Chemistry O Level Qn

POP222 SG

If steam and water has the same volume at 100 degree celsius and atmospheric pressure, will they have equal number of molecules.

Are the molecules able to move from water into the steam and from steam into water?Its in a clsed setup.

UltimaOnline SG

If steam and water has the same volume at 100 degree celsius and atmospheric pressure, will they have equal number of molecules.

Are the molecules able to move from water into the steam and from steam into water?Its in a clsed setup.

I specialize in the A levels, and we agreed that ChemGuide (my fellow Moderator) would handle O level Chem questions. So if you find my following comments not helpful to you at O levels, you can wait for others (eg. ChemGuide) to add in their comments.

(Also, it might make a difference whether you're an O level student yourself, or you're an O level teacher / tutor asking on behalf of your students).

At boiling point (for a given pressure) of a species, Gibbs free energy for the reaction in either direction is zero, and the system is at equilibrium between the liquid and gaseous states. Hence, the rate of the forward reaction (eg. vaporization of water) is equal to the rate of the backward reaction (eg. condensation of water).

Hence, the final answer to your final question is, yes of course.

Chemguide7 SG

Hi, for your first question, the answer is no. Steam is gaseous water and is a gas. A gas has weak forces of attractions between the molecules and the molecules are far apart. For water, the molecules are packed closer together since it is a liquid. So, given the same volume of water and steam at the same conditions, water will have more molecules packed into a given volume than steam.

At boiling point, liquid water molecules can escape the surface of the water and become gaseous water molecules (ie. steam). In a closed system, some steam can condense to become water. This is an equilibrium system. You may not have learned equilibrium concept yet. As an analogy, it is something like two rooms A and B, joined by a door. 50 people from room A can go into room B. Similarly, 50 people from room B goes into room A by the door. So, similarly, water (room A) and steam (room B) can "exchange" their molecules (50 people, number is used just as an illustration) in a dynamic equilibrium.

Ultimately, in a general Sec 3 States of Matter chapter scenario, given more heat energy from an external heat source, all the water can boil to become steam.