Question

State whether each of the following is a physical or a chemical change: (a) combustion (b) vaporization (c) sublimation (d) oxidation

Short answer

(a) Chemical change, (b) Physical change, (c) Physical change, (d) Chemical change.

Step-by-step solution

Define Physical Change

A physical change is a type of change that affects the form of a chemical substance, but not its chemical composition. Examples include changes in state such as from solid to liquid or liquid to gas.

Define Chemical Change

A chemical change is a process where one or more substances are altered into one or more new and different substances. This typically involves making or breaking chemical bonds, such as in combustion or oxidation reactions.

Analyze Combustion

Combustion is a reaction in which a substance combines with oxygen and releases heat and light, resulting in new substances such as carbon dioxide and water. Since new substances are formed, combustion is a chemical change.

Analyze Vaporization

Vaporization is the process by which a liquid changes into a gas. It does not alter the chemical structure of the substance. Therefore, vaporization is a physical change.

Analyze Sublimation

Sublimation is the transition from a solid to a gas without passing through the liquid state. This change affects only the state of the substance, not its chemical structure; thus, sublimation is a physical change.

Analyze Oxidation

Oxidation generally refers to the chemical reaction of a substance with oxygen to form an oxide. Since it results in the formation of new substances, oxidation is considered a chemical change.

Key concepts

Physical Change

Physical changes are transformations that alter the appearance or state of a substance without changing its chemical identity. Imagine ice melting into water; the ice changes from a solid to a liquid. The molecules remain the same, just arranged differently.
Examples of physical changes include:

• Melting (solid to liquid)
• Freezing (liquid to solid)
• Vaporization (liquid to gas)
• Condensation (gas to liquid)
• Sublimation (solid to gas)
• Deposition (gas to solid)

In all these processes, no new substances are created because the molecular composition remains unchanged.

Chemical Change

A chemical change occurs when substances transform into entirely different substances with new chemical properties. This involves breaking and forming chemical bonds, leading to chemical reactions.
Here are some key indicators of a chemical change:

• Color change
• Formation of a precipitate
• Gas production
• Temperature change
• Light or heat emission

Combustion and oxidation are prime examples. For instance, burning wood involves wood (a complex organic compound) reacting with oxygen to form carbon dioxide, water, and ash.

Combustion

Combustion is a type of chemical change where a substance reacts rapidly with oxygen and releases energy in the form of light and heat. It’s an exothermic reaction, producing new substances such as carbon dioxide and water. This process is what happens when fuels like gasoline burn in car engines.
Combustion requires three elements: a fuel, oxygen, and an ignition source, often referred to as the fire triangle. If you remove any one of these, the combustion cannot continue.
Due to its nature, combustion not only alters substances but also generates significant energy, making it crucial for cooking, heating, and powering engines.

Vaporization

Vaporization, a physical change, involves the transition of a substance from a liquid to a gas. This can occur through evaporation or boiling.
Evaporation happens at the surface of a liquid and can occur at temperatures below boiling point, while boiling occurs throughout the liquid at a specific temperature. Despite the change in state, the molecular structure remains unchanged.
Vaporization requires energy, often in the form of heat, to overcome molecular attractions and allow molecules to escape into a gaseous state. Because it doesn’t change the substance chemically, it’s classified as a physical change.

Sublimation

Sublimation is a fascinating process wherein a solid changes directly into a gas, bypassing the liquid phase. This is another example of a physical change.
Dry ice, which is solid carbon dioxide, sublimates into CO2 gas at room temperature, showing how the substance can change state without altering its chemical structure.
The process requires energy input to overcome the forces holding the molecules in the solid state, making sublimation an endothermic process. Yet, since the chemical identity remains untouched, it stays categorized as a physical change.

Oxidation

Oxidation is a chemical change involving a substance combining with oxygen, altering its chemical structure. Rusting of iron is a common example—where iron reacts with oxygen and moisture to form iron oxide.
This process often involves the loss of electrons from the oxidized substance, making it a part of redox (reduction-oxidation) reactions.
While oxidation primarily refers to interactions with oxygen, it can also involve other oxidizing agents, manifesting as a change in substances and showcasing the creation of new compounds through chemical reactions.