Solid carbon dioxide, also known as dry ice, undergoes a process called sublimation, where it changes directly from a solid to a gas without passing through the liquid phase. This is because the pressure at which carbon dioxide exists at normal atmospheric conditions is below its triple point, which is the temperature and pressure at which a substance can exist as a solid, liquid, and gas. Therefore, when solid carbon dioxide is heated, it changes directly to a gas without going through the liquid phase.