Most people would describe water like a colorless liquid. They __1__
( P' V' p# U+ K) F- H2 M% D would know that in very cold conditions it becomes a solid called
* a6 L, t' A- C0 {- |, ~$ s# }' e ice and that when heating on a fire it becomes a vapor called steam. __2__6 J7 H& w s1 E/ A n& G
However, water, they would say, is a liquid. We have learned that
1 z5 e; j7 R' @7 ~0 R3 V water consists of molecules composed with two atoms of hydrogen __3__8 i! ]- y. w% g- t1 o* J. | d
and one atom of oxygen, which we describe by the formula H2O.2 V2 ^) C# H8 C& u
This is equally true of the solid called ice and the gas called steam." `" `: N6 c5 d% D2 @$ ~" @6 h
Chemically there is no difference between the gas, the liquid, and
. v1 t: m- o& v the solid, all of which is made up of molecules with the formula H2O. __4__
6 e4 h) L9 Q( _+ A This is true of other chemical substances; most of them can exist as
4 q9 A) K5 ?: t8 D gases or as liquids or as solids. We may normally think of iron as a4 |/ S9 I! Q+ Q/ [: ?2 N& t
solid, but if we will heat it in a furnace, it will melt and become a __5__
. Q; G6 U+ }, j; ~5 S% W' q' C- u liquid, and at very high temperatures it will become a gas. Nothing/ z4 P2 E6 F" C* \- q! L
very permanent occurs when a gas changes into a liquid or a solid.2 @' |# d! s8 j% H2 D ~
Everyone knows that ice, which has been made by freezing water,
4 J3 B% |( N% K& [# w D can be melted again by warmed and that steam can be condensed __6__0 `8 h7 C* P2 i K* D5 F
on a cold surface to become liquid water. In fact, it is only because7 f! W7 O" j- x7 J0 [
water is so a familiar substance that different names are used for __7__
1 Z% ^' b5 U2 O+ m( {! h$ _/ G k the solid, liquid and gas. Most substances are only familiar with __8__
* c( t* a8 ~7 A$ C, c) ? us in one state, because the temperatures requiring to turn them __9__% y8 K4 x! O2 o
into gases are very high, or the temperatures necessary to turn them
2 C1 P3 y. B" X n+ v5 g. T0 E into solids are so low. Water is an exception in this respect, which
; ~* f4 B2 e% B9 E, X; g
5 {5 S8 T. c1 U2 i! W" p is another reason why its three states have given three different names. __10__ |