Some Ideas for Teaching about Dissolving,
Mixtures, and Separating Mixtures
RTSmith 2003.01.13
Goal:
We have aspects of two benchmarks – dealing with dissolving, mixtures, and
separating mixtures – that are not covered by the BSCS Investigating Heat and
Physical Changes module. The following pages include suggestions for
supplemental activities for addressing these benchmark ideas:
IV.2.e.1.
Describe common physical changes in matter—size, shape; melting, freezing (K-2);
dissolving, evaporating (3-5).
Key concepts:
States of matter—solid, liquid, gas. Changes in size and shape—bending,
tearing, breaking. Processes that cause changes of state: heating, cooling.
See EH-V.2 e.1 (water in three states).
Real-world contexts: Changes in size or shape of familiar objects, such as making snowballs,
breaking glass, crumbling cookies, making clay models, carving wood,
breaking bones; changes in state of water or other substances, such as
freezing of ice cream, or ponds, melting wax or steel, puddles drying up.
IV.2.e.2.
Prepare mixtures and separate them into their component parts.
Key concepts:
Mixture, solution. Separation techniques—(K-2) filtration, using sieves,
using magnets, floating vs. sinking; (3-5) dissolving soluble substances,
evaporating.
Tools:
Filter paper, funnels, magnets, sieves, beakers, solar stills.
Real-world contexts: Mixtures of various kinds—salt and pepper, iron filings and sand, sand and
sugar, rocks and wood chips, sand and gravel, sugar or salt solutions.
Pre-requisites:
Complete at least "Lesson 5: It's Melting!" (and preferably Lesson 8) in the BSCS
Investigating Heat and Physical Changes module before attempting to deal with
dissolving and mixtures.
Individual Pre-assessment: Use one of the following tasks with your class:
1.
As a “quick write” journal entry, write a paragraph describing how you would
explain to a third-grade student the difference between dissolving and
melting. Include an example of each of these kinds of (physical) change in
your explanation. If you wish, you can include a diagram to help with your
explanation.
2.
While discussing "Lesson 6: It's Freezing!" with the members of your team,
you realize that one is saying that the rock salt "melts" the ice and the other
says the rock salt "dissolves" the ice. With whom do you agree? Why?
With whom do you disagree? Why? As a “quick write” journal entry, write a
paragraph describing how you think the rock salt and ice interact. Include in
your description whether you think melting or dissolving or both are involved,
and why you think that. If you wish, you can include a diagram to help with
your explanation.
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Whole-Class Pre-assessment: Begin this activity as a whole-class discussion after
having done one of the above individual pre-assessment writing tasks. Have ready
a large sheet of chart paper with three columns (the left one initially labeled "Melt"
and the right one initially labeled "Dissolve").
A.
By show of hands, how many of you have heard or used the word “melt”
before today?
B.
By show of hands, how many of you have heard or used the word “dissolve”
before today?
C.
Consider (i.e., think for a minute about) the following:
Where (or when) does melting or dissolving occur?
Why might it be useful or important to know something about melting or
dissolving?
D.
OK, now let’s begin with a pretty straightforward question. Most of you
indicated that you’ve at least heard the words “melt” and “dissolve” before
today. Who can give me one example of something melting or something
that can melt?
E.
Record student’s example on a large Post-It note. Place on a large chart
table in the “Melt” category.
F.
Repeat steps “D” and “E” to get examples of melting from several students.
At this point, don’t bother with disagreements; simply record ideas.
G.
Then do steps “D” through” “F” for several examples of something dissolving
or something that can dissolve. Place these Post-It notes on the large chart
table in the “Dissolve” category.
H.
Now ask if any students disagree with the placement of any of the previous
examples. Have them explain why they disagree. Re-label the left column
"Agree Melts" and the right column "Agree Dissolves" and label the middle
column "Disagree." Move the corresponding Post-It notes to the “Disagree”
column on the table.
Continue this step until all disagreements have been established. Clarify that
anything that remains in the left or right columns indicates that these are
examples about which we agree, some being instances of melting and some
being instances of dissolving.
[Possibly encourage students to record this table in their science journals.]
I.
Ask students to consider for the moment just the examples in the “Agree
Melts” and “Agree Dissolves” columns; ignore the “Disagree” column. Can
we use the examples about which we agree to refine our explanation of how
melting and dissolving are different? Are there any things about melting and
dissolving that are alike or the same or similar? In other words, can we
“compare and contrast” these two kinds of events (or changes)?
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J.
Start a new table to record current ideas about melting and dissolving.
Include two columns: “Alike” and “Different.” Also start a third table as
needed with “Questions for Ongoing Consideration.”
Make explicit that what we are trying to do at this point – beyond learning
about, clarifying, and comparing and contrasting students’ ideas about
melting and dissolving – is to see if we can use the examples of melting and
dissolving about which we agree to come up with a way (that is, a method or
procedure) that someone could use to decide about those examples where
we currently disagree. If we can settle some of those disagreements now,
great. If not, then we can at least record our ideas and questions, and then
working on this issue can be part of what we investigate and learn about in
the days ahead.
Save each of the three tables generated today for future reference and further
refinement.
[Possibly encourage students to record the second and third tables in their
science journals as time and interest permit.]
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Making Iced Tea:
Demonstrate making iced tea. Heat water in a tea kettle (or microwave, etc.) and
then pour into a clear glass. Have a student describe what they observe. Add a tea
bag to the hot water. Have one or more students describe what they observe. Add
a couple of sugar cubes and stir (until the sugar cubes completely
dissolve/disappear). Have one or more students describe what they observe. Add a
couple of ice cubes and stir (until the ice cubes completely melt/disappear). Have
one or more students describe what they observe.
Ask the students what is similar and what is different with what happened for each of
the three things (i.e., the tea bag, sugar cubes, and ice cubes) that you added to the
glass of hot water. Ask the students which (if any) were an example of something
melting. Ask the students which (if any) were an example of something dissolving.
Have the students consider what conditions are necessary for something to melt and
what conditions are necessary for something to dissolve. For example, set just a
sugar cube on a desktop and ask if it will melt or dissolve or not change under this
condition (i.e., setting on a desk at room temperature). Then set an ice cube on a
desktop and ask if it will melt or dissolve or not change under this condition (i.e.,
setting on a desk at room temperature).
Guide the students' discussion to recall that melting requires a transfer of heat
causing a change of state. For a substance to melt, the temperature of the solid
substance must first be raised to its melting point. (For pure water, that temperature
is 0ºC.) Room temperature is not hot enough to melt the sugar cube, but it is hot
enough to melt the ice cube. (With sufficient heat, we could get the sugar cube to
melt, but it would require a temperature well above room temperature.) An object
can melt in isolation; all that is required is the transfer of a sufficient amount of heat
to the object to cause it to change state from a solid to a liquid. In contrast,
dissolving requires that one object/substance be placed/mixed in with a different
substance. (We usually think of dissolving as mixing a solid into a liquid. But any
molecular-level mixture is an example of dissolving, whether it is solid in solid, solid
in liquid, liquid in liquid, gas in liquid, etc.) When a substance dissolves, it breaks
into the tiniest pieces possible (i.e., molecules) and mixes in amongst the tiniest
particles of the substance in which it is being dissolved. (Also, heating is not
required for one substance to dissolve in another. For example, it is possible to
dissolve sugar into ice water, but it does require more stirring. Heating can increase
the rate at which dissolving occurs.) The dissolved particles are too small to be
seen with the naked eye; therefore the object may seem to disappear (e.g., as
happens with the sugar cubes in the glass of water). But we know the sugar is still
in the water because we can taste the sweetness when we drink the mixture. (And
the mass of the sugar-water solution is the same as the sum of the individual
masses of the sugar and water before they were mixed. In other words, mass is
conserved in a physical change like dissolving.) When some colored substances
dissolve in water (e.g., tea), while the individual particles of tea that dissolve in the
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water are too small to be seen individually, they still retain their color, and so color
the whole "solution."
Both melting and dissolving are kinds of physical changes. You still have the same
substances present after the change as you did before the change. And both
changes can be reversed. You can reverse melting by removing heat until the
substance freezes. You can reverse dissolving, for example, by evaporating or
boiling away the water to recover whatever you had dissolved in the water (e.g.,
sugar).
When you dissolve one substance in another substance (e.g., dissolve sugar in
water) you are making a mixture. A mixture is just mixing two different things
together in a way that both components (and their properties) are retained. In many
mixtures, the components are big enough to still see and distinguish, even after
mixing. For example, a serving of peas and carrots is a mixture (in which you can
still see and distinguish the individual peas and pieces of carrots). Dissolving is
simply a special case of creating a mixture in which the individual pieces of the
mixed substances are each too small to see (i.e., the substances are mixed at the
level of individual molecules).
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Separating Mixtures:
There are a number of ways of separating mixtures. Selecting which method to use
can depend on the properties of each of the components of the mixture. For
example, with a mixture of peas and carrots, it is relatively easy to tell the peas from
the carrots (based on their properties of color and shape), and you could separate
them one piece at a time using your fingers.
However, with some mixtures, even though the individual components are visible,
they may be too small or too numerous to separate one piece at a time by hand. For
example, imagine trying to separate a mixture of sand and salt by hand. In such
cases, we need to use other methods – based on differences in the properties of the
components of the mixture – to separate the components. Such methods can
include:
using a sieve
when the components differ significantly in size
(e.g., gravel and sand)
using (dissolving and then) filter paper
which is essentially a very small sieve
(e.g., sand and salt mixed/dissolved in water;
sand and sugar mixed/dissolved in water)
using a magnet
when one component is attracted to a magnet
and the other is not
(e.g., steel paper clips and aluminum soda pop
can tabs; iron filings and sand)
using electrostatic attraction
when one component is easily attracted to a
charged object and the other is not
(e.g., pepper and salt)
using floating and sinking
when one component will sink in water and the
other will float
(e.g., steel paper clips and wood chips, rocks and
wood)
using evaporation, boiling, or distillation
when one component can be changed from a
liquid to a gas (and thus removed) while leaving
the other component behind (typically as a solid)
(e.g., salt or sugar dissolved in water)
Consider allowing students opportunities to make and then separate examples of
mixtures (like those listed in the table above) using a variety of separation
procedures. This may best be done by using a "centers" approach and having
teams of students rotate through each of several centers where each center involves
a different mixture and method for separating it.
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