012 – Physical properties and identification of products

No two pure substances share the same set of physical properties. Each has its own boiling point and its own density, and these constants are catalogued in reference tables that chemists consult every day. This is why an unlabelled bottle is never identified by guesswork: measuring one or two well-chosen physical properties and comparing the results against known values is usually enough to name a substance. The same approach is used in quality control, in forensic laboratories, and in checking the purity of manufactured chemicals.

Two properties carry this laboratory. The boiling point is the temperature at which a liquid’s vapour pressure equals the surrounding atmospheric pressure; while a liquid boils, its temperature stops rising, because the heat supplied is consumed by the change of state rather than by warming the liquid. Density is the ratio of mass to volume, ρ = m/V. It is an intensive property — a large fragment and a small one give the same value — and for an irregular solid the volume is most easily measured by water displacement, the insight attributed to Archimedes.

In this laboratory you will identify four unknown substances. Two unknown liquids are heated on hot plates set to 105 °C while thermometers record their temperature: the plateau each liquid reaches is its boiling point, and its name follows from it. Two unknown solids are then weighed on the electronic balance and immersed in a full overflow vessel, the displaced water being collected in a graduated cylinder; the mass and the volume together give the density, and the density gives the identity.

Educational Goals

Familiarization with the laboratory environment

• Locate and use the equipment of a wet-chemistry bench: hot plates, universal stands and clamps, magnetic stirrers, the electronic balance, the overflow vessel and the recovery bin.

Use of protective equipment and safe laboratory conduct

• Wear the protective equipment appropriate to handling near-boiling liquids, and dispose of used samples in the recovery bin.
• Explain why fingers must not touch the water during a displacement measurement: any immersed object adds its own volume and distorts the result.

Measurement of volume and mass

• Measure liquid volumes at the meniscus with the appropriate graduated cylinder.
• Weigh solids on an electronic balance using a weighing boat, and record every measurement with its unit.

Determination of a boiling point

• Assemble a heating setup with a clamped thermometer, a magnetic stir bar and a timed record of temperature.
• Recognise the temperature plateau that marks boiling, and use it — not the bubbling itself — as the measured boiling point.

Determination of density by water displacement

• Measure the volume of an irregular solid from the water it forces out of a full overflow vessel.
• Calculate density as ρ = m/V from a measured mass and a displaced volume.

Identification of substances from physical properties

• Compare a measured boiling point or density against reference values and justify the identification.
• State the limits of the comparison: how close two reference values can be before a single property no longer separates the candidates.

Protocol

Identification of unknown liquids

1. Precisely measure 50 mL of unknown liquid 1 with the 50 mL graduated cylinder.
2. Pour the liquid contained in the graduated cylinder into a 100 mL beaker labeled A.
3. Rinse the graduated cylinder.
4. Repeat steps 1 to 3 with unknown liquid 2, with beaker B.
5. Place beaker A on the left hot plate and beaker B on the right hot plate.
6. Place a magnetic stir bar inside each beaker.
7. Attach a universal clamp to the stands above the center of each beaker.
8. Place a thermometer in each beaker by fastening them to the clamps attached to the universal stands.
9. Start the stopwatch.
10. Start the magnetic stirrers.
11. Adjust the temperature of the hot plates to 105 °C.
12. In the results table, check the change in temperature.
13. According to the boiling temperature, what could the unknown liquids be?
14. When the maximum boiling temperature for each beaker has been reached, stop the magnetic stirrers.
15. Stop the stopwatch.
16. Lower the temperature of the hot plate to 15°C.
17. Detach the thermometers from the clamps.
18. Remove the magnetic stir bars from the beakers.
19. Empty the contents of the beakers into the recovery bin.

Identification of unknown solids

1. Using the tweezers, place 5 pieces of unknown solid 1 on the balance using the weighing boat. Record the total weight of the 5 pieces.
2. Using tap water, fill an overflow vessel to its full capacity before overflow (500 mL). Let the excess water flow into the sink.
3. Place the overflow vessel on the counter right next to the 25 mL graduated cylinder so that the latter is positioned under the overflow opening of the overflow vessel.
4. Gently empty the pieces of solid 1 contained in the weighing boat into the overflow vessel. Attention ! Fingers must not come into contact with the water so as not to distort the results.
5. Using the overflow from the overflow vessel and the meniscus in the graduated cylinder, determine the volume of solid 1.
6. Wait for the flow to stop completely.
7. Record the volume occupied. With the weight of the pieces weighed in step 1, you will thus be able to calculate the density.
8. Using the tweezers, remove the pieces of solid 1 from the overflow vessel and throw them into the recovery bin.
9. Empty the liquid contained in the 25 mL cylinder into the recovery bin.
10. Using tap water, fill the overflow vessel again to its full capacity before overflow (500 mL). Let the excess water flow into the sink.
11. Place the overflow vessel back on the counter right next to the 25 mL graduated cylinder so that the latter is positioned under the overflow opening of the overflow vessel.
12. Using the tweezers, place 5 pieces of unknown solid 2 on the balance using the weighing boat. Record the total weight of the 5 pieces.
13. Gently empty the pieces of solid 2 contained in the weighing boat into the overflow vessel. Attention ! Fingers must not come into contact with the water so as not to distort the results.
14. Using the overflow from the overflow vessel and the meniscus in the graduated cylinder, determine the volume of solid 2.
15. Record the volume occupied. With the weight of the pieces weighed in step 12, you will thus be able to calculate the density.
16. You will thus be able to identify the 2 unknown solids using their density.

Anticipated Outcomes

Part A — the unknown liquids. Both beakers are heated on plates set to 105 °C, just above the boiling point of water, so each liquid eventually reaches a plateau it cannot pass. The plateau temperatures are the identification.

A liquid boils when its vapour pressure equals the pressure of the atmosphere above it. From that moment the heat delivered by the hot plate is spent converting liquid into vapour rather than raising the temperature, so the thermometer reading stops climbing — a plateau — even though the plate is set 5 °C higher. Because unknown liquid #2 boils at 78 °C, well below 100 °C, it begins to boil first; observing which beaker boils sooner is itself evidence, and the plateau values then settle the identification.

Part B — the unknown solids. Five pieces of each solid are weighed, then dropped into an overflow vessel filled to capacity (500 mL); the water they push out runs into the 25 mL graduated cylinder, giving the volume of the pieces directly.

The governing relationship is ρ = m/V. For solid #1: ρ = 14.5 g ÷ 5.4 mL = 2.7 g/mL, which matches calcium carbonate’s reference density of 2.71 g/mL to within the reading precision of the cylinder. For solid #2: ρ = 63.75 g ÷ 12.2 mL = 5.2 g/mL, matching iron(III) oxide at 5.24 g/mL. Density is intensive, which is why five pieces give the same value as one — using five simply makes both the mass and the displaced volume large enough to read accurately.

The displacement method works because a submerged solid pushes aside exactly its own volume of water — Archimedes’ insight. It requires the solid to sink and to be essentially insoluble, which both of these are. It also explains the protocol’s warning about fingers: anything else entering the water adds its own volume to the overflow and falsifies the reading. You will also notice the water taking an orange tint after unknown solid #2 is added — fine particles of iron(III) oxide, the pigment of rust, suspend in the water. The colour is a useful qualitative clue, but the density is the evidence.

Summary of Assignment by Grade Range

Grade 9–10

• Focus: physical properties as identifiers, and careful measurement with correct units.
• Activities: measure volumes and masses; observe the two liquids as they heat and record the plateau temperatures; drop the weighed solids into the overflow vessel and read the displaced volume; compute each density with ρ = m/V and match all four unknowns to reference values.

Grade 11

• Focus: the quantitative treatment of boiling and density.
• Activities: sketch temperature against time for both beakers and explain the plateau in terms of energy going into the change of state; explain why the plate is set to 105 °C; carry the correct number of significant figures through both density calculations; explain why five pieces are used rather than one.

Grade 12 / College Level

• Focus: single-property identification and when a second property is needed.
• Activities: discuss the pressure dependence of boiling points; assess when density alone cannot separate two candidates and propose an independent confirming test; classify the properties used as intensive or extensive and justify why only intensive properties identify substances.

Laboratory essentials

Instruments

• Beakers (100 mL)
• Electronic scale & weighing boat
• Graduated cylinders (25 mL & 50 mL)
• Hot plates
• Lab stands & clamps
• Magnetic stirrers & stir bars
• Overflow vessel (500 mL)
• Recovery bin
• Thermometers
• Timer
• Tweezers

Products

• Unknown liquids (#1 & #2)
• Unknown solids (#1 & #2)
• Tap water