Question

Burning Candle After burning for three hours, a candle has lost half of its mass. Explain why this example does not violate the law of conservation of mass.

Short answer

The burning candle losing half of its mass after three hours does not violate the law of conservation of mass because the mass of CO₂ and H₂O produced in the combustion is equal to the mass of the burned wax and the used oxygen. The total mass of the reactants is equal to the total mass of the products, thus maintaining the law of conservation of mass.

Step-by-step solution

Identify the components of the candle

A candle is made of wax (a hydrocarbon) and a wick. When the candle burns, it undergoes a chemical reaction with oxygen present in the air.

Understand the combustion process

In the combustion process, the hydrocarbon in the wax reacts with oxygen to produce carbon dioxide (CO₂) and water vapor (H₂O). This is an exothermic reaction, releasing heat in the form of light and heat. The wick absorbs the melted wax, which is drawn up into the flame and undergoes combustion.

Mass conservation during combustion

The change in mass of the candle is due to the wax being converted to CO₂ and H₂O. Even though the mass of the wax decreases, the mass of the CO₂ and H₂O produced is equal to the mass of the wax that was burned. Therefore, the total mass of the products (CO₂ and H₂O) is equal to the initial mass of the reactants (wax and oxygen).

Account for heat in the reaction

As the heat produced during combustion is absorbed by the candle and its surroundings, it does not contribute to the mass change in the candle. The heat produced is energy in the form of light and thermal energy.

Conclude why the example doesn't violate the law of conservation of mass

The burning candle losing half of its mass after three hours does not violate the law of conservation of mass because the mass of CO₂ and H₂O produced in the combustion is equal to the mass of the burned wax and the used oxygen. The total mass of the reactants is equal to the total mass of the products, thus maintaining the law of conservation of mass.

Key concepts

Chemical Reactions

Chemical reactions are the processes by which substances undergo chemical changes to form new substances. A classic example of a chemical reaction is the burning candle from our exercise. The wax in a candle, which consists of hydrocarbons, reacts with oxygen from the air to transform into entirely new substances: carbon dioxide (CO₂) and water vapor (H₂O). This transformation happens through a series of bonds breaking and new bonds forming between atoms. The intriguing aspect is that despite this complex molecular dance, the total amount of matter remains constant. This is a fundamental property of all chemical reactions, hinging on the principle that matter cannot be created or destroyed, only rearranged.

It's easy to be fooled into thinking the candle mass mysteriously disappears, but when you take a closer look, you realize that the wax doesn't vanish—it changes form. During a chemical reaction, atoms are not lost; they are simply reorganized into different molecules. That's key to understanding why a chemical reaction like the combustion of a candle doesn't break the law of conservation of mass.

Combustion Process

The combustion process, a type of chemical reaction, is more familiar to us than we might think. It's seen in car engines, power plants, and yes, our burning candle exercise. Put simply, combustion is a reaction between a substance and oxygen that produces heat and light. In the case of a candle, the hydrocarbon (wax) is the fuel, and as the wax burns in the presence of oxygen, it releases energy in the form of heat and light—what we see as the flame.

This process doesn't happen randomly; it follows a set of chemical rules. It starts off by heating the candle, which melts the wax. The liquid wax is then drawn up the wick by capillary action, vaporizes, and reacts with the oxygen in the air to produce an exothermic reaction—an event that releases energy. This interplay is crucial for a sustained combustion process, and it is an excellent illustration of how energy transformations are tied to chemical reactions.

Conservation During Combustion

When we light a candle and watch it diminish in size, it might be tempting to think that the law of conservation of mass isn't in play, but that's a misinterpretation. The law states that in a closed system, the mass of the system must remain constant over time. During the combustion of our candle, wax is used up, and its mass seems to 'disappear.' However, it's actually being converted into carbon dioxide and water vapor, which float away into the air and are not as immediately noticeable as the solid wax.

To truly appreciate this conservation, we have to consider the entire system, including the gases produced and the oxygen consumed. The total mass before and after the reaction remains equal - the mass is merely distributed in a different form. You can think of it like a magic trick where the magician hasn't really made something disappear; they've just moved it somewhere else where you aren't looking. Here, our 'magician' is the law of conservation of mass, and it ensures that even during the dynamic process of combustion, every atom is accounted for.