Which substance can be broken down by a chemical change? a. be b. b 3. h2o 4. mg?

A compound, which contains two or more kinds of atom, can be broken down by a chemical change. However, an element, which contains only one kinds of atom, cannot be broken down by a chemical change.

Among the four options, only H₂O are compounds, while the rest (Be, B and Mg) are elements. Therefore, only H₂O can be broken down in a chemical change.

...... The answer is : 3. H₂O