Sec 1 1 The Rate of R99ctl99 -5 5 When hac a chemical reaction tnken pliice'? A species ran low its identill, by decomposition. combination, ur isomcrizaIion. As a consequence of the different configurations, these two isomers display different chemical and physrcal propertres. Therefore. we consider them as two different xpecie~ even thnuph each has the same number af atoms of ezch element. We say that a chemical reactiort has tnken place when a detectable num- ber of mulecufes of one or more species have lost their identity and assumed a new form by a change in the kind or number of atoms in the compound and/or by a change in structure or configuration of these atoms. In this classical approach to chemical change, it is assumed that the total mass is neither cre- ated nor destroyed when n chemicill reaction occurs. The mass refemd to is the totar collective mass of a11 the different species in the system. However, when considering the individual species involved in a particular reaction,
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