Here's a problem to solve: Which is a chemical change?

Among the 4 following options, which is the correct answer?

- A: Element 1 is hammered into a thin sheet.
 
- B: Element 2 is heated and turns into a liquid.
 
- C: Element 3 turns a greenish color as it sits in air.
 
- D: Element 4 is ground up into a fine, slippery powder.
 