C. Structure and Properties of Matter · Structure and Properties of Matter

Investigation: Classifying Solids by Their Properties

C2.6 C2.5 A1.1 A1.4 A1.5 A1.6 A1.8 A1.10 A1.11About 75 minutes

Learning goals

I can plan a safe investigation to classify solids, identifying variables and a control.
I can use evidence from melting, solubility and conductivity tests to classify ionic, molecular, covalent network and metallic solids.
I can identify sources of error and suggest improvements.
I can write conclusions that link evidence to bonding theory.

Words to know

independent variable
The factor the investigator changes on purpose (here, the solid being tested).

dependent variable
The factor that is measured in response (here, properties such as conductivity).

control
A comparison test that leaves out the factor being studied, such as distilled water alone.

electrolyte / non-electrolyte
A substance whose solution conducts electricity / does not conduct.

semiconductor
A material, such as silicon, whose conductivity is between that of a metal and an insulator and increases with temperature or doping.

Be the detective

A technician hands you five unlabelled jars of white or grey solids and asks, "What type of solid is each one?" You cannot see the particles, so you must infer the bonding from properties: exactly the kind of reasoning analytical chemists, materials engineers and forensic scientists use every day.

In this lesson you will plan and carry out an investigation (or analyse results from a school lab), using the theory from the last lesson to interpret evidence.

Planning the investigation

Question: Which type of solid (ionic, molecular, covalent network or metallic) is each sample?

Hypothesis: write one for each sample with a reason, for example: "Sample A is ionic, because it looks like table salt, so I predict it will have a high melting point, dissolve in water and conduct when dissolved."

Variables: the independent variable is the identity of the solid; the dependent variables are the properties you measure; controlled variables include the amount of sample, the volume and temperature of water, the voltage of the conductivity tester and the heating time.

Tests (in a school lab):

Appearance and lustre. Look with a hand lens.
Hardness and malleability. Press with a spatula; try to bend.
Conductivity of the solid. Touch both probes to the dry solid.
Solubility. Add a small amount (about 0.5 g) to 10 mL of distilled water and stir.
Conductivity of the solution. Test the liquid from step 4, and also distilled water alone as a control.
Melting. Heat a small sample in a test tube or on a metal spoon over a flame, for a fixed time (for example 1 minute). Record whether and how quickly it melts.

Safety (school lab): wear goggles; heat only small amounts; never heat a closed container; use tongs; do not taste or smell samples directly; dispose of solids in the container your teacher provides. Some samples (for example, naphthalene) give off vapours: heat only in a fume hood.

A simple conductivity tester. The probes are dipped into a solution (or pressed onto a solid). Current flows only if the sample contains mobile charged particles: ions or delocalized electrons.

Sample	Appearance	Melts in 1 min of heating?	Conducts as solid?	Dissolves in water?	Solution conducts?
A	white cubic crystals	no	no	yes	yes
B	white grains	yes, then turns brown	no	yes	no
C	shiny grey strip, bends	no	yes	no	(not tested)
D	clear, glassy crystals	no	no	no	(not tested)
E	white waxy lumps	yes, quickly	no	no	(not tested)

Sample results from a school investigation (distilled water alone did not conduct).

Analysing the results

A: high melting point, soluble, solution conducts, solid does not: ionic (for example, sodium chloride).
B: melts easily, soluble, but its solution does not conduct: molecular, polar (for example, sucrose).
C: lustrous, malleable and conducts as a solid: metallic (for example, magnesium or zinc).
D: does not melt, insoluble, does not conduct: covalent network (for example, quartz sand).
E: melts easily, insoluble in water: molecular, non-polar (for example, paraffin wax).

Sources of error and limitations

A Bunsen burner flame is hot, but a sample heated in a test tube for a minute often stays well below 800 °C, so it cannot melt an ionic solid like NaCl (801 °C) quickly; "did not melt" therefore means "melting point above what we could reach", not an exact value. A melting-point apparatus would give better data for low-melting samples.
Tap water contains dissolved ions and conducts; using it would make a molecular substance's solution look conductive. That is why the control with distilled water matters.
Some ionic compounds are almost insoluble (calcium carbonate, for example), so solubility alone cannot rule out ionic bonding. Test a melted sample's conductivity in a school lab, or use several properties together.
Small samples and a simple LED tester detect only fairly good conductors; a multimeter gives a more sensitive, quantitative reading.

Communicating

A good report states the purpose, hypothesis, materials, method, observations in a table, analysis (each classification with the evidence for it), sources of error, and a conclusion that answers the question.

A key for classifying an unknown solid using simple tests. Real samples can be borderline (some ionic solids are insoluble), so confirm with more than one property.

Worked example

Example 1: Classify from data

An unknown solid is hard and brittle and does not conduct. It dissolves in water, and the solution conducts. Classify it and explain.

Conducts only when dissolved: it contains ions that are fixed in the solid but free in solution.
Hard and brittle fits an ionic lattice.
Molecular solids (non-electrolytes) would not give a conducting solution, unless they are acids.

Ionic: the solution conducts because the dissolved ions can move.

Worked example

Example 2: Improving a method

A student tested sugar solution with a conductivity tester and the LED glowed faintly. Suggest two explanations and how to check them.

Explanation 1: the student used tap water, which contains dissolved ions. Check: test the water alone (control), then repeat with distilled water.
Explanation 2: the probes were contaminated with salt from an earlier test. Check: rinse the probes with distilled water and dry them between tests.
Sugar is molecular and is a non-electrolyte, so a pure sugar solution in distilled water should not light the LED.

Most likely contamination by ions (tap water or dirty probes); use a distilled-water control and rinse probes.

Worked example

Example 3: Writing a conclusion

Write a one-sentence conclusion for Sample D in the table.

State the classification.
Give the evidence: did not melt, insoluble, non-conducting.
Link it to the theory: strong covalent bonds throughout the crystal.

"Sample D is a covalent network solid, because it did not melt in the flame, did not dissolve and did not conduct, which is consistent with a continuous network of strong covalent bonds that must be broken to melt it and that has no mobile charges."

Just for fun

A chemist walks into a lab and asks for a sample of a covalent network solid. The technician hands her a grain of sand and says, "Don't take it for granite." (Granite is a rock that contains a lot of quartz, so the joke is at least chemically accurate.)

Hands-on activity

Home investigation: five solids

You need: table salt, white sugar, a birthday candle or small piece of paraffin wax, clean sand, aluminium foil; 5 small clear cups; distilled water if available (otherwise tap water and a cup of tap water alone as a control); a spoon; a small steel pan; and a conductivity tester made from a 9 V battery, a battery snap with wires, and an LED with a built-in resistor (from an electronics or hobby store) as in the diagram.

Safety (grown-up note): an adult must do all stove heating. Use only a pea-sized amount of each solid, on low heat, with the stove fan on; turn off the heat as soon as the sugar starts to brown, and never touch hot sugar or wax (it sticks and burns). Keep the conductivity test in salt water to a few seconds: running current through salt water for a long time produces small amounts of hydrogen and chlorine gas. Do not put the battery in water. Wash your hands afterwards.

Solubility: stir a quarter teaspoon of each solid into separate cups of water.
Conductivity of solutions: dip the two probes (not touching each other) into each cup, and into the control water. Rinse and dry the probes between cups. Record whether the LED glows.
Conductivity of solids: touch the probes to a dry pile of each solid, and to the foil.
Melting (adult only): place a pea-sized amount of salt, sugar, wax and sand on separate corners of a small square of foil in the pan, warm on low for up to 2 minutes, and observe.
Record your results in a table like the sample one and classify each solid with evidence.

What to look for: salt water lights the LED (ionic); sugar water does not (molecular); wax melts quickly and does not dissolve (molecular); sugar melts and then browns (molecular, decomposing); salt and sand do not melt on a stove (high melting points); sand does not dissolve or conduct (network); foil conducts as a solid (metallic). If your control tap water makes the LED glow faintly, explain why and compare brightness instead.

Your turn

Why is distilled water used as a control in conductivity tests?

Identify the independent and dependent variables in this investigation.

A solid conducts electricity, is lustrous and bends. Classify it.

A solid melts quickly over a low flame and its water solution does not conduct. Classify it.

Why can't a school Bunsen burner test tell you the exact melting point of NaCl?

Give one safety precaution for heating unknown solids.

Why should probes be rinsed between tests?

More practice: warm-up

Which type of solid conducts only when molten or dissolved?

Which type of solid has the lowest melting points?

What is a control in an experiment?

Which property best identifies a metal?

Name one tool that gives a more precise conductivity reading than an LED tester.

More practice: on your own

An unknown solid does not dissolve in water. List two types it could be and one further test to decide.

Why might an insoluble ionic compound such as calcium carbonate be misclassified? How could you avoid this?

Write a testable hypothesis about the conductivity of potassium iodide solution.

Suggest a way to make the melting test fairer between samples.

A student says "Sample E is molecular because it is white." Improve the reasoning.

Challenge

Design a test to distinguish graphite from a metal, given that both conduct.

A sample conducts weakly as a solid, much better when heated, and is hard and brittle. Silicon behaves like this. What type of solid is silicon, and why is this behaviour useful?

Propose a quantitative extension to this investigation that would give stronger evidence than the LED tester.

Check your understanding

Quiz

A solid does not conduct, but its water solution does. What type is it most likely to be?

Why is distilled water used instead of tap water?

Which result best identifies a covalent network solid?

In this investigation, what is the dependent variable?

Which solid would you expect to melt first when heated gently?

Which is a limitation of an LED conductivity tester?

For parents and teachers

Big idea (C2.6, with strand A skills): properties are evidence for bonding. Your teen should be able to plan fair tests, use a control, classify unknowns with reasons, and discuss limitations.

How to help: please do all stove heating yourself and keep amounts tiny. LEDs with built-in resistors and 9 V battery snaps are inexpensive at electronics or hobby stores; if you cannot get one, the activity still works for solubility and melting, and your teen can use the sample results in the lesson for conductivity.

Common difficulties:

Classifying from a single property (insoluble does not always mean network).
Forgetting the control or contaminating probes.
Writing conclusions without evidence ("It is ionic." instead of "It is ionic because ...").