How can you tell if a sample contains baking soda and cornstarch or baking powder?

To find out if a sample has baking soda we can add vinegar.

Explanation:

When vinegar (acetic acid) is mixed with salt (sodium bicarbonate), water, carbon dioxide and sodium acetate are formed. We see the formation of bubbles (which is carbon dioxide, which is a gas).

it is stable as it is in group 0 which have full outer shells of electrons