Lab / experiment Grade 11 Science
Mixing Warm and Cool Water: A Thermal Energy and Oxygen Lab
A grade 11 lab where students mix warm and cool water, predict the final temperature from an energy balance, and connect it to reservoir layering and fish oxygen.
Objectives
- Students will predict a mixture's final temperature using an energy balance and compare it to a measured value.
- Students will connect thermal energy transfer to reservoir stratification and dissolved-oxygen availability for fish.
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, energy balance, specific heat, thermal stratification
Teacher overview
When warm and cool water are combined, they always settle at a single temperature somewhere between the two, and where that temperature lands is set by how much of each you started with. This lab has students predict that final temperature from a simple energy balance, measure it, and explain the gap. The payoff is a connection to real water bodies: the same physics explains why a summer reservoir splits into a warm top and a cold bottom, and why the colder, denser bottom water can hold more dissolved oxygen, at least until decay uses it up. Because 1 milliliter of water has a mass near 1 gram, students can read masses straight off a measuring cup.
Procedure
Set up. Label two foam cups A and B for each group. Foam insulates well enough that most of the heat stays in the water during the short mix.
Measure the cool water. Pour a measured mass of cool water (near 10 degrees Celsius) into cup A. For trial 1 use 150 grams (150 milliliters). Record the mass and its starting temperature.
Measure the warm water. Handle carefully, because water near 60 degrees Celsius can scald. Pour an equal 150 grams of warm water into cup B and record its starting temperature just before mixing.
Mix and stir. Pour the warm water from cup B into the cool water in cup A, stir gently and steadily, and watch the thermometer. When the reading holds steady for several seconds, record it as the final measured temperature.
Run trial 2 with unequal amounts. Repeat the steps using 100 grams of warm water and 200 grams of cool water so the two trials can be compared. Record all masses and temperatures.
Safety and cleanup. Do not drink any of the water. Wipe up spills immediately so no one slips, and let an adult handle refilling the warm-water supply.
Data & Analysis
| Trial | Warm water mass (g) | Warm start (degrees C) | Cool water mass (g) | Cool start (degrees C) | Final measured (degrees C) |
|---|---|---|---|---|---|
| 1 | |||||
| 2 |
The Sedgemoor Reservoir named below is a fictional reservoir. Answer each question.
Suppose a trial mixes 150 grams of water at 60 degrees Celsius with 150 grams of water at 10 degrees Celsius. Using the idea that the heat lost by the warm water equals the heat gained by the cool water, predict the final temperature.
Using your own trial 1 masses and starting temperatures, predict the final temperature, then write it down.
Compare your predicted final temperature from item 2 to the temperature you actually measured. Find the difference, and give one reason a real mixture does not exactly match the prediction.
Predict the final temperature if you mix 100 grams of water at 60 degrees Celsius with 300 grams of water at 10 degrees Celsius. Is the result closer to the warm or the cool starting temperature, and why?
In every trial the two starting temperatures end at one temperature between them, never further apart. Explain what this shows about the direction thermal energy moves when warm and cool water are combined.
In summer the fictional Sedgemoor Reservoir separates into a warm top layer and a cold bottom layer that do not readily mix. Using what you observed, explain why the cold bottom water can hold more dissolved oxygen, and why fish needing at least 5 mg/L of dissolved oxygen may be stressed if the warm surface layer loses oxygen.
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: 35 degrees Celsius. With equal masses, the final temperature is the average of the two: (150 x 60 + 150 x 10) / (150 + 150) = 10,500 / 300 = 35 degrees Celsius.
Answer: Answers vary. Accept any predicted value that equals (warm mass x warm start + cool mass x cool start) divided by the total mass, using the student's own recorded numbers.
Answer: Answers vary. The difference should be small; accept any reasonable value with a valid reason, such as heat lost to the cup, the air, or the thermometer, or slight measurement error in mass or temperature.
Answer: 22.5 degrees Celsius. (100 x 60 + 300 x 10) / (100 + 300) = 9,000 / 400 = 22.5 degrees Celsius. It is closer to the cool starting temperature because there is three times as much cool water, so the cool water dominates the balance.
Answer: Thermal energy moves from the warmer water to the cooler water until both reach the same temperature, spreading the energy more evenly through the mixture. It never spontaneously moves the other way to make one part hotter and the other colder.
Answer: Cold water can dissolve more oxygen than warm water, so at saturation the cold bottom layer can hold more. The warm surface layer holds less oxygen at saturation, and on hot days it can fall toward or below the roughly 5 mg/L that fish need, stressing fish that stay near the surface. Because the layers do not mix in summer, the bottom layer is cut off from the air, so decay can use up its oxygen over the summer, which leaves fish squeezed between a too-warm top and an oxygen-poor bottom.
Differentiation
Support
Pre-fill a data table with trial 1 masses already entered, and provide the averaging shortcut for equal masses before item 1. Demonstrate one full mix as a class so groups know what a steady final reading looks like.
Extension
Have students test whether stirring speed or cup insulation changes the gap between predicted and measured temperatures, then use their trial 2 unequal-mass data to argue how mass ratio controls where the final temperature lands.
ELL supports
Provide labeled picture cards for "warm," "cool," "mix," "final temperature," and "layer," and a cross-section image of a stratified reservoir. Allow items 5 and 6 to be answered with a labeled arrow diagram plus one sentence from a provided frame.