Testing for Bond Type

Six white solids, coded A to F. From across the room they are indistinguishable. You are going to work out, without ever seeing an atom, which of them are held together by transferred electrons and which by shared ones — and you are going to commit to an answer in writing before you test anything.

What you are trying to find out

Chemists talk confidently about ionic and covalent bonding as though they had watched it happen. Nobody has. The claim rests entirely on properties you can measure at a bench: what melts easily, what dissolves in what, and what conducts a current when.

So the real question today is not “which of these is ionic”. It is:

Which measurable property actually separates the two groups, and how much does one property on its own entitle you to say?

You also have a second, cheaper source of evidence. Electronegativity is tabulated for every element. If you know a substance’s formula, you can calculate the difference across each bond and predict the bond type on paper, before the bench opens. Whether the paper prediction and the bench evidence agree is the point of the whole period.

What you have to work with

Six coded solids. Their formulas are on the card at your station; their names are not.

• Distilled water, and a non-polar solvent — mineral oil, at room temperature and never heated.
• A battery-powered conductivity tester (the low-voltage probe with the LED). Nothing in this room plugs into the wall and touches a liquid.
• Hot plate with a stated maximum setting, a metal spatula, a crucible lid, tongs.
• Spot plate, small test tubes, stirring rod, thermometer.
• A table of electronegativity values.

Your design decisions, written down and checked before you run anything:

• The order of your tests. One of these tests discriminates far better than the others. Decide which, run it first, and say why.
• What “dissolves” means, operationally. A spatula tip in 5 mL, stirred for how long, judged how? Two groups using two definitions will disagree about the same solid, and neither will be wrong.
• How you will avoid a false conductivity reading. The probe must be rinsed with distilled water and dried between solutions. Think about what a wet probe carries into the next tube, and which way that error pushes your reading.
• What you will do about a solid that neither melts nor stays put. At least one of these will darken and smell rather than melt cleanly. That is data. Decide in advance how you will record it.

Do this part first, on paper: the electronegativity prediction

The difference in electronegativity across a bond, written $\Delta$EN, is a rough guide to how unequally the bonding electrons are shared. The usual classroom convention:

• $\Delta \text{EN}<0.4$ — essentially non-polar covalent
• $0.4\le \Delta \text{EN}\le 1.7$ — polar covalent
• $\Delta \text{EN}>1.7$ — ionic

Worked, for sodium chloride. Sodium has an electronegativity of 0.93 and chlorine 3.16, so $\Delta \text{EN}=3.16-0.93=2.23$, which is above 1.7. Predict ionic, and therefore predict a high melting point, solubility in water, and a solution that conducts.

Worked, for water. Oxygen is 3.44 and hydrogen 2.20, so $\Delta \text{EN}=3.44-2.20=1.24$ — polar covalent. A shared pair, pulled hard toward the oxygen but not handed over.

Now do all six of yours, and record the prediction before you go to the bench. The predictions you get wrong are the useful ones, which is the argument Mistakes Are Data makes at greater length.

One warning about those cut-offs, because you will meet the case. They are a convention somebody chose, not a law of nature. Hydrogen fluoride comes out at $3.98-2.20=1.78$, which the rule calls ionic — and hydrogen fluoride is a molecular substance that boils at around 20 °C, which no ionic compound does. The rule is a useful first guess with a known failure mode, and you should say so when you use it.

Hot plate, mineral oil, and six things that look like food

• You will recognise some of these solids. One of them may well be sugar. Nothing in this room is food, ever, including something that was food this morning — it has been in lab glassware, near lab chemicals, handled with lab spatulas. Nothing is tasted. This is the single most likely way today goes wrong.
• Hot solid and cold solid look identical, and so do hot and cold crucible lids. Move everything with tongs. A lid that has been off the plate for a minute is still hot enough to blister.
• Heat a spatula tip, no more, and do not take the hot plate above the setting written on your card. Sugars and organic acids darken and smoke if you push them, and the smoke is an irritant.
• Waft any odour toward you; never lean over a heated sample and inhale.
• The mineral oil is never heated. Not on the hot plate, not in a water bath, not “just to see”. It is there as a cold solvent only.
• The conductivity tester is battery-powered. Never substitute a mains-powered device, never test a solution with anything plugged into the wall, and dry your hands before you pick the probe up.
• Copper compounds are harmful if swallowed and are toxic to aquatic life. Solutions go to the labelled waste container, not the sink, and you wash your hands before you leave whether or not you think you touched anything.
• Unused solid goes to waste. Nothing goes back into a stock bottle.

The prediction you write first

For each of the six coded solids, before any testing:

1. Your electronegativity prediction — ionic, polar covalent, or non-polar covalent — with the $\Delta$EN you calculated.
2. The melting behaviour you therefore expect on a hot plate limited to a couple of hundred degrees: melts, or does not melt.
3. Whether you expect the solution to conduct, and whether you expect the solid to conduct.
4. Which solvent you expect it to dissolve in — water, oil, both, or neither.

Then one sentence: which single result would most surprise you, and what you would conclude if you got it.

What to collect

Code	Formula	$\Delta$EN	Predicted type	Melts below the limit?	Melting range (°C)	Soluble in water	Soluble in oil	Solid conducts	Solution conducts	Type from the evidence
A
B
C
D
E
F

Two things this table demands of you.

Units and resolution. A melting range is written as a range, in degrees Celsius, to the resolution your thermometer actually has — “melted between 52 °C and 58 °C” carries more information than “about 55”. Record the thermometer’s smallest division before your first reading.

A separate column for prediction and for evidence. Do not overwrite one with the other. The gap between the two columns is what you will be asked about.

What to bring to the consolidation discussion

• The completed table, with the prediction column untouched.
• Which single test separated the two groups best, and the evidence for saying so. Be specific: name the solid where that test settled an argument the other tests had left open.
• Every disagreement between your paper prediction and your bench result, with your best account of which one to trust and why.
• The solid that conducts as a solution but not as a solid, and what that combination rules out. This is the result with the most information in it and it is worth arguing about for ten minutes.
• Any solid that behaved like neither group.

What you should not claim

• One test is not an identification. Sugar dissolves in water and does not conduct; salt dissolves in water and does. Solubility alone separates nothing here, and a conclusion resting on it alone is a guess with a table around it.
• “Does not melt below the hot plate limit” is not “has a high melting point”. You measured the limit of your equipment, not the property of the substance. Write the first sentence.
• A solid that darkened and smelled did not melt. It decomposed, and a decomposition temperature is not a melting point. Recording it in the melting-point column would put a number in your table that is not the quantity the column names.
• Electronegativity difference is a prediction, not a measurement. It comes from a scale built to be useful, its cut-offs are a convention, and it fails on real substances — hydrogen fluoride above, and it is not the only one. Where it disagrees with your bench evidence, the bench wins.
• A damp probe pushes one way. Rinse water left on the electrodes dilutes the next solution, so a genuinely conducting solution reads weaker than it should. It never reads stronger. If one of your conductivity results was surprisingly faint, check when you last dried the probe before you start theorising.

Curriculum connection

B3.5

compare and contrast the physical properties of ionic and molecular compounds (e.g., $\mathrm{NaCl}$ and $\mathrm{CH}{}_4$; $\mathrm{NaOH}$ and $\mathrm{H}{}_2\mathrm{O}$)

Link to original

B2.5

predict the nature of a bond (e.g., non-polar covalent, polar covalent, ionic), using electronegativity values of atoms [AI]

Link to original

A1.5

conduct inquiries, controlling relevant variables, adapting or extending procedures as required, and using appropriate materials and equipment safely, accurately, and effectively, to collect observations and data

Link to original