Final answer:
The correct answer to exercise 7.2.1, which asks for an example of a chemical reaction, is b. A bicycle rusting. This is because rusting involves a chemical change when iron reacts with oxygen to form iron oxide or rust, unlike the other options which represent physical changes.
Step-by-step explanation:
The subject matter of the exercise 7.2.1 falls under the category of Chemistry, specifically dealing with chemical reactions. The question asks which of the listed options is a chemical reaction. A chemical reaction involves the transformation of substances through the making or breaking of chemical bonds. Out of the options provided:
- Painting a wall blue is not a chemical reaction. It is a physical action of applying paint to a surface.
- A bicycle rusting is a chemical reaction. It is the result of iron (Fe) in the bicycle reacting with oxygen (O2) in the air, particularly in the presence of water (H2O), to form iron oxide (Fe2O3), commonly known as rust. This process involves a change in the chemical composition of the material.
- Ice cream melting is a physical change and not a chemical reaction. It is simply a change from one state of matter (solid) to another (liquid).
- Scratching a key across a desk is a physical change. No new substances are formed and no chemical bonds are broken or made.
- Building a sandcastle is also not a chemical reaction. It is a physical process that does not involve a chemical transformation.
Therefore, in regards to exercise 7.2.1, b. A bicycle rusting is the correct answer that represents a chemical reaction.