HS-PS3-4: How Thermal Energy Evens Out
Also called NGSS HS-PS3-4 · Topic: Energy · Physical Science
The Official Wording
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).
Clarification statement: Emphasis is on analyzing data from student investigations and using mathematical thinking to describe the energy changes both quantitatively and conceptually. Examples of investigations could include mixing liquids at different initial temperatures or adding objects at different temperatures to water.
Assessment boundary: Assessment is limited to investigations based on materials and tools provided to students.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What HS-PS3-4 Means in Plain English
Students plan and run an investigation where a hot object and a cold one share a closed space, such as hot metal dropped into cool water. They record temperatures over time and calculate the energy each part gained or lost until both match.
The evidence supports the second law of thermodynamics: energy flows from hotter to colder objects until their temperatures match. Students use only the materials and tools provided. It builds on MS-PS3-4 and on energy models in HS-PS3-1.
HS-PS3-4 I Can Statements
- I can predict which way thermal energy flows between two objects.
- I can calculate energy gained or lost from a temperature change.
- I can explain why mixed objects end at one temperature.
HS-PS3-4 Worked Example
A calorimeter lab with hot brass, cool water and a temperature probe.
When hot metal goes into cool water, where do the two temperatures end up?
Set It Up: Two nested foam cups with a lid, 150 g of water at 21 °C, a 200 g brass mass heated in boiling water, and a temperature probe.
- Heat the brass in boiling water for five minutes so it starts near 100 °C.Brass about 100 °C, water 21 °C
- Move the brass quickly into the cup and close the lid with the probe in the water.Water temperature climbs fast
- Record the temperature every 10 seconds until it stops changing.Levels off near 29 °C in about 2 minutes
- Calculate the energy the water gained and the brass lost, using 4186 and 380 J/kg°C.Water: 0.150 × 4186 × 8 ≈ 5,000 J. Brass: 0.200 × 380 × 71 ≈ 5,400 J
- Compare the two energies.Close, with the gap lost to the cup and air
The brass cooled and the water warmed until both sat near 29 °C, and the water gained nearly all the energy the brass lost.
Why it works: Thermal energy flows from the hotter object to the cooler one until their temperatures match, so the energy ends up spread more evenly.
Common Mistakes with HS-PS3-4
Saying cold flows into the warm object
“The cold water moved into the brass.”
Try this: Only thermal energy moves, and it always flows from the hotter object to the cooler one. Cold is just less thermal energy.
Averaging the two temperatures
“100 °C and 21 °C, so it should end near 60 °C.”
Try this: The final temperature depends on each object's mass and specific heat. Water stores far more energy per degree, so it warms only a little.
Ignoring losses to the surroundings
One open paper cup, and surprise when the numbers do not match
Try this: Use a lidded, nested foam cup and move the metal quickly, then name the cup and air as places energy can escape.
Where HS-PS3-4 Fits
Practice HS-PS3-4
Quick Check
- A hot spoon goes into cool soup in a closed thermos. What happens to their temperatures?
- Why does 150 g of water warm only about 8 °C when 200 g of hot brass is added?
- Why do the water's gain and the brass's loss not match exactly?