Question

If you place a glass rod over a burning candle, the glass turns black. What is happening to each of the following (physical change, chemical change, both, or neither) as the candle burns? Explain. a. the wax b. the wick c. the glass rod

Short answer

a. The wax - Both (Physical and Chemical Changes): The wax melts (physical change) and reacts with oxygen to produce new substances like carbon dioxide and water vapor (chemical change). b. The wick - Chemical Change: The wick reacts with oxygen and turns into new substances like carbon dioxide and water vapor. c. The glass rod - Physical Change: The glass rod turns black due to the deposition of soot (carbon) particles, which is a change in appearance without altering its chemical composition.

Step-by-step solution

Understand Physical and Chemical Changes

A physical change occurs when a substance undergoes a change in its physical properties, such as shape, size, or state, without changing its chemical composition. Examples of physical changes include melting ice, breaking a glass, or boiling water. On the other hand, a chemical change involves a change in the chemical composition of a substance, producing new substances with different properties. This can be observed through changes in color, the release of gas, or the production of heat. Examples of chemical changes include rusting of iron, burning of wood, or reacting baking soda with vinegar.

Observe the Wax

As the candle burns, the wax near the flame melts and is drawn up the wick. This change in state from solid to liquid is a physical change. However, the wax also undergoes a chemical change when it reacts with the oxygen in the air as it burns, producing new substances such as carbon dioxide and water vapor. This is a chemical change. Therefore, the wax undergoes both physical and chemical changes. Answer: a. The wax - Both (Physical and Chemical Changes)

Observe the Wick

The wick is typically made of a material like cotton, which undergoes a chemical change as it burns. The solid wick reacts with oxygen in the air and turns into new substances like carbon dioxide and water vapor, which are gaseous. The wick does not undergo any significant physical changes during this process. So, only a chemical change occurs to the wick. Answer: b. The wick - Chemical Change

Observe the Glass Rod

As the glass rod is placed above the burning candle, it turns black due to the deposition of soot (carbon) particles produced from the burning candle. The glass rod itself remains chemically unchanged, as the carbon particles merely form a layer on its surface. This blackening of the glass rod is a physical change, as it involves a change in the appearance of the glass without altering its chemical composition. Answer: c. The glass rod - Physical Change

Key concepts

Chemical Reactions

Chemical reactions are fundamental processes by which substances interact to form new products, exhibiting different properties from the reactants.

Consider the burning candle in our exercise. The wax and wick, when heated, undergo chemical reactions with the oxygen in the air. During this process, both the wax and the wick are chemically transformed into new substances such as carbon dioxide and water vapor. Combustion is a type of chemical reaction where a substance combines with oxygen to give off heat, which is what's happening with the candle's wax and wick.

In general, chemical reactions can be identified by certain indicators such as change in temperature, production of gas, change in color, or the formation of a precipitate. These signs point to a reorganization of atoms and the creation of new chemical bonds, which is at the heart of a chemical reaction. Settling for an avoidant explanation by stating it's 'just burning' does not suffice; it's important to recognize and understand the complex interactions at a molecular level that result in these observable changes.

Properties of Matter

Matter comprises everything around us and is characterized by its properties, which can be categorized into physical and chemical properties.

Physical properties include aspects such as color, odor, density, melting point, boiling point, and hardness, which can be observed or measured without altering the substance's chemical identity. For example, when the candle wax melts, its physical state changes from solid to liquid, which is a physical change since its chemical composition remains unchanged.

Chemical properties are attributes that cannot be determined by touch or sight and involve the substance's intrinsic ability to undergo changes that alter its chemical structure. The wax's capacity to burn and transform into different chemicals like carbon dioxide and water vapor is a chemical property. Understanding these properties is crucial in predicting how substances will behave and interact with other substances. Misinterpreting physical changes as chemical changes (or vice versa) can lead to a flawed understanding of what's happening during a reaction or process.

States of Matter

Matter exists predominantly in three distinct states: solid, liquid, and gas; each state is determined by the arrangement and energy of the particles that constitute the matter. The wax in our exercise starts as a solid, featuring particles that are closely packed in a fixed position, but as it is exposed to heat, it transforms into a liquid with particles that are still in contact but can move around each other.

Transition between these states—also known as phase changes—occurs through the addition or removal of energy. Melting, freezing, vaporization, condensation, sublimation, and deposition are all processes that describe these transitions. The phase change of the candle's wax from solid to liquid as it melts is a classic example of a physical change, not involving a chemical reaction.

Understanding states of matter helps us grasp various natural phenomena and technological processes, ranging from water cycle dynamics to culinary transformations in cooking—though often overlooked, knowing the distinction is key to unlocking the underlying principles of material interactions.