Which of these represents a chemical change? A) two compounds combining to form a mixture B) a mixture being separated into two elements C) a compound being separated into two elements D) an element changing state from solid to liquid 364

the answer is C

Explanation:

A chemical change involves changing a sustance(s) into a different substance(s).

In (a), all you are doing is physically mixing two compounds. The compounds do not change their identities. You can separate them from each other if you want to by physical means (dissolution, filtering, etc.).

In (b), assuming that the mixture consists of only the two elements, nothing has changed its identity.  

In (d), changing from a solid to liquid is simply melting. If I melt an ice cube, solid H2O, into a liquid, the substance is still H2O.

In (c), to separate a compound into two elements, I have to break chemical bonds. Example:

2HgO(s) ==> 2Hg(l) + O2(g)

If I heat mercury(II) oxide, it will decompose into metallic Hg and oxygen gas. The substance changed its identity from HgO to Hg + O2.

A,C, is a chemical change as new substance is formed in these reactions and the reaction is not easily reversed

in B a mixture can formed again by mixing the two elemenets

in d the liquid can again turn to solid

b and d is a physical change

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.