Design and Construct a Device That Releases or Absorbs Thermal Energy Through a Chemical Process - Middle School Physical Science

SECTION 1

Historical Context & Motivation

Have you ever used a hand warmer on a cold day? Or placed a cold pack on a twisted ankle? These everyday items work because of chemical reactions that release or absorb heat. Scientists and engineers have studied these reactions for centuries. Understanding how chemicals exchange energy helped people invent useful devices.

The idea that chemical reactions involve energy changes is not new. Early chemists noticed that mixing certain substances made containers feel hot or cold. Over time, researchers learned to measure and control these energy changes. This knowledge led to real-world inventions that people still use today.

1780 • Lavoisier Measures Reaction Heat

Antoine Lavoisier built an ice calorimeter to measure heat released by chemical reactions. He showed that reactions follow predictable energy patterns.

1840 • Hess's Law of Heat Summation

Germain Hess discovered that the total heat change in a reaction is always the same, no matter how many steps the reaction takes.

1950s • Military Develops Portable Heat Packs

Engineers created iron-based hand warmers for soldiers in cold climates. These packs used an exothermic reaction between iron and oxygen.

1970s • Instant Cold Packs Hit the Market

Companies began selling disposable cold packs for sports injuries. These used ammonium nitrate dissolving in water—an endothermic process.

Today • Designing for Everyday Solutions

Students and engineers continue to design new thermal devices. Modern designs focus on safety, cost, and environmental impact.

Here is the big question: How can you design a device that uses a chemical process to release or absorb just the right amount of thermal energy? This lesson will help you understand the science behind hot packs and cold packs. You will also learn how engineers think about designing these devices.

SECTION 2

Core Principles & Definitions

Before you can design a thermal device, you need to understand a few key ideas. Every chemical reaction involves energy. Some reactions give off energy to the surroundings. Others pull energy in from the surroundings. The type and amount of substances you use determine how much energy changes.

Thermal Energy

Thermal energy is the total kinetic energy of all the particles in a substance. When particles move faster, the substance has more thermal energy and feels warmer.

Exothermic Reactions

An exothermic reaction releases thermal energy to the surroundings. The temperature of the surroundings goes up. Example: iron rusting in a hand warmer.

Endothermic Reactions

An endothermic reaction absorbs thermal energy from the surroundings. The temperature of the surroundings goes down. Example: ammonium nitrate dissolving in water.

Chemical Process

A chemical process is any change where substances interact and form new substances or dissolve. Bonds between atoms break and form, which involves energy.

Engineering Design

The engineering design process is a cycle of defining problems, developing solutions, testing, and improving. You use it to build your thermal device.

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KEY TAKEAWAY

Think of exothermic reactions like a campfire—they give off heat and warm your hands. Endothermic reactions are like ice melting in your lemonade—they absorb heat and cool your drink down. Your design challenge is to pick the right chemical process for the job!

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NGSS Connection

This lesson connects to MS-PS1-6: Undertake a design project to construct a device that either releases or absorbs thermal energy by chemical processes. You will also use the Crosscutting Concept of Energy and Matter and the Science and Engineering Practice of Constructing Explanations and Designing Solutions.

SECTION 3

Visual Explanation: Exothermic vs. Endothermic

The diagram below shows the difference between exothermic and endothermic reactions. Pay attention to the direction of the energy arrows. In an exothermic reaction, energy flows out of the reaction and into the surroundings. In an endothermic reaction, energy flows in from the surroundings.

Left: An exothermic reaction (like iron rusting in a hand warmer) releases energy outward, warming the surroundings. Right: An endothermic process (like ammonium nitrate dissolving) absorbs energy inward, cooling the surroundings.

Notice how the arrows in the exothermic panel point outward, away from the reaction. That shows energy leaving the chemical process. The arrows in the endothermic panel point inward, toward the reaction. That shows energy being absorbed from the surroundings. When you design your device, you choose which type of reaction to use based on whether you want to heat something up or cool something down.

SECTION 4

How It Works: Energy and Chemical Bonds

Where does the thermal energy actually come from? It comes from changes in chemical bonds. When atoms form new bonds, energy is released. When bonds break apart, energy is absorbed. The balance between breaking old bonds and forming new ones determines whether a reaction is exothermic or endothermic.

THERMAL ENERGY CHANGE

Q = m × c × ΔT

Q = thermal energy transferred (in joules, J), m = mass of the substance (in grams, g), c = specific heat capacity (for water, 4.18 J/g·°C), ΔT = change in temperature (final temperature − initial temperature, in °C).

This formula is very useful for your design. It tells you how much the temperature will change when a certain amount of energy is released or absorbed. If you want your cold pack to get colder, you need a process that absorbs more energy (larger Q). You can change the amount of chemical you use (m) to control how much energy transfers.

TEMPERATURE CHANGE

ΔT = Q ÷ (m × c)

Rearranging the formula helps you predict the temperature change. A larger Q means a bigger temperature change. A larger mass means the same energy is spread over more material, so the temperature changes less.

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ENERGY AND MATTER

The Crosscutting Concept of Energy and Matter is at work here. Energy is not created or destroyed—it just transfers between the chemicals and their surroundings. Think of it like pouring water between two cups. The total amount of water stays the same; it just moves from one cup to the other.

SECTION 5

The Engineering Design Process for Thermal Devices

Designing a hot pack or cold pack is not just about science—it is about engineering. The engineering design process helps you plan, build, test, and improve your device. You follow a cycle of steps, and you can go back to earlier steps whenever you need to.

The engineering design process is a cycle with six steps: Define the problem, Research materials and reactions, Design your device plan, Build and Test it, Evaluate your results, and Improve through redesign. You can iterate (repeat) any step.

Applying the Design Process

Define: What is your goal? Do you need a device that heats up or cools down? How hot or cold should it get? How long should it last?
Research: What chemicals are safe and available? Iron powder with salt and water is exothermic. Ammonium nitrate in water is endothermic. Baking soda and vinegar is also endothermic.
Design: Sketch your device. How will the chemicals stay separate until you are ready? How much of each chemical will you use?
Build & Test: Construct your device and measure the temperature change using a thermometer. Record your data carefully.
Evaluate: Did your device meet your design criteria? Was the temperature change big enough? Did it last long enough?
Improve: Based on your results, change one variable at a time. Try more chemical, different packaging, or insulation.

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Design Criteria vs. Constraints

Criteria are the goals your device must meet (for example, reach at least 40°C or cool below 10°C). Constraints are the limits you must work within (for example, cost under $2, must be safe to touch, must use only classroom-approved materials).

SECTION 6

Worked Example: Designing a Cold Pack

Let's walk through a complete example. Imagine you need to design a cold pack that lowers the temperature of 100 grams of water by 15°C. We will figure out how much energy must be absorbed and then connect that to the chemical process.

Designing a Cold Pack

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Step 1 — Define the Problem

We need a cold pack that absorbs enough thermal energy to lower 100 g of water from 25°C to 10°C. That is a temperature change of ΔT = 10°C − 25°C = −15°C. The negative sign means the temperature decreases.

ΔT = −15°C
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Step 2 — Calculate the Energy Needed

Use the formula Q = m × c × ΔT. Plug in the values: Q = 100 g × 4.18 J/g·°C × (−15°C).

Q = 100 × 4.18 × (−15) = −6,270 J (the cold pack must absorb 6,270 joules of energy)
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Step 3 — Choose a Chemical Process

We need an endothermic process. Ammonium nitrate (NH₄NO₃) dissolving in water absorbs about 25.7 kJ per mole. One mole of ammonium nitrate has a mass of 80 g.

NH₄NO₃ dissolving in water = endothermic
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Step 4 — Calculate How Much Chemical to Use

We need 6,270 J = 6.27 kJ of energy absorbed. Each mole (80 g) absorbs 25.7 kJ. So we need: 6.27 ÷ 25.7 = 0.244 moles. In grams: 0.244 × 80 g = about 19.5 g of ammonium nitrate.

Use approximately 20 g of NH₄NO₃
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Step 5 — Design, Build, and Test

Place 20 g of ammonium nitrate in a sealed inner bag. Put 100 mL of water in an outer bag. When you squeeze and break the inner bag, the chemical dissolves and absorbs heat. Measure the temperature with a thermometer. If it does not cool enough, add more chemical. If it cools too much, use less.

Test → Evaluate → Improve → Repeat

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DESIGN TIP

Real engineers test many versions. Your first design probably will not be perfect, and that is totally okay! Each test gives you data. Use that data to improve your next version. This is called iterating—like leveling up in a video game by learning from each round.

SECTION 7

Comparing Common Thermal Device Materials

Not all chemical processes produce the same temperature change. Some are safer than others. Some cost more. Engineers must weigh the strengths and limitations of each option before choosing materials for a design. The table below compares several common choices.

Chemical Process	Type	Strengths	Limitations
Iron + oxygen + salt water	Exothermic	Safe, inexpensive, long-lasting warmth	Slow to heat up; not reusable
Calcium chloride + water	Exothermic	Fast heat release, gets very warm	Can get too hot; slightly irritating to skin
Ammonium nitrate + water	Endothermic	Effective cooling, widely used in cold packs	Must handle carefully; not reusable
Baking soda + vinegar	Endothermic	Very safe, easy to get at home	Small temperature change; produces gas (bubbles)
Citric acid + baking soda + water	Endothermic	Safe, classroom-friendly, produces fizz	Moderate cooling only; produces CO₂ gas

Comparison of common materials for thermal energy devices

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CAUSE AND EFFECT

The Crosscutting Concept of Cause and Effect applies here. The cause is the chemical reaction. The effect is the temperature change. By changing the type of chemical or the amount you use, you directly change how hot or cold your device gets. It is like choosing ingredients in a recipe—different ingredients give different results.

SECTION 8

Connecting to Bigger Ideas

The thermal devices you design in this lesson connect to much bigger ideas in science. In high school chemistry, you will learn about enthalpy (a more precise way to measure energy changes) and thermodynamics (the study of energy flow). The table below shows how your middle school learning connects to advanced topics.

What You Learn Now	What Comes Next
Exothermic and endothermic reactions	Enthalpy (ΔH) calculations and Hess's Law
Q = m × c × ΔT	Calorimetry experiments with precise instruments
Engineering design process	Formal engineering design with cost analysis and optimization
Energy transfers between system and surroundings	Laws of thermodynamics and conservation of energy
Choosing safe materials	Green chemistry and environmental impact analysis

How middle school concepts connect to advanced chemistry and engineering

You are building a strong foundation right now. The skills you practice—measuring temperature, calculating energy, testing and improving designs—are the same skills that real scientists and engineers use every day. Whether it is designing better batteries, developing new medicines, or creating sustainable energy systems, it all starts with understanding how energy and matter interact in chemical processes.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

You mix two chemicals in a cup and the cup feels cold. What type of reaction is happening? A) Exothermic — the reaction is releasing heat B) Endothermic — the reaction is absorbing heat C) No reaction — the chemicals are just mixing D) Exothermic — the reaction is absorbing heat

Show answer

B) Endothermic — the reaction is absorbing heat. When the cup feels cold, the reaction is pulling thermal energy away from your hand and the cup. That means the reaction absorbs energy, which is the definition of endothermic. Option A is wrong because exothermic reactions release heat and would make the cup feel warm. Option C is wrong because a temperature change is evidence of a chemical or physical process happening. Option D uses the word exothermic incorrectly—exothermic means releasing, not absorbing.

Question 2: BASIC CALCULATION

A student heats 50 g of water and the temperature rises from 20°C to 35°C. How much thermal energy was transferred? (c = 4.18 J/g·°C) A) 3,135 J B) 6,270 J C) 2,090 J D) 150 J

Show answer

A) 3,135 J. Use Q = m × c × ΔT. First find ΔT = 35°C − 20°C = 15°C. Then Q = 50 × 4.18 × 15 = 3,135 J. Option B (6,270 J) would result from using 100 g instead of 50 g. Option C (2,090 J) uses 10°C instead of 15°C. Option D (150 J) ignores the specific heat capacity entirely.

Question 3: INTERMEDIATE

A student wants to design a hot pack that raises 200 g of water from 22°C to 40°C. How much thermal energy must the chemical process release? (c = 4.18 J/g·°C) A) 8,360 J B) 15,048 J C) 33,440 J D) 1,672 J

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B) 15,048 J. ΔT = 40°C − 22°C = 18°C. Q = 200 × 4.18 × 18 = 15,048 J. Option A (8,360 J) uses ΔT = 10°C, which is not the correct temperature change. Option C (33,440 J) uses ΔT = 40°C (forgetting to subtract the starting temperature). Option D (1,672 J) uses only 2°C for the temperature change.

Question 4: APPLIED

A team designed two cold packs. Cold Pack A uses 15 g of ammonium nitrate in 100 g of water. Cold Pack B uses 30 g of ammonium nitrate in 100 g of water. Both start at 25°C. After testing, Pack A drops to 20°C and Pack B drops to 15°C. Which statement best explains why? A) Pack B has more water, so it cools more B) Pack B uses more chemical, so more energy is absorbed from the water C) Pack A uses a different chemical than Pack B D) Pack B is exothermic, and Pack A is endothermic

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B) Pack B uses more chemical, so more energy is absorbed from the water. Both packs use the same chemical and the same amount of water. The only difference is the amount of ammonium nitrate. More chemical means more dissolving, which absorbs more energy from the water and causes a bigger temperature drop. Option A is wrong because both packs have 100 g of water. Option C is wrong because both use ammonium nitrate. Option D is wrong because both processes are endothermic—they both absorb heat.

Question 5: CRITICAL THINKING

A student designed a hot pack using calcium chloride and water. It reached 55°C, which was too hot to hold safely. The design criteria says it must stay below 45°C. Which change to the design would MOST likely solve the problem while still keeping the pack warm? A) Switch to an endothermic chemical B) Use more calcium chloride to spread the heat out C) Use less calcium chloride so less energy is released D) Remove the water from the design

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C) Use less calcium chloride so less energy is released. Using less of the exothermic chemical means less energy is released, so the temperature will not rise as high. This keeps the hot pack functional but safer. Option A would make a cold pack instead of a hot pack—that defeats the purpose. Option B is wrong because adding more chemical would release even more energy and make it hotter. Option D is wrong because water is needed for the dissolving reaction to occur. This is a great example of the engineering design cycle—test, evaluate, and improve!

SUMMARY

Lesson Summary

Summary

In this lesson, you learned how to design and build a device that uses a chemical process to transfer thermal energy. Exothermic reactions release heat to the surroundings (like hand warmers), while endothermic reactions absorb heat from the surroundings (like cold packs). The formula Q = m × c × ΔT helps you calculate how much energy is transferred and predict the temperature change.

You followed the engineering design process to define a problem, research materials, design a solution, build and test a prototype, evaluate results, and improve your design through iteration. The Crosscutting Concepts of Energy and Matter and Cause and Effect guided your thinking. By choosing the right chemical, the right amount, and the right design, you can engineer a device that meets specific temperature goals safely and effectively.