Ellen Weisa, age 11, of Albany, N  Y  0 for her question:

Can a gas be changed to a solid?

A very common gas in the air is water vapor  We cannot see it or smell 1t and it is mingled with the other invisible gases of the atmosphere  Yet with our own eyes we have seen a lump of solid ice which is the solid form of gaseous water vapor  The solid ice is liquid water and water vapor are all made from the same type of particle, a basic particle made from one atom of oxygen and two atoms~of hydrogen 

In the gaseous state, the small individual particles are free and widely separated  In the liquid, state, these same particles are next door neighbors with a tendency to cling to each other and follow the leader  In the solid state, these particles are looked together in a fragile lattice of tiny crystals, The gaseous water vapor becomes solid ice 

In the process of becoming a solid, the vapor loses a lot of heat energy  Its particles become less active, less free to rush and move around  This loss of activity is the same thing as loss of heat energy  At a certain temperature the vapor becomes chilled to liquid water  In the case of pure water, this temperature is 100 degrees centigrade  This is the boiling point of water, the temperature at which liquid water turns to vapor 

When liquid water is chilled to 0 degrees centigrade, it becomes solid ice  At this temperature, a block of ice begins to melt, In the case of water, the temperature of the freezing point is the same as that of the melting point  This is true of many other substances also  However, most gases can be turned to the solid state with the help of pressure added to the chilling temperatures 

This is because the solid form of most gases occupies less space than the warmer liquid state 

In the case of frozen water, 'tiny pockets of apace are interlocked in the

crystal lattice work  The ice takes up a little more space than water and compressing, or squeezing the particles together will not help them

become solid 

Compression, however, does help in the formation of cry ice, which is the solid form of the gee carbon dioxide  The freezing point at which carbon dioxide becomes solid drat ice is minus 8 degrees centigrade  Nitrogen and oxygen each have their own boiling points at which they change from a liquid to a gaseous stets and their own freezing points at which they become solid  Oxygen, the vital gas we breathe, becomes a solid when it reaches a temperature of minus 218 centigrade degrees  The nitrogen which makes up about 78 per cent of the air would become frozen solid should the earth lose the sun and all its heat and chill to a temperature of minus 23  C centigrade degrees 

Hydrogen, the lightest of the geese, becomes a liquid when it is chilled to a temperature of about minus 252 centigrade degrees and at about 258 centigrade degrees it becomes a frozen solid  Helium has the chilliest freezing point of all the gaseous elements  Its freezing point is around minus 272 degrees centigrade, which is absolute zero  At this temperature all heat energy is lost from any substance and its particles frozen to a solid standstill