1. Mathematics & Science Center
Today we will perform a lab that is part of a new web based lesson on the Mathematics & Science Center’s web site we call ‘Mystery Cargo’. I’ll tell you later in the lesson how you can go to this web site and complete activities like this at home or at school.
Mathematics & Science Center

2. Objectives Classify as homogeneous or heterogeneous.
Explain how light interacts with mixtures.
Define solution.
Explain “like dissolves like.”
Define mixture.
Objectives:
Define “mixture”. Can anyone give me an example of a mixture?
Classify mixtures as homogeneous or heterogeneous...two big science terms. How many of you have heard these words before?
Define “solution”. Can you tell me what kind of mixture a solution is? (Homogeneous…you will define solution further using slides 5 &6)
Explain how light interacts with mixtures. You will test some mixtures with a beam of light during your experiment today.
Explain “like dissolves like”. Have you heard that expression before? We will view some animations that will enable us to see how materials interact with water and understand why some substances do not dissolve in water.
Don’t give students all the answers at this point or even acknowledge the answers they give you as right or wrong–--keep them thinking with neutral facial expressions– ---and comment like ‘we will check your responses as we go through the lesson’.

3. Mixtures
Mixtures are an important part of your everyday life. You eat, drink and breathe mixtures. You use a mixture to brush your teeth and you walk on surfaces that are mixtures. Most natural substances are mixtures.

4. Properties of Mixtures
Substances may be separated physically.
Composition of mixture may vary.
No chemical reaction occurs.
Mixtures are formed when two or more unlike substances that do not react chemically are combined. An example would be iron filings and sulfur.
The amounts of each component in a mixture can vary. I can add more iron filings and still have the same type of mixture.
And mixtures can be physically separated back into the original substances. Which of these substances can I remove with this magnet?

5. Heterogeneous or Homogeneous?
Types of Mixtures
Heterogeneous or Homogeneous?
Scientists classify mixtures into various groups based on observable characteristics…. Do the particles dissolve? Do the particles separate? Are the particles visible to the human eye? Do the particles reflect light?
You will classify the mixtures you work with today as either homogeneous or heterogeneous. These pictures represent what you would see immediately after adding a small amount of flour to the glass on the left and a small amount of sugar to the glass on the right.
Homogeneous mixtures, commonly referred to as solutions, have a uniform appearance and composition throughout. The particles in a homogeneous mixture are the size of atoms or molecules – too small to be seen. Light passes straight through a homogeneous mixture giving it a clear appearance.
Heterogeneous mixtures consist of visibly different substances or phases. Some heterogeneous mixtures, called suspensions, contain individual solute particles that are too large to stick to the solvent particles and remain permanently suspended in the solvent. These particles may stay temporarily suspended. Light passing through this type of heterogeneous mixture will be dispersed by these larger particles so that the beam of light is visible in the mixture. Eventually, the particles will separate and sink to the bottom or float to the top.
Ask students based on what they see immediately after mixing to classify each as homogeneous or heterogeneous.
Click to show 2nd set of pictures. Look at the pictures one hour later. Were you correct? How do you know?
The sugar and water mixture is homogeneous. Beam of light is not visible in the mixture.
The flour and water mixture is an example of a heterogeneous mixture. The individual suspended particles are too small to be seen directly by the human eye but light is scattered by these particles and the mixture looks cloudy. Some of the flour has settled to the bottom of the glass.

6. Solutions Solute Solvent Solution
Now, can you tell me which kind of mixture a solution is?
Yes, homogeneous mixtures are referred to as ‘solutions’.
The solute is the substance dissolved in a liquid. The solvent is the liquid. The solvent is said “to do the dissolving.” For example, sugar is the solute and water is the solvent in a sugar-water solution.
Water is the most common solvent so let’s explore how water molecules interact with different substances.
Solvent
Solution

7. “All Mixed Up?” Team Report Mathematics & Science Center
Substances Mixed With Water
Dissolve?
Separate?
Reflect Light?
Homogeneous?
Heterogeneous?
Salt
Pepper
Tea Leaves
White Vinegar
Oil
Yes No No
Yes No No
Yes
Yes No
Yes No
Yes
Yes No
Yes
We will be careful to use very small amounts of each solid to determine whether or not the solid will dissolve because we have a small volume of water.
The ones that reflect light have particles suspended in the solution that may settle with time. If the light beam is not visible the solid has dissolved completely to form a solution or the substance may have separated from the water molecules. Careful observations will be important.
Yes No No
Yes No No
Yes No No
Yes
Mathematics & Science Center

8. Like Dissolves Like
The red ball represents an oxygen atom and the two gray balls represent hydrogen atoms. Do you know what molecule they will form? (water)
When all the electrons present in a atom exist in pairs, that atom is in its most stable state. How many electrons does an oxygen atom have? (If they don’t know, refer to atomic number, etc.) The graphics we will use today only show the electrons in the outermost energy level (Bohr ring) of the atom – the one furtherest from the nucleus.
Look at the picture. How many electrons does oxygen have in the energy level furtherest from its nucleus?
Atoms may become more stable by forming a union we call a bond.
Two hydrogen atoms and one oxygen atom overlap orbitals and share electrons to form a water molecule. This sharing is called a covalent bond. The oxygen atom has a stronger attraction for the electrons that are shared by the two atoms. Positive and negatively charged regions form on the molecule because of this unequal sharing of the electrons in the overlapped region. This kind of sharing results in a polar molecule. These charged regions cause the molecules to attract each other forming the liquid we call water. We will see later how these polar water molecules are also attracted to and able to dissolve other polar molecules.

9. Like Dissolves Like
Metal atoms that come in contact with nonmetal atoms are not able to overlap and share electrons.
Nonmetals have a stronger attraction for electrons. Metal atoms will transfer excess unstable electrons to the nonmetal atom. Both atoms become more stable charged particles. The atom that gained electrons become negatively charged. Can you tell me why? The metal atom becomes positively charged. The oppositely charged particles are now attracted to each other just like N and S poles on magnets. A new substance forms.
In this example the blue circles represent chlorine atoms and the yellow circles represent sodium atoms. A salt crystal forms as more of the oppositely charged particles beginning sticking together. The result of this type of attraction is called an ionic bond.
Water can dissolve this type of substance by interacting with the individual charged particles forming the crystal. As the crystal becomes smaller we can no longer see it and we say the salt has dissolved in the water.

10. What happens at the molecular level?
Sugar 
Oil 
Sand 
Let’s watch water interact with these 3 substances and see what we can learn about the particles these substances are made of.
Oil: molecule does not have any charges within it that are strong enough to cause an interaction with water that forms a solution. Ask students why the oil floats on top of the water. Oil is less dense (lighter) than water.
Sand: Grains of sand do not have any charges within them strong enough to interact with water and form a solution. Why do they sink?...Sand is more dense than water.
Sugar: Water surrounds each sugar molecule and separates it from other sugar molecules making up the crystal. Sugar molecules contain charges strong enough to interact with water molecules and form a solution. The crystals of sugar you can see with your eyes contain thousands of sugar molecules. As individual sugar molecules are taken away by water the crystals disappear and we say the sugar has dissolved. A solution has formed.

11. Colloids
Fill two jars almost full of water. Put some sugar in one jar and a small amount of instant coffee creamer in the other. Recap the jars and shake. The coffee creamer is used because it doesn’t sour as easily as milk. Will last o.k for several days. Place on overhead projector and remove caps from jars for the discussion.
Remember how we used the flashlights to test our solutions today? A light beam allows us to identify another special type of mixture we call a colloid. Is this a new word?
Colloids are a special class of heterogeneous mixtures containing particles that consist of clumps of molecules so tiny that they look homogeneous to the naked eye. Does this cup of milk look homogeneous?
The particles in a colloid generally do not settle over time. Most colloids are opaque. What do I mean by ‘opaque’? But some colloids may appear clear when very dilute. Just as with other heterogeneous mixtures, the suspended particles in a colloid are large enough to scatter light making the beam of light visible in the liquid mixture.
Sometimes there may be doubt as to whether a mixture that appears homogeneous is a solution or a colloid. A light beam is the easiest way to determine whether a mixture is a true solution or a colloid.
Let’s look at the two mixtures on the overhead. Which one is a true solution? A colloid? The milk looks like a homogenous mixture …. We can’t see boundaries between the different kinds of particles that make up the mixture. But what happens as the light passes though the milk? Yes, the light is scattered by the tiny particles suspended in the water and the cup appears to glow. All of the light passes through the red solution of sugar water.

12. Colloids
Some colloids are composed of a gas and a liquid –whipped cream is a colloid.
A marshmallow is an example of a colloid composed of a gas and a solid. Every bite of the marshmallow tastes the same because the particles are pretty evenly distributed throughout just like in a solution.
Milk and mayonnaise are examples of a colloid composed of two liquids.
Aerosol sprays , egg white, jellies, paint, and even your own blood are other examples of colloids.

13. Adding flour to water with stirring forms ________. a compound
a heterogeneous mixture
a homogeneous mixture
Let’s review what we have learned.

14. What happens when a beam of light passes through a homogeneous mixture?
The mixture appears clear.
The mixture appears opaque.
The mixture scatters the light and appears to glow.

15. The composition of a mixture _______.
is always constant
may vary
never varies

16. All mixtures contain a liquid and a solid.
True
False

17. A colloid is a special type of heterogeneous mixture.
True
False

18. Visit
for more adventures…