Re: re:mineral,ore,rock,metal

farooq_w_at_hotmail.com
Date: 12/29/04 

Date: 29 Dec 2004 03:56:40 -0800

edwin wrote:

>ok~thx you very much.You help me a lot...
>But I still have some questions to consult with you..
>1) Can we say that "because this is a chemical change , the >reactants
must involve the chemical property of themselves >unless this is
physical change so that the property for the >reactants is physical
property.

>In other words , chemical change must correspond with >chemical
property to make agreement
>and vice versa.....then physical change must correspond with >physical
property also to make agreement. Is it correct??

Check out this link and read it carefully.
http://antoine.frostburg.edu/chem/senese/101/matter/faq/physical-chemical.shtml

>2)Must the original reactants disappear in chemical >property/change??

Not necessarily complete disappearance, for example under equilibrium
conditions (reversible chemical reactions) there is a compromise
between the amount of reactants and products.

>3)Last question,
>sugar+water=sugar water (physical change)
>calcium oxide+water=calcium hydroxide (chemical change)

>I don't understand both are doing dissloving but why the >process of
producing sugar water is physical change and the >other one is chemical
change...please help me at all..thx:)

If you remember this criterion that a chemical change = formation of a
new substance/s which is/are *totally* different in chemical and
physical properties. Chemical changes are accompanied by large changes
in heat content i.e either heat is absorbed or evolved in a chemical
reaction.

In a physical change only *few* of the properties are transformed and
the change is such that we can not claim that a new substance has
formed.

Now taking your example:
a) sugar+water=sugar water (physical change)
When you dissolve sugar in water, do you notice the formation of a new
substance which is totally different from water and sugar? No. This
process involved sugar crystals being broken apart into their
individual molecules by water. So if you evaporate water you get back
sugar.

b)calcium oxide+water=calcium hydroxide (chemical change)

We call it a chemical change using the above mention criterion, i.e
when you add calcium oxide in water, a large amount of heat is evolved,
what you get is a white product, calcium hydroxide that neither
resembles water or calcium oxide in its physical or chemical properties.