Tieng anh nganh hoa

UNIT 5 :
SOLUTIONS
If sugar and water, two pure substances, are mixed together, a solution result, uniform throughout
in its properties, in which the sugar can neither be seen with a microscope nor filtered out. It is not
distinguishable from a pure substance in appearance.
The experimental distinction between a pure substance and solution is quite simple when the solute
/the dissolved substance/ is not volatile so that it is left behind when the solvent is evaporated. However,
when both are volatile the matter is not quite so simple and it is necessary to find out whether any change
in composition and hence in properties occurs during a change in state.
Suppose we wish to determine whether air is a pure substance or a solution. One method would be
to liquefy a certain amount and then observe what happens to it as it slowly evaporates. As the
evaporation proceeds one may observe that
a- The light blue color gradually becomes deeper
b- The temperature of the liquid slowly rises
c- The densities of both liquid and gas change.
Any one of these as well as other possible observations show that air must contain two or more
components whose relative amounts change during the evaporation, causing the observed changes in
properties due to differences between the components in color, volatility, density, chemical behavior. Still
other properties might have been used.
The term solution is not restricted to liquid solutions. All gases are completely miscible with each
other, forming but one phase, so that every mixture of gases is a solution. Alloys of silver and gold, no
matter what the relative amounts of the two metals, contain but one kind of crystal,/the properties of
which change continuously with the composition/, thus being a solid solution.
If liquid air is distilled in a scientifically constructed still, it is possible to separate it into two nearly
pure constituents. One of these constituents, nitrogen, is found to be slightly lighter than air; it can be
condensed to a colorless liquid boiling at -194 0 C; it is very inert chemically, reacting with but few other
substances. The other constituent, oxygen, is slightly heavier than air; it gives, when condensed at low
temperatures, a blue liquid boiling at -182.5 0 C, and it reacts readily with many substances.
As another illustration, suppose we have a solid metal, which appears to be perfectly homogeneous
under the microscope. We could determine whether it is a solution or a pure substance by melting it,
dipping into the melt a suitable thermometer and letting it cool slowly, taking temperature readings at
regular intervals, and plotting temperature against time.
EXERCISES
A. Read and translate into Vietnamese
result, throughout, microscope, appearance, experimental, distinction, solute, volatile, solvent, evaporated, matter,
occur, determine, liquefy, observe, evaporation, proceed, gradually, observation, relative, cause, changes,
volatility, behavior, restricted, miscible, completely, phase, alloy, metal, continuously, solid, scientifically,
constructed, separated, condensed inert, react, illustration, suppose, appear, perfectly, suitable, thermometer,
interval.
B. Answer the following questions
1. What is a solution?
2. Is it distinguishable from a pure substance in appearance?
3. When is the experimental distinction between a pure substance and a solution simple?
4. What is the difference between a solute and a solvent?
5. How can you determine whether air is a pure substance or a solution?
6. Is the term solution restricted only to liquid solutions?
7. What does it mean when a substance is volatile?
8. Give the constituents of air and compare them with each other.
9. Give some liquids that are miscible.
10. Give some examples of solids soluble in liquids.
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