* Chemical Change: Involves the formation of new substances with different chemical properties.

* Physical Change: Alters the appearance or form of a substance, but not its chemical composition.

Here's how etching glass with acid demonstrates a chemical change:

1. Reaction: The acid reacts with the silica (SiO2) in the glass.

2. New Substances: The reaction produces new compounds, like silicon fluoride (SiF4) and water (H2O). These compounds have different properties than the original silica.

3. Irreversible: The etching process is irreversible. You can't simply reverse the process to get the original glass back.

Therefore, etching glass with acid is a chemical change because it involves the formation of new substances with different chemical properties.