Are states of matter not inter-convertible?

States of matter—solid, liquid, gas, and plasma—are indeed inter-convertible through changes in temperature and pressure. For example, heating a solid can turn it into a liquid (melting), and further heating can convert that liquid into a gas (vaporization). Conversely, lowering the temperature can cause gases to condense into liquids and liquids to freeze into solids. Thus, the states of matter can transition between each other under the right conditions.