Classification of Elements and Compounds

Classifying substances from evidence

Melting point, boiling point, solubility in water and electrical conductivity provide clues about a substance’s particles, bonding and structure. In this investigation, the required substances are sodium chloride, magnesium sulfate, hexane, liquid paraffin, silicon(IV) oxide, copper sulfate and sucrose (sugar).

The aim is to collect or compare property data and use the combined pattern to classify each substance. A conclusion should follow from the results rather than being decided in advance. One observation is rarely enough: different structural types can share a property, so several pieces of evidence should be considered together.

What melting and boiling reveal

Melting and boiling require attractions between particles to be overcome.

• Ionic substances usually have relatively high melting and boiling points because strong electrostatic attractions act between oppositely charged ions throughout a giant lattice.
• Giant covalent substances usually have very high melting and boiling points because many strong covalent bonds must be broken.
• Metallic substances mostly have relatively high melting and boiling points because of the strong attraction between positive metal ions and delocalised electrons.
• Simple molecular substances usually have relatively low melting and boiling points. Melting or boiling overcomes intermolecular forces between molecules, not the strong covalent bonds within each molecule.

A substance may be unsuitable for direct heating with school equipment. In that case, reliable published melting-point or boiling-point data can be compared instead. Record what the data actually show, including any stated range or decomposition, rather than forcing every substance into a single-temperature result.

What a solubility test shows

Solubility describes whether a substance dissolves in a particular solvent; here, the solvent is water. A soluble solid appears to disappear as its particles spread through the water and form a solution. An insoluble solid remains visible, while an insoluble liquid may remain as a separate layer.

Solubility is useful evidence, but it cannot identify bonding by itself. Ionic compounds are not all equally soluble, and molecular substances can differ in their solubility. A clear solution also does not reveal whether it contains ions or neutral molecules; conductivity provides an additional test.

What conductivity reveals

An electric current requires charged particles that are free to move.

• Metals conduct as solids because their delocalised electrons can move through the structure.
• An ionic solid does not conduct because its ions are fixed in lattice positions.
• A molten ionic substance conducts because its ions are free to move.
• If an ionic compound dissolves in water, the resulting solution conducts because mobile ions carry charge.
• Typical simple molecular substances conduct poorly because they have no mobile ions or delocalised electrons.
• Most giant covalent substances conduct poorly. Graphite is an important exception to this general pattern.

Therefore, a conductivity result must always include the state tested. “Does not conduct as a solid” and “does not conduct in aqueous solution” are different observations.

Planning a fair solubility comparison

Use labelled samples of all seven named substances, together with water, labelled test tubes or wells, suitable measuring apparatus and a stirring rod. Wear suitable eye protection and follow the laboratory instructions for each chemical.

1. Put the same volume of water into each labelled container.
2. Add the same small mass of each solid, or the same small volume of each liquid.
3. Stir every sample for the same length of time at the same temperature.
4. Record whether a uniform solution forms, some material remains, or a separate liquid layer is present.

The quantity of substance, volume of water, temperature and stirring time are control variables. Keeping them consistent makes comparisons more meaningful.

Testing electrical conductivity

A low-voltage conductivity meter can test whether a material allows current to pass. An LED lighting indicates conduction under the conditions of the test.

1. Check the meter with a known conductor and use deionised water as a low-conductivity comparison.
2. Test each solid sample as instructed, recording that the solid was tested.
3. Test each original liquid sample separately.
4. Where a substance forms an aqueous solution, test that solution as a separate sample.
5. Rinse the electrode tips with deionised water and dry them between samples so that ions are not carried from one sample to the next.
6. Use similar solution concentrations where possible and record the strength of the indication consistently.

Poor contact between electrodes and a powdered solid can affect the observation. Interpret conductivity alongside the other property results rather than treating a single negative reading as decisive.

Recording the named substances

A results table should separate observations from classifications:

Write “not applicable” when no aqueous solution forms rather than inventing a solution-conductivity result. Numerical melting and boiling data should include units.

Turning results into a classification

Compare each complete row with these general patterns:

Use cautious language when the evidence is incomplete. For example: “The solid did not conduct, but its aqueous solution did. This supports an ionic classification because the charged ions are fixed in the solid and mobile in solution.” A strong conclusion names the observations, identifies the likely structure and explains the link using particles.

Evidence patterns

• Ionic: usually high melting and boiling points; does not conduct as a solid; conducts when molten or in aqueous solution because ions can move.
• Simple molecular: usually low melting and boiling points; poor conductor; water solubility varies.
• Giant covalent: usually very high melting point, insoluble and non-conducting; graphite is a conductivity exception.
• Metallic: conducts as a solid because delocalised electrons can move; most metals have relatively high melting points.

Required samples

Investigate sodium chloride, magnesium sulfate, hexane, liquid paraffin, silicon(IV) oxide, copper sulfate and sucrose. Record melting or boiling evidence, water solubility, conductivity as a solid or original liquid, and conductivity in aqueous solution where one forms.

Practical recall

Keep the sample quantity, water volume, temperature and stirring time constant. Test solids and solutions separately with a low-voltage conductivity meter. Clean and dry the electrodes between samples. Use reliable published transition-temperature data when direct measurement is unsuitable.

Drawing conclusions

Electrical conduction requires mobile charged particles. Use the combined results to classify each substance, and keep observations separate from conclusions. Do not assume that dissolving automatically produces a conducting solution.