Handling chemicals
Solids
- Powders: use a spatula or a folded paper chute. Tilt the test tube, slide the powder to the bottom, then slowly stand the tube upright.
- Lumps: use tweezers. Lay the test tube flat, place the lump at the mouth, then slowly raise the tube so the lump slides gently to the bottom without cracking it.
Liquids
- Pouring: put the stopper upside down on the bench, keep the label facing your palm (so drips don’t ruin it), and rest the bottle’s mouth against the test tube as you pour slowly.
- Adding drops: hold the dropper upright and just above the opening. Don’t put it inside or let it touch the walls.
If no amount is given, use 1–2 mL of a liquid, or just enough solid to cover the bottom of the test tube.
Reading a graduated cylinder
Set the cylinder on a flat surface and read with your eye level with the bottom of the meniscus (the curved liquid surface). If your eye isn’t level:
- Looking down from above: the reading is too high.
- Looking up from below: the reading is too low.
Drag the slider below to move your eye up or down and see where your line of sight meets the scale:
RuleReading a graduated cylinder: looking down gives a reading that’s too high, and looking up gives one that’s too low.
WhyA cylinder’s scale increases upward. Looking down, your line of sight slopes downward and crosses the wall above the bottom of the meniscus, where the numbers are larger.
Careful: when you’re measuring out a set volume (say, 6 mL), it works the other way around. Looking down, you stop at what looks like 6 mL but have actually poured less; looking up, you’ve poured more.
Heating
- Heat with the outer part of the flame.
- Liquid in a test tube: fill no more than 1/3, clamp the holder about 1/3 of the way down from the mouth, tilt the tube at about 45°, warm it by moving it back and forth before heating one spot, and never point it at yourself or anyone else.
- Solid in a test tube: tilt the mouth slightly downward, so condensed water can’t run back onto the hot glass and crack it.
Filtering
Filtering separates an insoluble solid from a liquid, such as removing mud from muddy water. In Chinese classrooms the key points are summed up as “one flat, two low, three touching”:
- One flat: the filter paper lies flat against the funnel with no air bubbles, or filtering will be very slow.
- Two low: the edge of the paper is below the rim of the funnel, and the liquid stays below the edge of the paper, so nothing runs down between the paper and the funnel.
- Three touching: the beaker’s lip touches the stirring rod; the rod’s tip rests against the triple-layered side of the paper; and the funnel’s stem touches the inside wall of the beaker below, so nothing splashes.
Before filtering, fold the round filter paper and set it in the funnel. The folded paper is three layers thick on one side and one layer on the other; that’s the “triple-layered side” in the “three touching” rule:
① Take a circle of filter paper and fold it in half along the dashed line.
② Fold the half-circle in half again along the dashed line.
③ You now have a quarter circle, four layers thick.
④ Open it into a cone by separating one layer from the other three: three layers on one side, one on the other.
⑤ Set the cone in the funnel and wet it with a little water so it lies flat against the glass. The stirring rod rests on the three-layer side.
The three-layer side is thicker, so the stirring rod can rest there while you pour without tearing the paper.
Evaporating
To get a solid back from a solution, such as salt from salt water, evaporate it in an evaporating dish:
- Stir constantly with a glass rod while heating, so no spot overheats and spits out drops.
- Stop heating once a good amount of solid appears, and let the leftover heat dry off the rest.
- A freshly heated dish is very hot. Pick it up with crucible tongs and set it on a wire gauze.
Example: two misreadings
A student measures a liquid in a graduated cylinder. The first time, looking up, they read 20 mL. After pouring some out, they look down and read 12 mL. Did they actually pour out more than, less than, or exactly 8 mL?
- First reading, looking up. Looking up gives a reading that’s too low, so the actual starting volume was more than 20 mL.
- Second reading, looking down. Looking down gives a reading that’s too high, so the actual amount left is less than 12 mL.
- Conclusion. Volume poured out = starting volume − volume left, which is more than 20 − 12 = 8 mL. They poured out more than 8 mL.
- Setting the stopper right side up on the bench while pouring
Set the stopper upside down so it doesn't pick up dirt from the bench.
- Reading a cylinder with your eye level with the bottom of the meniscus
That's the correct way to read it.
- Putting the dropper inside the test tube while adding drops
Hold the dropper upright just above the opening.
- Filling a test tube no more than 1/3 full before heating the liquid
Any more and it can boil over.
- Letting the liquid rise above the edge of the filter paper
Liquid then runs between the paper and the funnel, so the filtering fails.
- Heating an evaporating dish until every drop of water is gone
Stop once plenty of solid appears and let the leftover heat finish, or it will spit.
- Laying a test tube flat before adding a lump of solid
The lump can then slide gently to the bottom without breaking the tube.
Common mistakes
Mistake 1: Mixing up looking down and looking up. When reading, looking down gives a reading that’s too high, and looking up gives one that’s too low.
Mistake 2: Stirring inside the funnel while filtering. You’ll tear the filter paper. While filtering, the glass rod only guides the liquid.
Mistake 3: Pointing the mouth upward while heating a solid. Tilt it slightly down so condensed water can’t run back and crack the hot tube.
Quick check
Looking down at a graduated cylinder, you read 30 mL. The actual volume is
Looking down gives a reading that's too high, so the actual volume is less than 30 mL.
What is the glass rod for when filtering?
When filtering, the rod only guides the liquid. Never stir inside the funnel.
Summary
- Powders: tilt, slide, stand up. Lumps: lay flat, place, raise slowly. When pouring, stopper upside down and label to your palm. Hold droppers upright above the opening.
- Read a cylinder at eye level with the bottom of the meniscus. Looking down reads high; looking up reads low.
- Heat with the outer flame, keep liquids under 1/3 of the tube, never point it at anyone, and tilt it down for solids.
- Filtering: paper flat, levels low, three contact points. Evaporating: keep stirring, and stop heating once plenty of solid appears.
On to AP (for later)
AP labs use more precise tools: pipets, burets, and volumetric flasks. You still read them at the bottom of the meniscus, and you estimate one more digit (lesson 1.1.5). AP lab questions often ask whether a mistake makes a result too high or too low, which is exactly the kind of thinking you just practiced with the graduated cylinder.