Which material is often used to test the distinction between aluminum and aluminum alloy?

The material used to test the distinction between aluminum and aluminum alloy is caustic soda. This is primarily due to the reaction that occurs between caustic soda (sodium hydroxide) and the components of aluminum and aluminum alloys. Pure aluminum has a higher resistance to corrosion than aluminum alloy and will react differently when exposed to caustic soda.

When immersed in a caustic soda solution, pure aluminum will not show significant reactivity, whereas aluminum alloys, particularly those containing copper, may show signs of attack or corrosion. This reaction helps in identifying the material type based on the response to the caustic environment, making it a useful testing medium for analysis.

The other substances listed do not provide the same level of distinction; for example, hydrochloric acid may react with both materials but is not specific to differentiating between them, while water and ethanol do not specifically test for corrosion resistance distinction in this context.