Lab / experiment Grade 10 Science

Why Lakes Warm Slowly: A Water Specific-Heat Mixing Lab

A grade 10 lab where students mix measured hot and cold water, predict the final temperature with a weighted average, and link it to water's high specific heat.

Grade: Grade 10
Subject: Science
Material: Lab / experiment
Time: 50 min
Prep: Moderate prep
Materials: Two foam cups per group, nested to form one insulated mixing cup, Two thermometers reading 0-100 degrees Celsius per group, Two graduated cylinders or clear measuring cups marked in milliliters (one for hot water, one for cold), Hot tap water about 60 degrees Celsius, poured by an adult (about 300 mL per group), Cold water chilled with ice to about 10 degrees Celsius (about 300 mL per group), Stirring stick and a timer or clock

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 combine water at two different temperatures in a closed cup and gather evidence that the mixture reaches one uniform temperature between the two, exactly this standard's thermal-energy investigation.

All materials for HS-PS3-4 →

HSN-Q.A.1 Common Core Math

Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas; choose and interpret the scale and the origin in graphs and data displays.

How this material aligns: Students carry milliliters and degrees Celsius through the weighted-average prediction in analysis questions 1 and 2, choosing consistent units so the mass-weighted calculation is valid.

All materials for HSN-Q.A.1 →

Download Word file

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 the final temperature of mixed hot and cold water using a weighted average.
Students will compare measured and predicted final temperatures and account for heat loss.
Students will explain how water's high specific heat moderates lake temperature and 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: specific heat, thermal energy, weighted average, equilibrium temperature

Teacher overview

Water resists changing temperature because it has a high specific heat, and that property helps explain why a deep lake stays cooler than the air on a hot afternoon. In this lab, students mix measured amounts of hot and cold water in an insulated cup, record the final temperature, and compare it to a prediction from a mass-weighted average. Because both samples are water, the predicted final temperature is simply the weighted average of the two starting temperatures. The classroom setup is a simplified stand-in for a lake, not a precise calorimeter, but it shows the same energy-sharing principle at work.

Why Lakes Warm Slowly: A Water Specific-Heat Mixing Lab — Grade 10 Science

Name: ____________________________ Date: ______________

Procedure

Step 1 (safety, before mixing): The hot water should be about 60 degrees Celsius and must be poured by an adult. Hot water can scald, so handle cups carefully, keep the work area dry, and do not drink any water used in this lab.

Step 2 (set up the insulated cup): Nest one foam cup inside the other to slow heat loss. This nested cup is where you will mix the water.

Step 3 (measure the cold water): Using the graduated cylinder, measure 100 mL of cold water. Read and record its temperature in degrees Celsius. This is your cold starting temperature for Trial 1.

Step 4 (measure the hot water): Measure 100 mL of hot water in a separate container and record its temperature in degrees Celsius. This is your hot starting temperature for Trial 1.

Step 5 (mix and read): Pour both samples into the nested cup at the same time, stir gently for 10 seconds, and read the temperature every 15 seconds until it stops changing. Record the steady final temperature as your measured final for Trial 1.

Step 6 (run Trial 2 with unequal amounts): Empty and dry the cup. Repeat Steps 3 through 5, but this time use 150 mL of hot water and 50 mL of cold water, recording both starting temperatures and the measured final temperature.

Step 7 (record everything): Enter all measurements in the data table before starting the analysis.

Data & Analysis

Trial	Hot volume (mL)	Hot temp (degrees C)	Cold volume (mL)	Cold temp (degrees C)	Measured final temp (degrees C)
1 (100 mL + 100 mL)	100		100
2 (150 mL + 50 mL)	150		50

For Trial 1, calculate the predicted final temperature using the weighted average: (hot volume x hot temp + cold volume x cold temp) divided by total volume. Assume the hot water started at 60 degrees C and the cold at 10 degrees C.
For Trial 2, calculate the predicted final temperature the same way, using 150 mL of hot water at 60 degrees C and 50 mL of cold water at 10 degrees C.
Compare your measured final temperature in Trial 1 to the predicted value from question 1. Was the measured value higher or lower, and by how many degrees?
Using your results, explain why the mixed water settled at a temperature between the hot and cold starting values instead of at either extreme.
Explain how water's high specific heat helps a deep lake stay at a steadier temperature than the air above it during a hot day.
Most fish need dissolved oxygen above 5 mg/L, and cold water holds more oxygen than warm water. Using that fact, explain why a shallow pond that warms up quickly in summer could put fish at risk.

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 C. Predicted final = (100 x 60 + 100 x 10) divided by 200 = (6,000 + 1,000) divided by 200 = 7,000 divided by 200 = 35 degrees Celsius.
Answer: 47.5 degrees C. Predicted final = (150 x 60 + 50 x 10) divided by 200 = (9,000 + 500) divided by 200 = 9,500 divided by 200 = 47.5 degrees Celsius.
Answer: Varies by group's data. The measured value is usually a few degrees lower than the predicted 35 degrees C because some heat escapes to the cup and the air. Accept any answer that correctly states the direction and size of the difference from the student's own data.
Answer: The hot water lost thermal energy and the cold water gained the same amount until both reached one shared temperature. Because both samples are water with the same specific heat, equal masses changed temperature by equal amounts in Trial 1, so the mixture settled near the middle rather than staying hot or cold. Water's high specific heat matters when water exchanges energy with something else, such as air or sunlight, as in question 5.
Answer: A deep lake holds a large mass of water with a high specific heat, so it absorbs a lot of solar energy for only a small temperature rise. That large heat capacity lets the lake warm and cool much more slowly than the air, keeping its temperature steadier through the day.
Answer: Warm water holds less dissolved oxygen, so as a shallow pond heats up its oxygen drops. If it falls below the 5 mg/L that most fish need, the fish can suffocate, which is why fast summer warming in shallow water threatens fish.

Differentiation

Support

Pre-fill the starting temperatures (60 and 10 degrees C) on the data table and work Trial 1's predicted value with the class before groups run their own trials. Provide the weighted-average formula with each term labeled.

Extension

Have students design a third trial that predicts a final temperature of exactly 25 degrees C using the same 60-degree and 10-degree water, solve for the volumes needed, then test their prediction and explain any difference from the measured result.

ELL supports

Pair "specific heat," "thermal energy," and "equilibrium temperature" with simple pictures of a hot cup and a cold cup combining into one warm cup. Allow students to record measurements as numbers in the table and to answer questions 4 through 6 with a labeled sketch plus a short phrase.