Problem 54 - Introductory Chemistry Atoms First

Question

When a candle burns, the solid wax melts and burns. In the process, the wax is combined with oxygen from the atmosphere and converted into carbon dioxide and water vapor. Explain what is going on in terms of chemical change and physical change.

Step-by-Step Solution

Answer

During the burning of a candle, a physical change occurs when solid wax melts into liquid wax due to heat provided by the flame. A chemical change occurs when the liquid wax reacts with oxygen in the presence of heat, undergoing combustion, resulting in the production of carbon dioxide and water vapor. The mass of the system is conserved, and energy is released in the form of heat and light.

Step 1: Identify the components taking part in the process

First, we need to identify the main components involved in the burning process of the candle: solid wax, heat, oxygen, carbon dioxide, and water vapor.

Step 2: Physical change: Melting of the wax

As the candle burns, heat is provided by the flame. The heat causes the solid wax to melt, converting it from a solid state to a liquid state. This is a physical change since it only involves a change of state and does not create any new substances.

Step 3: Chemical change: Wax reacts with oxygen

In the presence of heat, the melted wax reacts with the oxygen in the air surrounding the candle, undergoing a chemical reaction known as combustion. During this process, the wax molecules are transformed into carbon dioxide (CO2) and water vapor (H2O). This is a chemical change because new substances (CO2 and H2O) are formed as a result of the reaction.

Step 4: Conservation of mass

During the burning process, the mass of the system remains constant. The mass of the candle (wax) decreases as it is converted into CO2 and H2O. However, the mass of the products (CO2 and H2O) increases, and the total mass of the system (candle + surrounding atmosphere) remains conserved.

Step 5: End products and energy release

At the end of the process, solid wax has been converted into CO2 and H2O via a chemical reaction. The reaction also releases energy in the form of heat and light. The heat released sustains the burning process, and the light provides illumination. To summarize, during the burning of a candle, a physical change occurs when solid wax melts into liquid wax, and a chemical change occurs when the liquid wax reacts with oxygen to produce carbon dioxide and water vapor.

Key Concepts

Combustion Reaction

When we talk about burning a candle, we're diving into the world of combustion reactions. A combustion reaction is a type of chemical reaction where a substance combines with oxygen, releasing energy. In the case of a candle, the wax (composed mainly of hydrocarbons) reacts with the oxygen in the air. This reaction generates carbon dioxide and water vapor as new products.

Key features of a combustion reaction include:

• The presence of oxygen as a reactant.
• The formation of new substances (in this case, carbon dioxide and water vapor).
• Release of energy, usually in the form of heat and light.

The combustion of the wax provides the fire needed to keep the candle flame alive. This isn't just about burning wax; it's an intricate dance of chemistry resulting in light and heat.

Physical Change

While the flame dances on a candle, the solid wax beneath is quietly undergoing a physical change. A physical change involves a change in the state or appearance of a substance without altering its composition. In this process, the heat from the flame melts the solid wax, transforming it into a liquid.

Characteristics of a physical change involve:

• Change of state (e.g., solid to liquid).
• No new substances formed.
• Often reversible.

In the case of the candle, the melted wax can re-solidify as it cools, demonstrating its reversibility. This differentiation between melting and burning helps understand the breadth of transformations occurring around a candle flame.

Conservation of Mass

One of the fundamental principles of chemistry, the conservation of mass, is elegantly displayed when a candle burns. This principle states that in a closed system, mass is neither created nor destroyed, it only changes form. As the candle burns, the wax seems to disappear, but what's truly happening is its transformation into gas form – mainly carbon dioxide and water vapor.

Important points about conservation of mass:

• The total mass of reactants equals the total mass of products.
• Involves balancing what goes into a reaction with what comes out.
• In a burning candle, the decrease in candle mass matches the increase in gases produced.

Although the wax seems to vanish, its mass is conserved; now it's just part of the air around us. This is why when measuring closed reactions in labs, accuracy is key to maintaining the mass balance.

Energy Release in Chemical Reactions

Energy release is a fascinating aspect of chemical reactions. When a candle burns, it reflects this principle perfectly. During this chemical change, energy stored in the bonds of wax molecules is released as heat and light, providing illumination and warmth.

Some essential aspects of energy release are:

• Chemical reactions like combustion release stored energy.
• This energy often manifests as heat and light (as observed in a burning candle).
• Energy release is necessary to maintain ongoing reactions.

Without the energy released during a candle's burning, the flame would extinguish quickly. It's this energy that's harnessed to keep the candle alight, casting a gentle glow in the surroundings. It’s a magnificent example of how chemical reactions power everyday phenomena.