Most people would describe water like a colorless liquid. They __1__+ @* t- p& ?, v, u# a
would know that in very cold conditions it becomes a solid called
5 N# S0 i2 c8 Q" E8 P4 b ice and that when heating on a fire it becomes a vapor called steam. __2__. y4 T) U" a* T7 q4 U, _! {
However, water, they would say, is a liquid. We have learned that
" n- L; {) E/ W$ J# X water consists of molecules composed with two atoms of hydrogen __3__1 f i" `' f$ A; q
and one atom of oxygen, which we describe by the formula H2O.& m. M, D& H D3 K) L/ }3 A7 {% }; X; ?
This is equally true of the solid called ice and the gas called steam.; b- F, H$ ~( {1 M- u$ z1 R$ w
Chemically there is no difference between the gas, the liquid, and' g. {6 q0 v: C! m! w5 V6 G t
the solid, all of which is made up of molecules with the formula H2O. __4__5 G `& N- t; G8 q% J+ m$ i1 L
This is true of other chemical substances; most of them can exist as
( u$ d) }' |$ S2 u- L, w7 C% Y4 ~ gases or as liquids or as solids. We may normally think of iron as a
/ U* j& Q) x* w* v solid, but if we will heat it in a furnace, it will melt and become a __5__6 W) }9 f8 P, v) t9 u
liquid, and at very high temperatures it will become a gas. Nothing$ e, `0 [9 b1 M+ f7 |$ `
very permanent occurs when a gas changes into a liquid or a solid.
( T7 x5 w9 [7 ?% H+ q/ q& A Everyone knows that ice, which has been made by freezing water,4 D9 v; r) {5 W# G6 |- E
can be melted again by warmed and that steam can be condensed __6__
& V; L! L; b8 S J9 v, b4 m on a cold surface to become liquid water. In fact, it is only because
" s" g, z! c' R0 {) m6 o$ T water is so a familiar substance that different names are used for __7__
( D; S) e# Z) F+ Z the solid, liquid and gas. Most substances are only familiar with __8__
' k, S% J; n, D/ A8 w3 e& s us in one state, because the temperatures requiring to turn them __9__# h3 S, X" l2 y
into gases are very high, or the temperatures necessary to turn them
' j2 O* ]2 l0 ~7 d' Z. j3 i8 n into solids are so low. Water is an exception in this respect, which
4 D8 ]6 Z+ y ~
l+ B1 s* u. _. [0 G, G 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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