Problem 18 - Chemistry and Chemical Reactivity

Question

Which observations below describe chemical properties? (a) Sodium metal reacts violently with water. (b) \(\mathrm{CO}_{2}\) does not support combustion. (c) Chlorine is a green gas. (d) Heat is required to melt ice.

Step-by-Step Solution

Answer

(a) and (b) describe chemical properties.

Step 1: Understanding Chemical Properties

Chemical properties describe how a substance interacts with other substances, changing its composition. This contrasts with physical properties, which are observable without altering the substance's identity.

Step 2: Analyze Observation (a)

Observation (a) states that sodium metal reacts violently with water. This is a chemical property because it describes sodium's ability to undergo a chemical change by reacting with water.

Step 3: Analyze Observation (b)

Observation (b) states that \(\text{CO}_2\) does not support combustion. This is a chemical property because it describes how \text{CO}_2\ interacts (or rather, does not interact) with combustible materials, affecting chemical reactions.

Step 4: Analyze Observation (c)

Observation (c) describes chlorine as a green gas. This is a physical property because it is an observable attribute that does not involve a chemical change in chlorine.

Step 5: Analyze Observation (d)

Observation (d) states that heat is required to melt ice. This is a physical property because melting is a phase change (from solid to liquid) that does not involve a chemical change in the substance's composition.

Key Concepts

Chemical Change

A chemical change involves a transformation in the substance's chemical structure. When a chemical change occurs, the original materials are transformed into new substances. This process is often irreversible.
Some common signs of a chemical change include:

• Change in color
• Production of a gas
• Formation of a precipitate
• Temperature change
• Emission of light

For instance, when sodium metal reacts with water, a violent reaction occurs: sodium turns into sodium hydroxide, releasing hydrogen gas. This is an indication of a chemical change because new substances are formed.

Reaction with Water

The ability of a substance to react with water is an important chemical property. This interaction often leads to a chemical change. For example, sodium's reaction with water is a classic demonstration of how vigorous such interactions can be. When sodium encounters water, it reacts to form sodium hydroxide and releases hydrogen gas, which is why the reaction is visibly violent.
Reactions with water can vary:

• Some substances like lithium also react but less violently
• Certain metals like gold do not react with water at all
• Other materials might dissolve without a noticeable chemical change

Understanding how a substance interacts with water can help predict its behavior in natural or industrial processes.

Combustion

Combustion is a chemical process where a substance reacts rapidly with oxygen, releasing energy in the form of light and heat. It is a common form of exothermic reaction. Not all substances can support combustion, which is also a chemical property.
For example, carbon dioxide ( ext{CO}_2) does not support combustion. This means it cannot react with oxygen to produce fire, often making it useful as a fire extinguisher component. When evaluating a substance's combustion ability:

• Check if it burns or helps other materials burn
• Determine the amount of oxygen required
• Identify what kind of energy release is expected

Understanding combustion helps in managing fire safety and energy production efficiently.

Physical Properties

Physical properties are characteristics of a substance that can be measured or observed without changing the substance's chemical structure. Unlike chemical properties, these do not involve transforming the substance into something new.
Some examples of physical properties include:

• Color and odor
• Melting point and boiling point
• Density and volume
• Mass and weight

Observing chlorine as a green gas is a physical property, just as melting ice requires heat but doesn't change water chemically.