Which of the following is considered not to be matter?

Heat is not considered matter because it does not have mass or occupy space. Matter is defined as anything that has mass and takes up space, and it is made up of particles. In contrast, heat is a form of energy that is transferred between systems or objects and is associated with the kinetic energy of particles, but it itself does not consist of particles that have definable mass or volume.

On the other hand, water, iron, and oxygen all qualify as matter because they possess both mass and volume. Water is a liquid composed of H₂O molecules, iron is a solid metal made up of iron atoms, and oxygen is a gas made up of O₂ molecules, all of which occupy physical space and can be measured.