Energy Changes: Heat and Light

Introduction

A lit match flares with a bright yellow flame. A hand warmer grows hot in your pocket. A cold pack goes icy when you squeeze it. Each of these is a chemical change, and in each case you notice it through an energy change : something gets hotter, colder or gives out light. This page looks at heat and light as signs of reaction, and at why they happen, as well as the cases where they can mislead you.

Core explanation

Every reaction involves energy. Substances store energy in the way their atoms are joined together. During a chemical change, bonds in the starting materials (the reactants) are broken and new bonds are formed in the products. Breaking bonds takes in energy; making bonds gives energy out. The two amounts are almost never equal, so the reaction as a whole either releases energy to the surroundings or takes energy from them.

Temperature changes. The easiest way to detect an energy change is with a thermometer. If a reaction releases energy, the mixture and its container warm up. If a reaction takes in energy, the mixture cools down. Chemists measure the starting temperature, mix the reactants, and record the highest or lowest temperature reached.

Change What you observe Typical temperature change --- --- --- Magnesium ribbon in dilute acid Tube feels warm, fizzing Rise of several °C Neutralising an acid with an alkali Solution warms Rise of a few °C to over 10 °C Citric acid with sodium hydrogencarbonate Solution feels cold, fizzing Fall of several °C Burning methane in a gas cooker Flame, strong heating Flame temperature well over 1000 °C

Light. Some reactions release so much energy, so quickly, that the products glow. Burning is the most familiar example: the flame of a candle or a gas hob gives out both heat and light. Magnesium burns in air with a dazzling white light. A few reactions produce light with very little heat, such as the chemicals inside a glow stick; this is called chemiluminescence.

Physical changes also involve energy. Melting ice takes in energy, and condensing steam gives energy out. Dissolving some solids in water, such as ammonium nitrate, makes the water noticeably colder, while dissolving others makes it warmer. An electric light bulb glows without any chemical change at all: electrical energy heats the filament. So a temperature change or a glow is a clue that a reaction may have happened, not proof. It is strongest when it appears together with other signs, such as a new colour or a gas.

Step-by-step reasoning

To use an energy change as evidence:

1. Measure the temperature of the starting materials. 2. Mix them and record the temperature again after a short time. 3. Note any light, flame or glow. 4. Ask whether a physical process, such as dissolving, heating from outside or an electric current, could explain the change. 5. Look for other signs of a new substance before concluding that a reaction has happened.

Visual explanation

Picture two thermometers in two beakers. In the first, acid and magnesium are reacting, and the red line climbs as energy flows out of the reaction into the water. In the second, citric acid and bicarbonate are fizzing, and the line drops as energy flows from the water into the reaction. Arrows show the direction of energy flow.

Real-world analogy

Energy in a reaction is like money in a shop. Breaking bonds is like paying for goods; making bonds is like receiving payment. If a shop takes in more than it spends, it has money left over to give out, like a reaction that heats its surroundings. If it spends more than it receives, it must borrow, like a reaction that cools things down.

Real-world example

Disposable hand warmers contain iron powder, salt, water and other materials sealed in a pouch. When the pouch is opened, air reaches the iron and it slowly rusts. The rusting releases energy steadily, keeping the pouch warm for several hours. The warmth is the sign that a chemical change is taking place.

Why?

Why do reactions give out light as well as heat? When a reaction releases a large amount of energy in a very small space, some particles become extremely hot or are left in high-energy states. As they lose this energy, some of it is emitted as visible light, which is why flames and burning metals glow.

Common misconception

"If something gets hot, a chemical reaction must be happening." Heating water on a stove, rubbing your hands together and dissolving some salts all cause temperature changes without making new substances. An energy change is only one piece of evidence.

Worked example

Question: A student adds a spatula of white powder to water at 20 °C. The temperature falls to 14 °C, the powder disappears, and nothing else is seen. When the water is evaporated, the same white powder is left. Was this a chemical change?

Reasoning: The temperature fell by 6 °C, so energy was taken in. However, no gas, colour change or new solid appeared, and the original substance was recovered by evaporation. The energy change was caused by dissolving.

Answer: No, it was a physical change (dissolving), even though the temperature fell.

Quick check

1. What two energy-related observations might suggest a reaction is happening? Answer: A change in temperature, either warming or cooling, and the release of light.

Exam focus

When asked for evidence of a reaction, give a specific, measurable observation: "the temperature rose from 18 °C to 27 °C" is better than "it got hot". Remember that the change in temperature is calculated as the final reading minus the starting reading, and always include the unit °C.

Advanced insight

The energy released or absorbed by a reaction can be measured accurately in a calorimeter, an insulated container that reduces energy loss to the air. Chemists express the result as an enthalpy change, in kilojoules per mole (kJ/mol). Burning methane, for example, releases about 890 kJ for every mole burned.

Summary

Chemical reactions involve energy changes because bonds are broken and new bonds are made. A temperature rise, a temperature fall or the emission of light can all signal a reaction. However, physical changes such as dissolving, melting and electrical heating also involve energy, so heat and light should be combined with other evidence before deciding that a new substance has formed.

Practice questions

1. Why do chemical reactions usually involve an energy change? Answer: Breaking bonds takes in energy and making bonds releases energy, and the two amounts are rarely equal. 2. The temperature of a reaction mixture goes from 21 °C to 35 °C. Calculate the temperature change. Answer: 35 − 21 = 14 °C rise. 3. Give one example of a reaction that produces light. Answer: Burning, for example magnesium burning with a bright white light or methane burning in a gas flame. 4. Explain why a glowing electric bulb is not evidence of a chemical change. Answer: The filament glows because an electric current heats it; no new substance is produced.