When heated, oxygen reacts with copper to form copper oxide. + • + • oxygen molecule copper atoms copper oxide molecules Key: Oxygen atom Copper atom If this reaction occurs in a sealed container, will the mass of the container and everything in it increase, decrease, or stay the same and why? The mass will stay the same because the number of each kind of atom stays O A the same. The mass will decrease because two substances combine to form one B substance. The mass will increase because a new kind of molecule is formed.

When heated, oxygen reacts with copper to form copper oxide. + • + • oxygen molecule copper atoms copper oxide molecules Key: Oxygen atom Copper atom If this reaction occurs in a sealed container, will the mass of the container and everything in it increase, decrease, or stay the same and why? The mass will stay the same because the number of each kind of atom stays O A the same. The mass will decrease because two substances combine to form one B substance. The mass will increase because a new kind of molecule is formed.