The observation of a burning candle placed on a scale shows a steadily decreasing weight reading. This apparent loss of mass seems to defy logic, suggesting that matter is vanishing into thin air. The science behind this effect reveals not a destruction of material, but a transformation of the solid wax into invisible substances that are no longer supported by the scale’s platform. Understanding why the weight goes down requires looking closely at the chemical process of burning and the physics of how a scale measures objects.
The Chemistry of Combustion
A candle’s combustion is a chemical reaction requiring two core ingredients: fuel and an oxidizer. The fuel is the wax, which is primarily made of hydrocarbons. The oxidizer is the oxygen gas found in the surrounding air.
The heat from the flame melts the solid wax near the wick, turning it into a liquid. This liquid wax is then drawn upward through the wick’s fibers by capillary action. Once it reaches the hottest part of the flame, the liquid wax is vaporized, transforming into a hot gas.
It is this wax vapor, not the liquid or solid wax, that actually reacts with the atmospheric oxygen. The molecules in the wax vapor combine and rearrange with the oxygen molecules, a process that releases energy in the form of heat and light. This continuous process sustains the flame, with the heat generated radiating back down to melt and vaporize more wax.
Where the Mass Goes
The reason the scale reading drops is directly linked to the products created by the combustion reaction. The chemical transformation of the wax and oxygen produces new substances, specifically carbon dioxide (\(\text{CO}_2\)) and water vapor (\(\text{H}_2\text{O}\)). The wax, a solid, is converted almost entirely into these two gaseous compounds.
Any mass that changes state from solid to gas and then leaves the platform’s surface will cause the reading to decrease. The carbon dioxide and water vapor are significantly warmer than the surrounding air due to the heat of the flame. This difference in temperature makes them less dense, causing them to rapidly rise and dissipate into the atmosphere above the scale.
These newly formed gaseous products are no longer physically pressing down on the scale, effectively removing their mass from the system being measured. The mass lost is the wax that has been chemically converted and physically carried away by the buoyant hot gases.
The Principle of Conservation of Mass
While the scale indicates a loss of weight, this does not mean that mass is being destroyed. The Law of Conservation of Mass states that matter can neither be created nor destroyed in a chemical reaction; it is merely transformed. The atoms from the wax and the oxygen are simply rearranged to form the new molecules of carbon dioxide and water vapor.
This scientific law applies perfectly to the burning candle, but the apparent contradiction arises because the experiment is conducted in an open system. If the candle were placed on a scale inside a completely sealed, airtight container, the total mass recorded by the scale would remain constant as the candle burns. Even though the wax changes from a solid to a gas, all the atoms, including the consumed oxygen and the gaseous products, would be trapped inside the container.
In a closed system, the initial mass of the wax and the oxygen consumed would exactly equal the final mass of the carbon dioxide, water vapor, and any remaining wax. The scale’s reading drops only because the system is open, allowing the buoyant gaseous products to escape the immediate area of the scale, thereby removing their weight from the measurement.