Problem 34 - 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: Understanding Physical vs Chemical Changes

A physical change is a change in the state or properties of a substance without forming a new substance. Examples include changes in state (like melting or boiling) and separation of mixtures. A chemical change results in one or more new substances being formed, evidenced by changes like color shift, gas formation, or temperature change.

Step 2: Analyzing Desalination of Sea Water

Desalination involves separating pure water from salts, which are dissolved in it. The chemical identity of water and salt remains unchanged because there is no formation of a new substance; therefore, this is a physical change.

Step 3: Evaluating the Formation of SO2 from Burning Coal

When coal with sulfur is burned, the sulfur reacts with oxygen to form sulfur dioxide (\( ext{SO}_2\)). This is a chemical change because a new substance is formed.

Step 4: Identifying Silver Tarnishing

Silver tarnishing occurs when silver reacts with sulfur compounds in the air, forming a black layer of silver sulfide. This is a chemical change because a new substance, silver sulfide, is produced.

Step 5: Considering Iron Heated to Red Heat

Heating iron to red heat does not change the substance chemically, it only increases the temperature and can change its physical appearance (color due to heat). Since no new substance is formed, this is a physical change.

Key Concepts

Desalination

Desalination is the process of removing dissolved salts and other impurities from seawater to obtain fresh, drinkable water. This process doesn’t involve a chemical reaction between the substances; it simply separates them through physical methods like distillation or reverse osmosis.
One common method of desalination is reverse osmosis, where water is forced through a semi-permeable membrane. This membrane allows water molecules to pass while blocking salts and other impurities.

Desalination is considered a physical change because:

• There is no formation of a new substance. The component substances (water and salt) remain chemically identical before and after the process.
• It involves the separation of components in a mixture.
• The process can be reversed (e.g., you can add salt back to the desalinated water).

Understanding desalination is important for water scarcity solutions across the globe.

Formation of Sulfur Dioxide

When coal containing sulfur burns, a chemical reaction takes place. This involves sulfur atoms reacting with oxygen in the air to form sulfur dioxide (\( ext{SO}_2\)), a gas known to contribute to air pollution.
This change is chemical in nature because:

• A new substance, sulfur dioxide, is produced.
• It involves a chemical reaction between sulfur and oxygen.
• The process is irreversible under normal conditions. Burning coal alters the composition permanently.

Formation of sulfur dioxide is a key example of a chemical change, with significant environmental implications due to its role in acid rain formation and respiratory health issues.

Silver Tarnishing

Silver tarnishing is an everyday example of a chemical change that occurs when silver reacts with sulfur compounds present in the air. This reaction results in the formation of silver sulfide, which appears as a black layer on the silver's surface.
Key points about silver tarnishing include:

• It is a chemical change because a new compound, silver sulfide, is formed.
• Tarnishing is characterized by a visible color change to black.
• The change cannot be easily reversed; polishing removes the tarnish but not by converting silver sulfide back to elemental silver.

Understanding this process is important for maintaining silver items in households and industries.

Red Heat of Iron

When iron is heated to a temperature where it glows red, you are witnessing a physical change. At this point, the energy supplied makes the iron atoms vibrate more vigorously, causing the iron to emit light and appear red.
This phenomenon is termed as a physical change because:

• No new chemical substance is formed; iron remains iron.
• The change is reversible; once cooled, the iron returns to its original color without any alteration in its chemical composition.
• It is a change of state or condition due to temperature variation.

Understanding the red heat of iron is essential in metallurgy and when discussing temperature effects on materials.