The marks are on the outside of the tube, the bottom of the meniscus is on its axis. With your eye 8 cm above the level (looking down), your line of sight lands on a mark above the liquid: a true reading of 12.34 mL is seen as 12.16 mL, 0.18 mL too small. Looking up, it reads 12.52 mL. A burette has 0 at the top, so looking down reads low and looking up reads high; a pipette has one mark, and lining it up from above actually delivers only 19.99 mL.

For 20.00 mL of acid (sample A), the pH one drop before equivalence is 3.92; one more drop (0.05 mL) takes it to 10.08, beyond phenolphthalein’s range, and the solution turns red. So the last step needs half a drop (0.025 mL): pH 9.78, pale pink that lasts half a minute. With 21.73 mL of 0.1000 mol/L NaOH used, c(HCl) = 0.1086 mol/L; the label says 0.1085 mol/L.

When 0.1000 mol/L hydrochloric acid is titrated with NaOH of the same concentration, the pH within ±0.1 % of equivalence jumps from 4.30 to 9.70, and methyl orange, methyl red and phenolphthalein all change inside the jump. Acetic acid’s jump is only 7.75–9.70, with pH 8.73 at equivalence (the solution is alkaline): methyl orange changes at pH 4.4 after only 6.10 mL, an error of about −70 %, so only phenolphthalein will do. At 0.0100 mol/L the hydrochloric acid jump shrinks to 5.30–8.70 and even methyl orange is off by −0.79 %.

Every error passes through c(unknown) = c(standard)·V(standard) / V(unknown). Water in the flask does not change the amount of acid, so the result is unchanged. A burette not rinsed with the standard dilutes it, V(standard) grows and the result is 1.01 % high; a flask rinsed with the acid holds extra acid, 2.49 % high; a tip bubble that disappears during the run, 0.46 % high; reading the end from above, 0.83 % low, from below, 0.83 % high; a pipette not rinsed with the acid, 1.01 % low. Two mistakes can add up or partly cancel.

Adding 2 mol/L NaOH to 2 mL of 0.1 mol/L AlCl₃, the Al(OH)₃ is greatest at the 6th drop and has all dissolved as [Al(OH)₄]⁻ by the 8th: precipitating and then dissolving is the mark of Al³⁺. Fe²⁺ also first gives white Fe(OH)₂ with NaOH, but it soon turns grey-green and finally red-brown. A clearer test for Fe²⁺ is KSCN first (no red), then chlorine water (red).

Sodium carbonate solution also gives white Ag₂CO₃ with AgNO₃, and white BaCO₃ with BaCl₂. So acidify with dilute nitric acid before testing for Cl⁻, and add dilute hydrochloric acid first, see nothing, then BaCl₂ when testing for SO₄²⁻. Dropping dilute hydrochloric acid into sodium carbonate, the first two drops only make HCO₃⁻ and give no bubbles; from the third drop CO₂ is released and turns limewater cloudy.

Sodium’s 589 nm yellow is so intense that a trace of sodium in the sample hides potassium’s colour. Potassium’s characteristic lines are at 766.5 nm and 404.4 nm; blue cobalt glass passes only 2.0 % at 589 nm but 74 % at 766.5 nm and 77 % at 404.4 nm, so you see lilac. Dipping the wire in dilute hydrochloric acid and heating until colourless turns residues into volatile chlorides that burn off, so last time’s sodium does not get mixed in.

Among 13 candidate salts, a single test can only split them into a few groups. Each test rules out the salts that do not match: sample number 7, tested in the order “base, heat, KSCN, flame, BaCl₂”, leaves only NH₄Cl after 5 tests. The predicted observations for every candidate come from the same ion reaction table, so the reason for each elimination is what you saw.

A measuring cylinder has 0 at the bottom and its marks increase upwards; a burette has 0 at the top and its marks increase downwards. Looking from above, your sight line lands above the liquid in both, so the cylinder reads high and the burette reads low.

A random error in a single run cannot be noticed. Only when two or more volumes agree within 0.10 mL is the mean reliable; an overshot run cannot be corrected, only redone.

No. The equivalence point is where acid and base have exactly reacted; the end point is where the indicator changes colour and we stop. A well-chosen indicator keeps the difference within ±0.1 %.

The bubble is pushed out of the tip by liquid; that volume counts in the reading difference but never reaches the flask. V(standard) reads too large, so c(unknown) comes out high.

The NH₃·H₂O formed from NH₄⁺ and OH⁻ only releases NH₃ when heated; NH₃ dissolves in the water on the paper and makes it alkaline, so red litmus turns blue.

If the solution contains SO₃²⁻, nitric acid oxidises it to SO₄²⁻ and misleads you. So add dilute hydrochloric acid first to remove interference from CO₃²⁻, SO₃²⁻ and Ag⁺.

No. Electrons in the metal atoms absorb energy, jump to higher levels and emit light of particular wavelengths as they fall back: a physical change.

Start with the test that splits the candidates into the most groups. One round of NaOH separates those with a precipitate from those without, and the precipitate colour splits them further; then tell the anions apart with BaCl₂, AgNO₃ or dilute acid.