Many of the substances which we use every day are solutions. This unit provides students with a chance to see how the properties which we value in the substances we use are produced by mixing two or more pure substances. Students will compare solutions to other types of mixtures and observe some of the chemical and physical properties of solutions in this unit.
It also provides an opportunity to examine the benefits and problems of water as a solvent. For example, dissolved oxygen is vital to many aquatic organisms. But the same water may also carry contaminants which may harm these same organisms, or which may prevent the water from carrying a sufficient supply of dissolved oxygen.
The expectation is that symbols for elements and formulas for compounds will be used to build on the experiences the students had with these tools in grade six. Word equations will be sufficient to describe the components of chemical reactions.
Science writing and reading activities, as discussed in this Guide, should be incorporated into each lesson. Writing in personal, reflective journals, reading from newspapers and journals, and reporting on the activities of science class using varied formats such as recipes, directions, stories, and charts are ways through which students may refine their understanding of the concepts of science and develop their ability to communicate through the written word.
Science challenge, as described in this Guide, is meant to extend students' critical and creative thinking abilities in the context of the science concepts being studied. Activities involving science challenge should be incorporated into science lessons in each unit. The challenge is intended to give each student a chance to investigate an area of interest in more depth than would be possible for all students in a class to do. Individual or small group projects give students a chance to pick a topic of personal interest, and through research, bring back to the class information and perspectives which it would be impossible for everyone in the class to discover individually. Science challenge is a key strategy for bringing the Adaptive Dimension to the classroom, and for encouraging independent learning.
Factors: A5, B16, C8, C12, E9, F2, G2
Assessment Techniques: anecdotal records, written assignment, short answer test items
Instructional Methods: inquiry, synectics, compare and contrast
Factors: A4, A5, B3,
C2, C15,
F5, G6
Assessment Techniques: peer assessment, rating
scale
Instructional Method: problem solving
Continue to add each pile of starch to the mixture, stirring
as it is added. How do the properties of the mixture change?
When enough starch has been added so that the mixture can't
be stirred any more, remove the mixture from the jar and
knead it to mix the starch and water. One or two more drops
of water may be added if the mixture becomes crumbly. Try to
discover all the properties of this mixture, which is a
colloid.
Repeat the above procedure, using Epsom salts (or table salt
if Epsom salts aren't available). How do the starch/water
and the salt/water mixtures compare?
Factors: A1, A4, B16,
C1, C12, E9,
F2, G2
Assessment Techniques: observation checklists,
presentation
Instructional Methods: inquiry, problem solving
Why is a knowledge of properties of solutions and mixtures
necessary for a career in cosmetology?
The mixtures containing starch are examples of colloids. A
colloid which you might want to try is made by mixing 10
parts ethanol (ethyl alcohol) with 1 part saturated calcium
acetate solution. Saturated calcium acetate is made by
slowly adding 100 mL of water to 40 grams of solid
calcium acetate, stirring as the water is added. The colloid
produced is a gel of the type that is used to produce solid
fuels such as Sterno. The acetate/alcohol colloidal
gel will burn with a hot, blue (almost colourless) flame. A
good way to burn it is to place a piece on a wire screen
supported well above the surface of a table. Caution
students to stay back from the flame. Hair, eyebrows, and
eyelashes singe easily.
Predict what will happen when you float the test tube in a
250 mL beaker containing 200 mL water. Try it and
see.
Mark the water level on the wall of the test tube with wax
marker or piece of masking tape. Place the tube in a beaker
of alcohol. Change the volume of alcohol used. Compare the
way the test tube floats in the alcohol to the way it floats
in the water.
Put the test tube in a beaker of saturated Epsom salt
solution. How does it float there? Take half the Epsom salt
solution and mix it with the same volume of water. Predict
the height that the test tube will float in this mixture.
What use could such a test tube have?
What uses are made in industries of the relationship between
solubility and temperature change?
Into a reused styrofoam meat tray, pour a 0.5 cm to
1 cm layer of the solution. Stand the objects in the solution.
Let the tray sit undisturbed for several days. Observe the
formation of crystals. Keep a journal to record the observations
and development over time. A polaroid camera,
video camera or 35 mm camera with slide film could be used to
record the progress of the crystal formation. On which objects do
the crystals form best? How do the crystals vary?
Extension: Using a large tray and the objects which promote
the best crystal formation, create a garden, scene or
abstract display which will be crystallized with the above
solution.
Experiment with different types of solutes. Epsom salts,
iodized (table) salt, calcium chloride, copper sulfate
(bluestone), or potash are possibilities. Also vary the
proportions of the components, substitute food colouring in
water for bluing, the temperature at which the trays are
stored and other variables which the students may
identify.
Using the saturated solution produced in the investigation
above, or a saturated solution prepared for this part of the
activity if none was produced in the first part, investigate
the properties of the solution. What happens if an ice cube
and 25 mL of the solution are mixed in a baby food jar
(or in a 100 mL beaker)? Compare the rate of diffusion
of a drop of food colouring, a crystal of potassium
permanganate or a drop of coloured liquid dishwashing
detergent through the saturated salt solution and through a
sample of water at the same temperature. Petri dish halves
or jar lids are good containers for this activity. Will a
crystal of pickling salt or another crystal of Epsom salts
dissolve more quickly if left undisturbed?