Handle a substance you cannot identify
An unknown gets the protection the worst plausible hazard would need. Learn what you never do to identify something, and what you write on the tube.
- Unknown means you have no hazard information: There is no safety data sheet for a substance you cannot name, so the only safe assumption is the most protective one.
- Your senses are not test instruments: Smell and taste give you almost no identification and they cost you an exposure you cannot take back.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Now protect the run itself: identifiers on containers, and unknowns kept apart from each other.
Four tubes hold four visually identical powders. What has to be true before the first reagent is added to any of them?
Reviewed- A.The rack is placed away from the edge of the bench
- B.Each tube carries its own written identifier
- C.The tubes are all the same size and shape
- D.The tubes are lined up in the order given by the procedure
Show the worked solution ▾
Answer: B. Each tube carries its own written identifier
- Step 1: Ask what survives a knock: Position in a rack does not survive being bumped. Writing on the tube does.
- Step 2: Tie the tube to the record: An identifier is only useful if it matches something in the table you are filling in.
Why it's right: Position in a rack is destroyed by one bump, and four identical powders cannot be told apart afterwards. Writing on each tube ties it to a row in the table, so a knocked rack costs nothing.
- A: Keeping the rack away from the edge reduces the chance of a knock, which is good practice and not the same as surviving one.
- C: Matched glassware makes a tidy bench and gives you no way at all to tell one powder from another.
- D: Order in the rack is exactly what a bump destroys, and nothing on the tubes would let you rebuild it.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
To free up rack space, a group wants to tip two leftover unknowns into a single container. Why is that refused?
Reviewed- A.Mixing changes the colour and makes the powders harder to photograph
- B.The combined volume would exceed what one tube holds
- C.The container would then need two identifiers instead of one
- D.The identities are gone and a reaction cannot be ruled out
Show the worked solution ▾
Answer: D. The identities are gone and a reaction cannot be ruled out
- Step 1: Ask what you can still say about the mixture: Two unidentified substances combined give one substance with even less known about it.
- Step 2: Ask what could happen on contact: Ruling out a reaction requires knowing both identities, and here you know neither.
Why it's right: Combining two unidentified substances produces a third thing that is even less known, and ruling out a reaction between them would require knowing what both are. Neither the safety question nor the disposal route can be answered afterwards.
- A: Photography is a documentation preference. The objection is the unpredictable chemistry and the lost identity.
- B: Volume is a container-choice problem with an easy fix. It is not why the mixing is refused.
- C: A mixture cannot honestly carry two identifiers, because it is no longer either of the two substances.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
- A student labels every tube in the Activity 1.2.3 run before adding hydrochloric acid or ferric nitrate to any of them.
Fill these in as you work through the lesson.
- Unknown (no identity, so no hazard data):
- Presumptive test (narrows the possibilities, does not confirm):
- Incompatible (reacts badly with something else):
- Unique identifier (what makes one tube different from three others):
An unknown is handled as though it were the plausible hazard, and it is never identified by or by mixing it with another unknown.
- Give two reasons smelling an unknown is not a test.
- Explain what has to be on four tubes before any reagent is added.
- Say what you would do with an unknown still on the bench at the bell.
Four tubes of pale powder look identical. Before any reagent is added, each tube gets its own written identifier that matches a row in your data table. Now a spilled tube costs you one row, not the whole run, and nobody has to guess which powder produced which result.