Blot a sample before you put it on the balance
Water clinging to the outside weighs the same as water that moved inside. Learn why the instruction says dry it, and why it also says do not press and do not leave it.
- A balance weighs everything on it: It cannot tell water inside a bag from water clinging to the outside. Both arrive as the same number.
- Water crosses this membrane in both directions: The bag is deliberately permeable to water, which is what makes the experiment work and what makes squeezing or wicking it a problem.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Now take the second half of the instruction seriously, and see what an inconsistent method does.
The instruction also says not to leave the bag in the towel. What does leaving it there do?
Reviewed- A.The towel keeps drawing water out, now from inside the bag
- B.The towel sticks to the membrane and tears it when you pull it away
- C.The bag cools too much
- D.The towel leaves fibres on the bag that then show up on the balance
Show the worked solution ▾
Answer: A. The towel keeps drawing water out, now from inside the bag
- Step 1: Ask when the towel stops working: It does not. It keeps taking water while it is touching the bag.
- Step 2: Ask where that water comes from once the outside is dry: Through the membrane, from inside.
Why it's right: A towel in contact with a permeable bag goes on drawing water after the surface is dry, and at that point it is pulling from the inside. The instruction is bounding the job: take the surface water and stop.
- B: Wet membrane does cling to paper a little, and tearing is not what the instruction is guarding against.
- C: A minute in a paper towel does not change the temperature of the contents in any way that matters.
- D: Loose fibres weigh almost nothing, and the concern is water leaving rather than paper arriving.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
A group weighs their bag before soaking without blotting it, then blots it before the second weighing. What has gone wrong?
Reviewed- A.The bag was heavier at the start than it should have been for this experiment
- B.The balance needed re-zeroing between the first reading and the second one
- C.The two readings were taken different ways, so the change includes surface water
- D.Nothing has gone wrong here, because only the final mass is used in the calculation
Show the worked solution ▾
Answer: C. The two readings were taken different ways, so the change includes surface water
- Step 1: Ask what the calculation subtracts: The first reading from the second.
- Step 2: Ask what is different between them: Not just the soaking. Also whether the outside was dry.
Why it's right: A change in mass only means something if both readings were produced the same way. Blotting one and not the other builds the difference in method straight into the number they will report as their result.
- A: The starting mass was whatever it was. The problem is the comparison rather than the value of either reading alone.
- B: Re-zeroing between readings is reasonable practice, and it would not fix two readings taken by two different methods.
- D: The final mass on its own says nothing. The whole experiment is the difference between two readings.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
- A student blots the bag the same way before both weighings without being reminded.
Fill these in as you work through the lesson.
- Surface water (clinging outside, not part of the result):
- Blot (loosely wrap, do not press):
- Change in mass (what the experiment actually reports):
- Wicking (what a towel keeps doing if you leave it there):
Dry the of the bag by loosely wrapping it in a paper towel. Do not on it and do not it in the towel. You only want to remove liquid to the outside.
- Say what surface water does to a mass reading.
- Explain why pressing removes part of the result.
- Describe what a towel keeps doing if the bag stays in it.
You lift the bag out, lay a paper towel loosely around it, turn it once, and take it straight to the balance. The outside is dry and nothing has been squeezed out of the inside.