Problem 48 - Chemistry Matter and Change

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.

Step-by-Step Solution

Answer

The mass of the candle transforms into gaseous products, aligning with the law of conservation of mass.

Step 1: Understanding the law

The law of conservation of mass states that mass cannot be created or destroyed in a chemical reaction. When a candle burns, the mass must be accounted for in the form of other substances.

Step 2: Identifying the chemical reaction

Burning a candle involves a chemical reaction where the wax (primarily hydrocarbons) reacts with oxygen in the air to produce carbon dioxide, water vapor, heat, and light.

Step 3: Tracking the mass

Although the candle loses half of its mass, it is important to recognize that the mass is not lost but transformed. The wax is converted into gaseous products like carbon dioxide and water vapor, which escape into the air.

Step 4: Concluding with the law

Since the mass of the gaseous products equals the initial mass of the part of the candle burned, the overall mass of the system (candle plus air) remains constant, thus adhering to the law of conservation of mass.

Key Concepts

Chemical Reaction

A chemical reaction is a process where substances are transformed into new substances. This process involves the breaking and forming of chemical bonds. When you light a candle, a chemical reaction occurs as the wax, made mostly of hydrocarbons, interacts with the oxygen in the air.

Key points about chemical reactions:

• Involve rearrangement of atoms.
• Result in new substance formation.
• Typically include energy changes, such as the release of heat or light.

The reaction in a burning candle is a combination of physical and chemical changes occurring together. As the candle burns, it not only produces heat and light but also converts the wax into different compounds.

Mass Transformation

Mass transformation refers to the change of form or state a substance undergoes during a chemical reaction. This concept is crucial to understanding why the burning candle seems to lose mass. As the candle burns, the wax undergoes a transformation into gaseous products.

Highlights of mass transformation:

• Substances change form but total mass remains constant.
• Solid wax transforms into gases such as carbon dioxide and water vapor.
• Explains why the physical appearance might change, but the actual mass does not vanish.

It's an observable principle in closed systems where all the products and reactants are contained, further showcasing the law of conservation of mass.

Gaseous Products

When a candle burns, gaseous products are formed as part of the chemical reaction. The main gaseous products are carbon dioxide and water vapor. These gases are the result of the wax combining with oxygen.

Important facts about gaseous products:

• Gases are less visible, which might give the impression of losing mass.
• They are dispersed into the surrounding air, contributing to the unseen conservation of mass.
• Combusting wax results in these gases carrying the weight of the initial solid wax.

Even though these products are invisible, they maintain the balance of mass in the system.

Combustion Process

The combustion process is a specific type of chemical reaction that involves burning a substance in the presence of oxygen. It is exothermic, meaning it releases energy in the form of heat and light. In the case of a candle, the wax fuels the combustion process.

Combustion details:

• The wax melts and vaporizes near the wick, starting the reaction with oxygen.
• Produces energy, making the process self-sustaining as long as wax and oxygen are available.
• Efficiently results in the transformation of solid material to energy and gaseous byproducts.

Understanding the combustion process reveals the full cycle of how a simple candle can demonstrate fundamental scientific principles such as the conservation of mass.