Explain why a steam burn from gaseous water at 100 °C is worse than a water burn involving the same amount of liquid water at 100 °C.
Burns or, more accurately, burn effects (blisters, redness, etc) are caused by heat, not temperature. The short answer is that steam at 100 C contains more heat than water at 100 C. Thus more, and more severe, burn effects come from the steam than from the water.
The reason steam has more heat, in the same amount of mass, is because water takes on additional heat to change from its liquid state into its gas state (i.e., into steam). That is, up to 100 C the heat raises the temperature of the water. But at 100 C, as the water changes to steam, more heat is added without changing the temperature.
That extra heat is called the heat of vaporization...it stays stored in the steam as a latent heat that will be released when the steam liquefies. And that extra heat is passed on into your body as the steam condenses on your skin. That extra, latent heat is why the burn effects are more severe.
When water at 100 degree C is converted into steam at 100 degree C, an additional heat energy of 540 Calories per gram of water is utilised. Hence, the energy possessed by steam is much higher than that of boiling water at 100 degree C. This additional energy (which is called "LATENT HEAT OF STEAM) is the cause of the severe burn.