Most people would describe water like a colorless liquid. They __1__; ~& @& l# s4 @' E0 v% \
would know that in very cold conditions it becomes a solid called
: V+ `- \3 f( a4 ` ice and that when heating on a fire it becomes a vapor called steam. __2__9 D- e' `6 C& z; y0 j0 f* p9 T
However, water, they would say, is a liquid. We have learned that
; k+ m; X% Q$ M; V water consists of molecules composed with two atoms of hydrogen __3__) g# p5 A, N9 a3 i
and one atom of oxygen, which we describe by the formula H2O.
) W0 t8 n; l; w; x0 H7 w This is equally true of the solid called ice and the gas called steam.; L. o+ Q! ]0 ?# z
Chemically there is no difference between the gas, the liquid, and) V( |& H! Z' b8 G
the solid, all of which is made up of molecules with the formula H2O. __4__7 d }# y6 _ L B, S# Z1 m( t
This is true of other chemical substances; most of them can exist as, Q8 X$ z- x) S: `9 Z
gases or as liquids or as solids. We may normally think of iron as a
! c# R! V& X5 r" }& L3 f5 j solid, but if we will heat it in a furnace, it will melt and become a __5__
: K6 s" n% ~2 g; K4 v liquid, and at very high temperatures it will become a gas. Nothing
/ Q8 [6 I: |* K* c: z very permanent occurs when a gas changes into a liquid or a solid.4 A4 U- ?0 x: X5 {0 l
Everyone knows that ice, which has been made by freezing water,
9 |" H5 u% G* {$ G6 d$ i2 N5 h can be melted again by warmed and that steam can be condensed __6__
7 \4 E( Y3 T3 ~# D on a cold surface to become liquid water. In fact, it is only because" @5 U$ |; e9 m
water is so a familiar substance that different names are used for __7__
: d- |2 g4 V6 {- P# s s the solid, liquid and gas. Most substances are only familiar with __8__
; o& _8 F+ w8 ]1 g9 f& K( m us in one state, because the temperatures requiring to turn them __9__2 n# T2 Y7 m* s# a
into gases are very high, or the temperatures necessary to turn them% V5 Q, F0 d6 W) E+ w+ { v
into solids are so low. Water is an exception in this respect, which% J5 ] p8 q j) z I
4 ~4 q. i" Y! N5 U- b3 w
is another reason why its three states have given three different names. __10__ |
发表于 2012-8-14 11:29:10
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