Layer alcohol down the side of the tube
Alcohol and water mix the moment anything stirs them. Learn why the tube is held at an angle, why the pour is slow, and what the boundary between the layers is for.
- The lighter liquid floats on the heavier one: Two liquids left alone settle with the less dense one on top. That is what puts the alcohol above the extract in this tube.
- Mixing takes energy: Two liquids that can dissolve in each other will still stay apart if nothing stirs them. A fast pour supplies the stirring without anyone meaning to.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Now look at the two ways the boundary gets destroyed, one at the pour and one afterwards.
A student adds the alcohol in one quick squirt straight down the middle. The tube now holds a single cloudy liquid with no visible line. What has been lost?
Reviewed- A.The boundary, so there is no one place for the strand to gather
- B.The alcohol, which has now been diluted below the strength it needs
- C.Nothing important, since the strand will still form, just more slowly
- D.The cells, which have been torn apart by the force of the squirt
Show the worked solution ▾
Answer: A. The boundary, so there is no one place for the strand to gather
- Step 1: Ask what the procedure promised: A clump appearing where the layers meet.
- Step 2: Ask what is now missing: There are no layers, so there is no meeting point.
Why it's right: The activity puts the clump at the meeting point of two layers. Blend the layers and that meeting point stops existing, so there is nowhere for material to gather and nothing for the stick to collect.
- B: Mixing does spread the alcohol through the whole tube, and that does work against the DNA coming out of solution. But the loss you can actually see and name here is the boundary itself.
- C: Waiting does not rebuild a boundary. Two liquids that have been stirred together stay together.
- D: The cells were already broken open on purpose in an earlier step, so a rough pour is not what damages them.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
After the pour the procedure says to let the tube stand undisturbed for two or three minutes. A group taps and rocks the tube to hurry it along. What are they undoing?
Reviewed- A.The separation they just built, since agitation is what blends these two
- B.The seal on the cap, which can loosen and let alcohol leak out onto the bench
- C.The timing, which has to be restarted from zero after any movement
- D.The cold of the alcohol, which warms when a tube is handled
Show the worked solution ▾
Answer: A. The separation they just built, since agitation is what blends these two
- Step 1: Ask what is holding the layers apart: Nothing but stillness.
- Step 2: Ask what tapping supplies: Exactly the stirring that stillness was withholding.
Why it's right: Stillness is the only thing keeping these two liquids in separate layers. Tapping and rocking supply the energy that mixes them, so the group is dismantling the arrangement the careful pour just created.
- B: A loose cap is worth checking, but rocking a capped tube is not what loosens it and a leak is not what a tap causes.
- C: There is no rule about restarting a clock. The problem is physical, not procedural.
- D: A hand on the outside of a tube for a few seconds is not what decides this, and the temperature was set before the pour.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
- A student sets the tube in a rack and steps back rather than holding it and fidgeting.
Fill these in as you work through the lesson.
- Interface (the line where the two layers meet):
- Density (why the alcohol ends up on top):
- Miscible (able to mix, which these two are):
- Undisturbed (what the tube has to be for two or three minutes):
Hold the tube at an and run the alcohol slowly down the . The alcohol is less than the extract, so it sits on top, and the strand appears where the two layers .
- Explain why these two liquids stay apart even though they can mix.
- Say what a fast pour straight down the middle destroys.
- Describe where the wooden stick goes and why there.
You tilt the tube, rest the pipette tip against the inside wall, and let the alcohol run down. It spreads across the top of the extract instead of driving into it, and a clear line appears between the two. That line is where you will be looking in three minutes.