Density Tubes - Prof Bunsen

Density Tubes Kit
This is a very special density column. Students drop a mixture of
blue and white beads into a tube, add alcohol, water and salt and . . . the beads will autoseparate.
But then amazingly the two bead layers slowly move closer together to the middle of
the liquid layer . . . How can beads float in the middle of the liquid
What part does the salt play
You are supplied with:
Plastic pre-form tubes 6
Pre-form lids 6
Blue PVC beads 24
Clear LDPE Beads 24
Pure salt (sodium chloride)
30g
Measuring spoon 1
PVC: Poly vinyl chloride
LDPE: Low density polyethylene
The clear beads are also UV sensitive so they will change colours in the sun.
Required: You will need at least 20 mL isopropyl alcohol (isopropanol or 2-propanol) per
tube from the hardware store. Also known as ‘rubbing alcohol’. The “Diggers” brand is 100%
pure and is sold in 125 mL bottles. So one bottle will suffice for 6 tubes. This solution can not
be shipped to you due to its flammability.
Safety: Isopropyl alcohol is extremely flammable and should be kept away from all sources of
ignition. Avoid contact with eyes and skin.
Here’s how to get the students going
They can work in six groups.
Each group gets one tube, one lid, 4 blue & 4 clear beads, 20 mL alcohol and salt.
What is happening
Alcohol (iso-propanol) and water are miscible in each other (alcohol dissolves in water). This
is because water and alcohol both are polar and hydrogen bonds form between the water and
the alcohol molecules.
Water has a density of 1 g/mL and isopropyl alcohol a density of 0.86 g/mL. When mixed they
have a shared density of around 0.97 g/mL. The blue beads have a density of 1.16 g/mL and
the clear beads, 0.92 g/mL.
Salt is an ionic compound. In water it dissociates into sodium cations (Na + ) and chloride
anions (Cl - ). They are MORE strongly attached to the polar water molecules than the alcohol.
So they “push” the alcohol molecules out of solution. When the salt is added, the alcohol is
pushed (separated) into its own layer (0.86 g/mL) and becomes immiscible in the salt water
(1.20 g/mL). The beads float between the alcohol and salt-water layers as they have densities
intermediate to these. Because both liquid layers are colourless, the interface between the
alcohol and water-salt solution is hard to detect.
This technique is known as “salting-out” and is used by biochemists in separating proteins.
As the students say: “Water will rather hang-out with salt than with alcohol”
Original idea by science teacher, Lynn Higgins of Polymer Ambassadors in the USA.
Do not take a short cut by adding the salt to the water beforehand. Or the salt to the alcohol.
Stick to the procedure: First alcohol, then water, then salt.
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DENSITY TUBES
STUDENT WORKSHEET
Procedure
The object is to separate the two types of beads using two liquids and salt and the
principle of density.
1. Add 4 blue and 4 clear beads to the tube.
2. Fill the tube half way with the alcohol (isopropanol).
3. Cap and shake the mixture.
4. Stop shaking, observe and sketch where the beads and liquid are in figure 1.
5. Complete: The _______________ are more dense than the _______________.
6. Add water up to the protruding lip of the tube. Cap, shake, observe.
7. Sketch the position of the liquid and beads in figure 2.
8. Complete: The liquid (water-alcohol solution) is more / less dense than the clear
beads but more / less dense than the blue beads.
9. If a water-alcohol solution has a density of 0.97 g/mL, what would you guess the
density be of the
blue beads __________ g/ml
clear beads __________ g /ml
10. Measure 5 g of the salt and pour all of it into the tube. (This is about ⅔ of the
medicine measuring spoon supplied). Cap and shake . . . and shake . . . and
shake. Until most of the salt has dissolved. This may take several minutes.
11. Invert the tube (turn around) on its cap and watch closely. If the beads do not
separate and move as in the photo on p 1, dissolve more salt or add a few crystals.
12. Sketch the layers and position of the beads after 2 minutes (Figure 3).
13. Put names to the densities in Table 1.
14. Can you see where the two clear liquid layers meet in the tube (the interface)
15. Can you explain the unexpected movement of the beads when salt was added
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DENSITY TUBES
Group Name: _______________________________ Date: __________________
Group members: _____________________________________________________
____________________________________________________________________
Figure 1
Alcohol & beads
Figure 2
Alcohol, water
& beads
Figure 3
Alcohol, water,
salt & beads
(After 2 minutes)
Below we listed the densities of the substances / mixes in the test tube in Figure 3.
Can you put names to the densities
Table 1
0.86 g/ml ____________________
0.92 g/mL ____________________
1.16 g /mL ____________________
1.20 g/mL ____________________
Figure 4
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