Copper(II) oxide is a black solid. It dissolves in acids to make copper(II) salts. It can react with carbon monoxide or hydrogen to make copper metal. It dissolves in very concentrated alkalis. It dissolves in ammonia to make a dark blue solution, similar to copper(II) hydroxide.
Copper(II) oxide is used to color ceramics. It is also used to make thermite. It can react with many toxic chemicals to oxidize them, so they are not toxic any more. It can be added to vitamins to get copper into the body. It can also be added to clothing to kill germs.
Copper(II) oxide is toxic when eaten. It can be eaten in very small amounts, such as in vitamins. This adds copper to the body, which is needed.