygen

A grade 9 lab where students mix warm and cold water to find equilibrium temperature, modeling a power plant's thermal mixing zone and its effect on dissolved oxygen.

Grade: Grade 9
Subject: Science
Material: Lab / experiment
Time: 45 min
Prep: Moderate prep
Materials: Three foam or insulated cups per group, one holding at least 400 mL, Two thermometers reading 0-100 C per group, Safety goggles, Warm tap water about 40-50 C, Cold water, Graduated cylinder or measuring cup marked to 250 mL, Stopwatch or timer, Stirring stick

Standards

HS-PS3-4 NGSS

Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution among the components in the system (second law of thermodynamics).

How this material aligns: Students mix measured volumes of warm and cold water, predict the equilibrium temperature, and compare it with the measured value in the data table, evidence that thermal energy spreads toward a more uniform distribution.

All materials for HS-PS3-4 →

HSN-Q.A.3 Common Core Math

Choose a level of accuracy appropriate to limitations on measurement when reporting quantities.

How this material aligns: Analysis item 1 asks students to report their predicted temperature to an accuracy matched to the thermometer's precision rather than overstating the decimal places.

All materials for HSN-Q.A.3 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will predict and measure the equilibrium temperature reached when warm and cold water are mixed.
Students will use a weighted average to explain unequal-volume mixing and connect warming to lower dissolved oxygen.

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.

Also: thermal energy, equilibrium temperature, weighted average, mixing zone

Teacher overview

When a power plant releases warm cooling water into a cooler river, the two flows mix and settle to an in-between temperature, forming what scientists call a thermal mixing zone. This lab models that process on a small scale: students mix warm and cold water, predict the temperature the mixture will reach, and compare it with the measured result. Because cold water holds more dissolved oxygen than warm water, and most fish need dissolved oxygen above 5 mg/L, the lab ends by connecting the temperature change to the health of a river like the fictional Verrin River.

Verrin River Thermal Mixing: Heat, Equilibrium, and Oxygen — Grade 9 Science

Name: ____________________________ Date: ______________

Procedure

Step 1. Put on safety goggles. Use warm tap water no hotter than about 50 C, and let your teacher handle any hotter water. Do not drink any water used in this lab.

Step 2. Measure 200 mL of cold water into one insulated cup. Place a thermometer in it, wait until the reading holds steady, and record the cold temperature.

Step 3. Measure 200 mL of warm water into the second cup. Measure and record its temperature the same way.

Step 4. Before mixing, predict the temperature the combined water will reach. For these equal volumes, average the two temperatures and record your prediction.

Step 5. Pour both cups into one insulated cup, stir gently for a few seconds, wait 30 seconds, and record the measured mixed temperature.

Step 6. For Trial 2, repeat steps 2 through 5 but use 300 mL of warm water and 100 mL of cold water, so the volumes are unequal. Record all four temperatures.

Step 7. Empty the cups, dry your station, and return the thermometers.

Data & Analysis

Trial	Cold volume (mL)	Cold temp (C)	Warm volume (mL)	Warm temp (C)	Predicted mix temp (C)	Measured mix temp (C)
1
2

For Trial 1 with equal volumes, what temperature did you predict for the mixture, and how did you calculate it? Report your prediction to the same precision as your thermometer readings (for example, the nearest 1 C), and explain why more decimal places would not be meaningful.
Compare your predicted and measured mixed temperatures for Trial 1. Was the measured value close to, above, or below your prediction, and give one reason for any difference.
Suppose a group mixed 200 mL of water at 40 C with 200 mL of water at 20 C. Calculate the predicted equilibrium temperature.
Suppose a group mixed 300 mL at 40 C with 100 mL at 20 C. Calculate the predicted equilibrium temperature using a weighted average that accounts for the different volumes.
The warm and cold water settled to one in-between temperature. State what the second law of thermodynamics says about combining components at different temperatures in a closed system, and connect it to what you observed.
A power plant releases warm water into the cooler Verrin River. Warmer water holds less dissolved oxygen, and most fish need dissolved oxygen above 5 mg/L. Explain how the warm mixing zone could stress fish, using your lab results as a model.

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: The average of the two temperatures. For equal volumes, the predicted mix is (cold temp + warm temp) / 2. Accept any value that matches the average of the student's own two recorded temperatures, reported to about the thermometer's precision. Extra decimal places would claim more accuracy than the thermometer readings have.
Answer: Answers will vary. The measured value is usually slightly below the prediction because some heat is lost to the cup and the surrounding air, so the closed-system ideal is not perfectly met. Accept any answer consistent with the student's data and a valid reason.
Answer: 30 C. With equal volumes, the equilibrium is the simple average: (40 + 20) / 2 = 30 C.
Answer: 35 C. Weighted by volume: (300 x 40 + 100 x 20) / (300 + 100) = (12,000 + 2,000) / 400 = 14,000 / 400 = 35 C. The larger warm volume pulls the mixture closer to 40 C.
Answer: The second law says thermal energy moves from the warmer to the cooler water until the energy is more uniformly distributed, giving one in-between temperature. Accept any answer stating that heat spread until both parts reached the same temperature, matching the single mixed reading observed.
Answer: Accept any answer explaining that adding warm water raises the river's temperature in the mixing zone, and warmer water holds less dissolved oxygen. If the oxygen drops near or below 5 mg/L, fish can be stressed or forced to leave. The lab models this because mixing warm and cold water produced a warmer result than the cold water alone.

Differentiation

Support

Pre-measure the warm and cold water for each group and provide a data table with Trial 1 volumes already filled in, so students focus on reading thermometers and averaging. Give a worked averaging example with different numbers before Step 4.

Extension

Have students test a third trial that predicts, before mixing, the exact volumes of warm and cold water needed to hit a target mixed temperature, then check their prediction by measurement and explain any gap using heat loss.

ELL supports

Provide labeled picture cards for "warm," "cold," and "mix," and a sentence frame for the analysis: "I predicted ___ C and measured ___ C because ___." Allow students to record temperatures in the table before writing full sentences.

Extensions

Have students look up the temperature and dissolved-oxygen readings from a nearby river or lake monitoring station across the seasons, and describe how the summer warming they see in the real data matches the pattern their small mixing experiment demonstrated.