STP is a way of keeping measurements of temperature and pressure constant so that when different formulas called "Laws" which predict the way things behave in nature, are applied, there will be a way to find out the value of things you don't know. For instance, STP is used mostly in the Ideal Gas Law. The formula for this is PV = nRT where P is pressure, V is volume, n is the number of moles of something, R is the ideal gas constant (a number that never changes) and T is temperature. With STP, you know you are always going to be talking about how many atmospheres of pressure you have. And with temperature, you will always be talking about how many degrees on the Kelvin temperature scale you have. R never changes. So you can arrange the formula to solve for the volume in liters or for moles of a substance. (There are equivalents for atmospheres and liters and degrees Kelvin, but to be correct you would use atm, L, & K). There are other laws in chemistry that have to do with the relationships between pressure, volume and temperature and combinations of them. The Ideal Gas Law which uses STP as part of it's calculation relates volume, pressure, temperature and moles of a gas under 1 set of known conditions.
Sometimes you can tell if something is a solid, liquid or gas by looking at the formula or what you know from personal experience. If the formula talks about oxygen, it's usually a gas and steel is usually a solid. The phases (solid, liquid and gas) of all elements can change given the right conditions which usually has to do with temperature. For example even metals melt but some take over a thousand drgrees to do so. Some even melt at minus degrees temperature. I don't know of anyone who knows the phases of every element and what it takes to make them change. The periodic table has arranged elements so that different columns or "groups" share common properties so there are groups that are gases, groups that are metals, nonmetals or combinations. Basically if you want to know what phase an element is and under what conditions, you just need to look up that particular element to find out.
· 1 decade ago