Why Does a Disposable Hand Warmer Get Warm When You Open the Bag?
― Inside, Iron Powder Is Rusting Very Fast
There is no battery and no fuel inside, yet a few minutes after you open the bag, the warmer turns gently warm. What is going on inside is the same "rust" you see on a bicycle left out in the rain. Rusting actually gives off heat. The warmer is built to release that heat fast enough for your hand to feel it, and slowly enough to last half a day.
It's a cold morning. You tear open the outer bag and take out the warmer. At first it feels cool, but as you knead it in your pocket, it slowly warms up.
On the back of the bag it says "Ingredients: iron powder, water, activated carbon, vermiculite, salts." Nothing in that list looks like it could burn.
Meanwhile, a nail left out in the garden stays cold, even when it is rusty red all over. It is the same iron, so where does the difference come from?
There are only two reasons it gets warm
Rusting is iron combining with oxygen in the air. This releases the same kind of heat that a campfire gets from burning wood. A nail simply rusts so slowly that the heat escapes right away.
The iron is ground into fine powder to widen the surface that touches air, and salt water and carbon speed up the rusting. Then tiny holes in the bag limit how much oxygen gets in, so it never gets too hot.
In other words, a hand warmer does two things inside one bag: it makes rusting faster, and it keeps rusting from getting too fast. Let's look at each in turn.
Why is a rusty nail cold, but iron powder warm?
Rusting happens only on the surface of the iron. Oxygen cannot get deep into a lump of iron. So how much rusts in a given time, and so how much heat comes out, is set almost entirely by how much surface touches the air.
Look at Figure 1. Take 1 gram of iron as a 1 cm cube, and its surface is smaller than the face of a 1-yen coin. But make the same 1 gram into powder with grains about 0.05 mm across, and the surface grows to about the size of one postcard. That is about 200 times the lump. Move the slider to change the grain size, and you can see the surface widen as the grains get finer.
Powdering is not the only trick for speeding up rust. The salt water inside makes iron rust many times faster. This is why bicycles by the sea rust so quickly. The black powder, activated carbon, holds air in its gaps and hands oxygen over to the iron.
So why doesn't it get hot all at once?
If speed were the only goal, you could just grind the iron even finer. In fact, steel wool, which is bundled fine iron threads, throws sparks and burns when you bring a flame near. Burning and rusting are the same change, iron combining with oxygen. Only the speed differs.
In a hand warmer, the bag itself acts as the brake. As Figure 2 shows, the inner bag is made of cloth with holes too small to see. Oxygen can enter only through those holes. The number and size of the holes set how much oxygen gets in, and so keep the heat output steady. The outer bag, which seals the warmer until it is sold, keeps air out so the rusting stays stopped.
Work it out, and an ordinary-sized warmer gives off about 3 watts on average. That is less power than a smartphone charger, but it is spread over a palm-sized area for half a day, so it feels plenty warm.
Rust is iron with oxygen attached. The contents gain weight by exactly the oxygen that attaches. By calculation, if 20 grams of iron all rust, it gains nearly 9 grams. From what looks like empty air, the warmer takes in weight over half a day.
In places where air hardly changes, such as inside a shoe or under piled-up bedding, oxygen runs short and the warmer struggles to heat up. Warmers made for the toes are said to have a different mix inside so they can work with less air.
Summary
A disposable hand warmer uses the heat that iron gives off when it rusts. Fine powder, salt water and carbon speed up rusting hundreds of times, and tiny holes in the bag limit the oxygen to keep the warmth just right for half a day.
Inside a hand warmer is iron rusting over half a day.
The source of the warmth is oxygen in the air.
Just because it feels warm, don't keep it against your skin for long. We explain why in Why do hand warmers and hot-water bottles cause burns? For another story of metal combining with air and changing colour, see our article on the green patina on copper. For heat that builds up and speeds itself along, see our article on oily rags that catch fire.
- Put a warmer that has started heating into a zip bag, press out as much air as you can, and close it. After a while, you will notice the warmth fading.
- Open the bag and let air back in, and it starts to warm up again. Whether oxygen can get in switches the heat on and off.
- On a kitchen scale, compare the warmer's weight just after opening with its weight the next day, once it has gone cold. If it has gained a few grams, that is proof that oxygen attached to the iron.
A little of the water inside escapes, so the weight gain can come out smaller than the calculation. Do not take out the contents; keep it in its bag for the test.
For those who want to know more ― terms, formulas and links to textbooksWe mark which level each part belongs to, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school "Chemistry"
- High school+Advanced high school content, or treated as a textbook sidebar
- UniversityNot taught in high school; university specialist courses (electrochemistry, reaction engineering)
- ResearchNot yet taught as settled fact even at university; being studied by researchers now
Junior highTerms: this phenomenon has a name
- Oxidation: a change in which a substance combines with oxygen. Both rusting and burning are oxidation. They differ only in how fast they go.
- Exothermic reaction: a reaction that gives off heat to its surroundings when it happens. Iron oxidation is a classic example, and it appears in junior high science as a hand warmer experiment.
- Surface area: how wide the surface of an object is. Even for the same amount, the finer you divide it, the larger the surface gets, and changes that happen on the surface go faster.
Junior highHigh schoolChecking with a formula: how many watts does a hand warmer give?
We estimate the heat released when all the iron in the warmer rusts, then divide it by the time it lasts, to get the average heat output in watts. One watt is a heat output of 1 joule per second.
| In symbols | P ＝ ( m ÷ M ) × Q ÷ t |
| In words | Heat output ＝ amount of iron (moles) × reaction heat per mole of iron ÷ time it stays warm |
| Where it comes from | Conservation of energy. When iron combines with oxygen to become rust, the difference in bond energy comes out as heat. The heat released is proportional to the amount of iron that reacts |
| Symbol | Meaning and unit |
| P | Average heat output (watts) |
| m | Mass of iron in the warmer (grams) |
| M | Mass of one mole of iron (grams) |
| Q | Heat released when one mole of iron rusts (kilojoules) |
| t | Time it stays warm (seconds) |
| Iron in an ordinary-sized warmer | said to be around 20 grams |
| Mass of one mole of iron | 55.8 grams |
| Heat when one mole of iron becomes red rust (iron oxide) | about 400 kilojoules (varies somewhat with the type of rust) |
| Time it stays warm | about 12 hours (varies by product) |
| Density of iron | 7.87 grams per cm³ |
| Convert the iron to moles | 20 ÷ 55.8 ≒ 0.36 moles |
| Heat when it all rusts | 0.36 × 400 ＝ 144 kilojoules |
| Convert 12 hours to seconds | 12 × 3600 ＝ 43200 seconds |
| Average heat output | 144000 ÷ 43200 ≒ 3.3 watts |
| Oxygen used in rusting (0.75 mole per mole of iron) | 0.36 × 0.75 ＝ 0.27 moles |
| Weight gained (1 mole of oxygen is 32 grams) | 0.27 × 32 ≒ 8.6 grams |
| Density × grain size (0.005 cm) | 7.87 × 0.005 ≒ 0.0394 |
| Surface area of 1 gram of powder | 6 ÷ 0.0394 ≒ 152 cm² |
| Surface area of 1 gram as a 1 cm cube | 6 ÷ 7.87 ≒ 0.76 cm² |
| How many times more surface the powder has than the lump | 152 ÷ 0.76 ＝ 200 times |
The result is that the warmer gives off a little over 3 watts of heat for half a day. A human body, even sitting still, is said to give off about 80 to 100 watts. The warmer's heat is a few tens of times smaller, but it is concentrated on one spot on your palm or lower back, so it feels clearly warm. In practice not all the iron may rust, so this value is only a guide.
High schoolHigh school+Why does rusting give off heat?
High schoolThe reaction in which iron becomes iron oxide is strongly exothermic, with a large heat of reaction (heat of formation). Between before and after the reaction, the bond energy between atoms drops, and the difference comes out as heat. The heat of reaction does not depend on the path taken, only on the first and last states (Hess's law). Whether the iron rusts slowly or burns all at once, the total heat from the same amount of iron is the same.
High school+What differs is how fast the heat comes out. The speed of a reaction depends on how much surface is in contact, the temperature, and the amount of water and salt around. Powdering the iron enlarges the surface area, and salt water lets electricity flow more easily, which helps the reaction along. The warmer sets up all the conditions that raise the speed, then narrows only the oxygen entrance to fix the overall speed.
UniversityIron and carbon form tiny batteries in salt water
In electrochemistry, the corrosion of iron is treated as a reaction driven by local cells. At one spot on the iron's surface, iron gives up electrons and dissolves (the anode), and at another spot, oxygen takes those electrons and reacts with water (the cathode). The activated carbon in a warmer conducts electricity, and is explained as taking on the role of this cathode over a wide area, speeding up the reaction. Salt helps electricity pass through the water. Inside the powder, both the speed at which oxygen seeps through the gaps (diffusion) and the speed of the reaction at the surface decide the overall speed. In reaction engineering, this is treated as a problem of heterogeneous reactions and specific surface area.
📖 Derivations and further reading: Hess's law (Wikipedia) ／ Specific surface area (Wikipedia)
ResearchWhat is still not well understood
- What the rust is made of changes with conditions. Iron rust comes in many kinds, including ones that hold water and ones with different crystal shapes. Which one forms depends on moisture and the amount of salt, and the fine details are still being studied.
- It is hard to predict exactly how oxygen moves through the powder. A slight difference in grain size, packing or the spread of water changes how the warmer heats. Product recipes are said to be decided not by calculation alone but through repeated trials and measurements.
- What happens in the first instant of rusting. The very early stage, when a film of water forms on the iron's surface and corrosion begins, is being studied, partly in research aimed at making bridges and ships last longer.
In other words, the contents of this article are also "an explanation within what is known today." The reason heat comes out is clear, but how the reaction spreads inside the powder still has parts we cannot know without measuring.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science, Year 2: "Chemical changes and heat," "Oxidation" | Heat comes out when iron rusts |
| High school | Chemistry: "Heat of reaction," "Hess's law" | Whether slow rusting or burning, the total heat is the same |
| High school+ | Chemistry: "Reaction rates" | Speeding up with powder and salt water, limiting with holes |
| University | Electrochemistry (corrosion), reaction engineering | Local cells, heterogeneous reactions and specific surface area |
| Research | Corrosion science | Types of rust, how rusting begins |
| ― | Links to daily life | Why a warmer fades in a shoe, why bicycles rust quickly by the sea |
- Kairo (hand warmer) - Wikipedia (Japanese)
- Japanese Industrial Standard JIS S 4100, "Disposable body warmers" (日本産業規格 JIS S 4100「使いすてカイロ」)
- Chemical Society of Japan (日本化学会), ed., Handbook of Chemistry, Basic Edition (化学便覧 基礎編), Maruzen Publishing (enthalpy of formation of iron oxide)
- Japan Society of Corrosion Engineering (腐食防食学会), ed., Handbook of Corrosion and Corrosion Protection (腐食・防食ハンドブック), Maruzen Publishing (corrosion by local cells)
- Ministry of Education, Culture, Sports, Science and Technology (文部科学省), Commentary on the Junior High School Course of Study: Science (中学校学習指導要領解説 理科編) (chemical changes and heat)
※This article is a popular-science explanation. The numbers given are rough guides to help you understand how it works. Do not take out the contents of a warmer, and follow the product's instructions for use.