Question

Which one is a physical change? (1) Burning of \(S\) in air (2) Burning of \(C\) in air (3) Conversion of white phosphorous to red phosphorous (4) Corrosion of metals

Short answer

Option 3: Conversion of white phosphorous to red phosphorous is a physical change.

Step-by-step solution

Understanding Physical Change

Physical changes are changes in the form or state of a substance without any change in its chemical composition. Examples include changes in state (like melting or boiling) and changes in shape or size.

Step-by-Step Evaluation of Options

Evaluate each option to determine if it represents a physical change.

Option 1: Burning of \(S\) in Air

Burning sulfur in air leads to a chemical reaction forming sulfur dioxide \(SO_2\). This is a chemical change as new substances are formed.

Option 2: Burning of \(C\) in Air

Burning carbon in air forms carbon dioxide \(CO_2\), which is also a chemical change due to the formation of new substances.

Option 3: Conversion of White Phosphorous to Red Phosphorous

This conversion involves a change in the physical form of phosphorus but no new substances are formed. Hence, this is a physical change.

Option 4: Corrosion of Metals

Corrosion involves a reaction with environmental substances (like oxygen) forming oxides or other compounds. This is a chemical change.

Conclusion

Among the given options, only the conversion of white phosphorous to red phosphorous is a physical change.

Key concepts

Physical Change

A physical change refers to a change in the state or properties of matter without any accompanying change in its chemical composition. This means that no new substances are formed. Examples include:
• Melting ice to water
• Boiling water to steam
• Shredding paper into smaller pieces
In a physical change, the identity of the substance remains the same before and after the change. For instance, water remains water whether it is in the form of ice, liquid, or steam. The conversion of white phosphorus to red phosphorus is an example of a physical change, where the form of phosphorus changes, but its chemical composition does not.

Chemical Change

A chemical change occurs when a substance undergoes a chemical reaction and forms one or more new substances with different properties. This process involves a change in the molecular structure of the substances involved. Some key characteristics of chemical changes are:
• Formation of new substances
• Release or absorption of energy (heat, light, etc.)
• Change in color
• Production of gas or formation of a precipitate
Examples of chemical changes include the burning of sulfur in air (forming sulfur dioxide), the rusting of iron, and the digestion of food in our bodies. During these processes, the original substances are altered chemically to form entirely new compounds.

Properties of Matter

The properties of matter are characteristics that allow us to recognize and differentiate substances. They can be classified into two main types: physical properties and chemical properties.
• **Physical properties**: These properties can be observed or measured without changing the substance's chemical identity. Examples include color, density, melting point, boiling point, and hardness.
• **Chemical properties**: These properties describe a substance's potential to undergo chemical changes. Examples include flammability, reactivity with acids or bases, and oxidation states.
Understanding the properties of matter is crucial for identifying substances and predicting their behavior during physical and chemical changes.

Phosphorus Conversion

Phosphorus exists in several allotropes, the most common being white phosphorus and red phosphorus. The conversion from white to red phosphorus is a physical change because it involves a change in the structure of the phosphorus atoms without altering the chemical composition.
• **White Phosphorus**: It is highly reactive and stored under water to prevent reaction with oxygen.
• **Red Phosphorus**: It is more stable and less reactive compared to white phosphorus.
The process of converting white phosphorus to red phosphorus involves heating white phosphorus in the absence of air. This rearranges the phosphorus atoms into a more stable structure, resulting in a change in physical form but not composition. Thus, no new substance is formed.