Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: philonossis on April 14, 2010, 08:47:18 AM
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"Classify the following as 1) a heterogeneous mixture, 2) heterogeneous, but not a mixture, 3) homogeneous mixture, or 4) homogeneous, but not a mixture.
(a) an undissolved sugar cube in water
(b) a partially dissolved sugar cube in water
(c) a completely dissolved sugar cube in water
(d) an ice cube in water
The book's answers:
a an undissolved sugar cube in water= heterogeneous mixture
b a partially dissolved sugar cube in water= heterogeneous mixture
c a completely dissolved sugar cube in water= homogeneous mixture
d an ice cube in water= heterogeneous mixture. PROBLEM
My answer: d = heterogeneous, but not a mixture.
The book (Intro. to Chemical Principles, Stoker) states that "A heterogeneous mixture contains two or more visually distinguishable phases..." p. 92
It also states: "A mixture is a physical combination of two or more substances."
So, an ice cube in water cannot be a mixture according to the book's own definition, since water and ice are the same substance.
Can anyone explain who is right here?
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How does the book define substance?
But I agree with you that it is a little bit fishy.
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Since the ice cube and liquid water are two distinct phases, the book seems correct to state the system as a Heterogeneous mixture.
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Ice in water meets the first criteria:
"A heterogeneous mixture contains two or more visually distinguishable phases..."
You have Ice which is H2O(s) and water which is H2O(l) so you have a mixture of liquid and solid = 2 visually distinguishable phases
It's pretty bad wording in the book the second statement "A mixture is a physical combination of two or more substances" is a poor use of english
Ice and water are not necessarily the same substance in the literal dictionary definition of the word and therefore do fit the second description of a mixture.
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How does the book define substance?
The book defines substance as follows: "Substance is a general term used to denote any variety of matter. Pure substance is a specific term that applies only to matter that contains a single substance." (p. 90 Intro. to Chemical Principles, Stoker.)
Elsewhere the book states: "In addition to its classification by physical state, matter can also be classified in terms of its chemical composition as a pure substance or a mixture. A pure substance is a single kind of matter that cannot be separated into other kinds of matter using physical means. all samples of a pure substance contain only that substance and nothing else....
A mixture is a physical combination of two or more substances. It has variable composition. Its properties may vary as composition varies. Its components can be separated using physical means.
A compound is a pure substance that can be broken down into two or more simpler pure substances using chemical means. Water is a compound. Hydrogen peroxide is a compound.