Problem 24 - Chemistry and Chemical Reactivity

Question

Identify the following as either physical changes or chemical changes. (a) The desalination of sea water (separation of pure water from dissolved salts). (b) The formation of \(\mathrm{SO}_{2}\) (an air pollutant) when coal containing sulfur is burned. (c) Silver tarnishes. (d) Iron is heated to red heat.

Step-by-Step Solution

Answer

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

Step 1: Understand Physical Change

A physical change involves a change in state or appearance without altering the composition. Examples include phase changes like melting or evaporation.

Step 2: Understand Chemical Change

A chemical change involves a transformation that alters the substance's composition, often resulting in the formation of new products, as indicated by an emission of gas, change in color, or release of energy.

Step 3: Analyze Desalination of Sea Water

Desalination involves the physical separation of water from dissolved salts, usually through evaporation and condensation. Since no new substance is formed, this is a physical change.

Step 4: Analyze Formation of SO2

The burning of coal that contains sulfur produces sulfur dioxide (\(\mathrm{SO}_{2}\)), a new substance. This process is a chemical change as it involves a change in molecular composition.

Step 5: Analyze Silver Tarnishing

When silver tarnishes, it reacts with sulfur compounds in the air to form silver sulfide, a black coating. This is a chemical change, as new compounds are formed.

Step 6: Analyze Heating Iron to Red Heat

Heating iron to red heat causes it to glow red due to increased energy, but it does not change its composition. Thus, this is a physical change.

Key Concepts

Physical Change

A physical change refers to a modification in the form, state, or appearance of a substance, but not its chemical composition. For instance, process changes like melting, boiling, freezing, and shredding all qualify as physical changes. Even though these processes might seem drastic—like ice melting into water—they don’t result in any new substances.
Physical changes are often reversible. For example:

• Melting and Freezing: Water can change from ice to liquid water and vice versa without altering its molecular structure.
• Dissolving: Sugar dissolving in water is a physical change as the composition of sugar and water remains unchanged.

When discussing desalination of sea water, this is a prime example of a physical change. The process separates pure water from salt through evaporation and condensation, but it does not change the inherent composition of the water or salt.

Chemical Change

A chemical change is entirely different from a physical change. It occurs when substances are transformed into different substances due to breaking and forming of chemical bonds. Indicators of a chemical change can include color change, gas production, and change in temperature or energy, often making the process irreversible.
Examples of chemical changes include:

• Burning: When wood burns, it turns into ash and releases gases, indicating a chemical transformation.
• Rusting: The formation of rust on iron involves iron reacting with oxygen to form iron oxide, a completely new substance.

In the exercise example, when coal that contains sulfur burns, it forms sulfur dioxide (\(\mathrm{SO}_2\)), representing a clear chemical change as the sulfur integrates into a new molecular composition.

Desalination

Desalination is a process that removes salts and minerals from saline water to produce fresh water. It is essential for providing potable water in areas where freshwater is not readily available, especially in coastal and arid regions. The most common desalination methods are distillation and reverse osmosis.

• Distillation: This involves evaporating the water and then condensing the steam back into liquid, leaving the salt behind.
• Reverse Osmosis: This method forces water through a semi-permeable membrane, filtering out the salt.

Both of these methods are forms of physical change because they involve changing water from one state to another without altering its chemical composition. The goal is simply to separate the water from its dissolved salts, with no new substances created during the process.

Sulfur Dioxide Formation

Sulfur dioxide (\(\mathrm{SO}_2\)) is a colorless gas with a pungent odor, commonly produced from burning coal or oil at power plants and during metal smelting. Its formation is a notable chemical change in which sulfur oxidizes to produce sulfate compounds.
The chemical reaction often appears as:\[\text{Sodium} + \text{Oxygen} \rightarrow \text{Sulfur Dioxide}\]This substance is not just a simple byproduct; it has significant environmental implications. Sulfur dioxide can further react with water to form sulfuric acid, contributing to acid rain issues.
Addressing sulfur emissions is crucial for reducing air pollution, emphasizing the importance of understanding its formation as a chemical change. The alteration from sulfur to sulfur dioxide presents a prime example of matter transforming into a completely new composition.