Problem 23 - Chemistry and Chemical Reactivity

Question

Identify the following as either physical changes or chemical changes. (a) Dry ice (solid \(\mathrm{CO}_{2}\)) sublimes (converts directly from solid to gas phase). (b) Mercury's density decreases as the temperature increases. (c) Energy is given off as heat when natural gas (mostly methane, \(\mathrm{CH}_{4}\)) burns. (d) NaCl dissolves in water.

Step-by-Step Solution

Answer

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

Step 1: Understanding Physical and Chemical Changes

Physical changes are changes in the state or appearance of a substance that don't alter the chemical composition, such as melting, freezing, or dissolving. On the other hand, chemical changes result in the formation of new substances with different chemical compositions, often accompanied by energy changes (like heat or light) and changes such as color, taste, and odor.

Step 2: Analyzing Dry Ice Sublimation

When dry ice sublimates, it transitions from a solid state directly into a gaseous state without passing through the liquid phase. Since this process involves only a change in state and not in chemical composition, it is a physical change.

Step 3: Evaluating Mercury's Density Change

The density of mercury decreases with increasing temperature, which indicates a change in its physical properties due to thermal expansion. Since the chemical nature of mercury is not altered, this is a physical change.

Step 4: Examining Methane Combustion

When methane (natural gas) burns, it reacts with oxygen to form new substances, carbon dioxide and water, and releases energy as heat. This process results in the formation of new chemical products, characteristic of a chemical change.

Step 5: Considering NaCl Dissolution

When NaCl dissolves in water, it dissociates into sodium and chloride ions. Despite this change in the state of the compound, the ions are still the same chemical entities, and no new substances are formed. Thus, dissolving is primarily a physical change, though it can be argued there is a minor chemical component to the ionic interaction with water.

Key Concepts

Physical Change

Whenever we talk about physical change, it helps to think about changes that don't alter what a substance fundamentally is. Consider a piece of paper. If you tear it into pieces, it's still paper. Physical changes are all about changes in state or form.
A few typical examples are:

• Melting
• Freezing
• Dissolving
• Boiling

These changes don't create something new at the molecular level. It's critical to remember that the substance remains the same, chemically speaking, which makes this an important concept to grasp.

Chemical Change

Chemical changes are transformations that result in forming a new substance. Imagine baking a cake - you mix ingredients and bake to create something new that isn't just flour and sugar anymore.
This type of change usually involves:

• Change in color
• Formation of gas
• Emission or absorption of heat or light
• Presence of a distinct smell

It often involves breaking bonds in molecules and forming new ones. Remember, chemical changes lead to new substances with different properties from the original ones.

Sublimation

Sublimation is a fascinating process where a substance turns from a solid directly into a gas, bypassing the liquid state. Think of dry ice, which is frozen carbon dioxide. Instead of becoming a liquid, it goes straight to gas as it warms.
Key characteristics of sublimation include:

• No change in chemical composition
• Change only in the physical state

Sublimation is considered a physical change because the substance itself remains chemically the same - a crucial point that supports our understanding of material transformations.

Density

Density is a measure of how much mass is contained in a given volume. People often refer to it when they want to understand how tightly matter is packed in a substance. For example, mercury becomes less dense as it heats up because the atoms spread out more.
Understanding density is key because:

• It helps identify substances
• It's crucial for understanding buoyancy
• Provides insights into a material's properties

This property is usually a physical characteristic, as it deals with changes in temperature and pressure without affecting the substance's core identity.

Combustion

Combustion is a type of chemical change where a substance reacts quickly with oxygen to release energy in the form of heat and light. Burning a candle or wood are classic examples of combustion. Noteworthy points about combustion:

• Involves new substances (like carbon dioxide and water)
• Exothermic reaction (releases heat)
• Typically involves flames

This process showcases how chemical changes create new substances. Understanding combustion is essential in fields ranging from energy production to everyday cooking.