Pre Diagnostic Assessment
Name: ________________________________
Date: _________________________
Your lab assistant is working on an experiment and
made two different mixtures that appear very
similar. However, she forgot to label them so she
needs your help to determine the difference between
the two. You decide to shine a light through the
samples to get a closer view of the mixture.
What type of mixture is in Tube A and B?
Correct answers:
1) Tube A is a homogenous mixture, and Tube B is a heterogeneous mixture
2) Tube A is a solution and Tube B is a colloid.
Partially correct:
3) Tube A and B are both solutions.
Why? Explain your thinking in detail. Please include scientific terms. (Hint: suspension,
colloid, solution, etc.)
Correct Answers:
1)
Two substances are not visible in Tube A. I infer that Tube A contains a homogenous
mixture because the two substances that were mixed together blended smoothly. In this
case if the solute was a solid, like salt, then it was soluble and dissolved in the solvent
evenly. If the solute was a liquid, then the substance is said to be miscible. When a
solute dissolves and a solution is uniform throughout, it is homogenous and light is not
scatter by the solution particles. Tube B is heterogeneous because the mixture appears
cloudy; it seems that there are two substances in the tube. This may be due to an
insoluble solid or immiscible liquid. Since the mixture particles are not uniform through,
they scatter light. This is the Tyndall effect.
2)
The mixture in Tube A is a solution or homogenous mixture, and the mixture in Tube B
is a colloid. Perhaps if Tube B was left longer to stand, the particles might sink down,
resulting in a suspension. Colloids and suspensions are two types of heterogeneous
mixtures. The beam of light helps us determine the types of mixture. Heterogeneous
mixtures scatter light, homogenous mixtures do not. Homogenous mixtures do not
scatter light because the mixture composition is uniform throughout the mixture. There
are no two substances visible in Tube A so one substance must have dissolved in the
other.
Reasoning:
The students may think that the mixture on the left is homogenous because there are no visible
particles floating in the liquid. If there was a solute in the left tube, then the substance was
soluble and dissolved in the solvent. For that reason it would be the better choice of the two
tubes. Nevertheless, the other mixture seems nicely blended as well, but what sets them apart is
the ability for light to shine light through the mixtures. The physical properties of the mixtures
set them apart. Student A knows the difference between a colloid and a suspension.
Partially correct:
3)
These mixtures are both solutions. Solutions can either be a combination of liquidliquid, liquid-gas, solid-liquid, solid-solid, or gas-gas. Tube A may be a mixture of two
liquids and Tube B may be a mixture of a solid and a liquid compound. The light is
shining through Tube B because not all the particles have dissolved yet.
Reasoning:
Since students are not sure what exactly is in the tubes, they have to make inferences. This
student may have the misconception that all mixtures are solutions. Since there are no two
distinct phases in both tubes, the students may think there are just different concentrations of
solute in the tubes. Or the student thinks that two liquids cannot cause a cloudy mixture. This
question is partially correct because the student realizes that at least one of the mixtures is a
solution, but the other is a heterogeneous. The student may not know that solutions are also
characterized as being insoluble (not dissolvable), thus making it a heterogeneous mixture that
can be separated by filtration.