Difficulty: Introductory | Prerequisites: None, though familiarity with basic algebra and unit conversions will help.
Density and chromatography are two of the most fundamental lab techniques you will encounter in introductory chemistry. Density connects mass and volume into a single measurable property that identifies substances, while chromatography lets you pull apart mixtures to see what is actually in them. Both topics test your ability to measure carefully, handle lab equipment, and interpret results quantitatively. If you missed earlier labs, start here: these two concepts underpin nearly every separation and identification experiment that follows.
Density is mass divided by volume, and whether something sinks or floats in water depends on whether its density is above or below 1 g/mL. Chromatography separates mixtures by exploiting differences in how strongly each component sticks to a stationary phase versus how readily it dissolves in a mobile phase. Know how to calculate an Rf value and when to use displacement versus direct measurement for volume.
Density
A physical property of all matter, defined as mass per unit volume. In simple terms, it tells you how tightly packed the stuff inside an object is.
Mass
The amount of matter in an object, measured with an electronic balance in grams. Think of it as how much "stuff" is there, regardless of shape or size.
Volume
The amount of space an object occupies. For liquids and regular solids you measure directly; for irregular solids you use water displacement.
Meniscus
The curved surface of a liquid in a graduated cylinder. You read the volume at the bottom of the curve, not the edges.
Chromatography
A technique for separating a mixture into its individual pure components. Think of it as a race where different substances travel at different speeds through a medium.
Retention factor (Rf)
The ratio of the distance a substance travels to the distance the solvent front travels. It is a dimensionless number between 0 and 1 that helps identify what is in a mixture.
Stationary phase
The fixed medium (such as paper or a silica plate) that components cling to during chromatography. The more a substance likes this phase, the slower it moves.
Mobile phase
The solvent that moves through the stationary phase, carrying components along with it. In the MSU lab, a dilute salt solution (1% NaCl) served as the mobile phase.
Capillary action
The ability of a liquid to flow upward through narrow spaces without external force. This is the mechanism that draws solvent up the chromatography paper.
Tartrazine
The chemical name for Yellow No. 5, a synthetic food dye. In the lab, it was identified by extracting dyes from yarn and running chromatography.
A physical property, meaning it does not depend on the amount of substance you have.
Calculated as D = mass / volume.
The density of pure water is 1 g/mL, which serves as the reference point for sinking and floating.
An object with density greater than 1 g/mL sinks in water.
An object with density less than 1 g/mL floats.
Use an electronic (analytical) balance.
Units are grams (g).
Always zero (tare) the balance before measuring.
Liquids and graduated cylinders: Read the volume at the bottom of the meniscus, with your eye level at the liquid surface.
Regular (geometric) solids: Measure length, width, and height, then calculate V = l x w x h.
Irregular solids (water displacement method):
Record the initial volume of water in the graduated cylinder.
Submerge the solid.
Record the new volume.
Subtract the initial volume from the final volume to get the volume of the solid.
Chromatography separates a mixture into its pure components based on differences in solubility and affinity.
The solvent rises through the paper by capillary action.
Stationary phase: The paper (or silica plate). It stays fixed. Components that interact strongly with this phase move slowly.
Mobile phase: The solvent moving through the paper. Components that dissolve readily in the mobile phase travel further and faster.
Separation happens because each component partitions differently between the two phases.
Thin-layer chromatography (TLC): Uses a thin coating of adsorbent on a plate. The type used in the MSU lab.
Column chromatography: Uses a column packed with adsorbent material. Good for larger-scale separations.
Gas chromatography (GC): The mobile phase is a gas. Used for volatile compounds.
Rf = distance travelled by the spot / distance travelled by the solvent front.
Each compound has a characteristic Rf under the same conditions, which helps with identification.
Rf is always between 0 and 1.
Dye extraction: 5% acetic acid was used to extract dyes from yarn. Ammonia was then used to release the extracted dye.
Chromatographic separation solvent: 1% NaCl solution served as the mobile phase.
Key identification: Yellow No. 5 = Tartrazine. This was the target dye identified in the lab.
D = \frac{m}{V}Where D = density (g/mL or g/cm³), m = mass (g), V = volume (mL or cm³).
R_f = \frac{\text{distance travelled by spot}}{\text{distance travelled by solvent front}}Rf is unitless and always falls between 0 and 1.
V_{\text{solid}} = V_{\text{final}} - V_{\text{initial}}Used in the water displacement method for irregularly shaped solids.
Density is the reason oil floats on water and why ice sits on top of a frozen lake rather than sinking to the bottom. Engineers use density measurements to verify the purity of metals, detect counterfeit coins, and design objects that need to float (ships, life jackets) or sink (anchors, submarine ballast).
Chromatography is used in forensic science to analyse ink samples, in food safety testing to identify artificial dyes (like the Tartrazine in this lab), and in pharmaceutical quality control to confirm that drugs contain exactly what they should.
Students often think a heavier object is always denser. It is not. A small steel ball bearing is denser than a large block of Styrofoam, even though the Styrofoam block may weigh more.
Students sometimes read the meniscus at the top of the curve instead of the bottom. Always read at the lowest point of the concave meniscus for water and most lab solvents.
A common mistake is assuming that a high Rf means a substance is "better" or "more concentrated." Rf only reflects how far a component travels relative to the solvent front. It says nothing about concentration.
Students frequently confuse the stationary and mobile phases. The stationary phase stays put (the paper). The mobile phase moves (the solvent).
⚠️ Density and chromatography are both marked with double asterisks in the source notes, meaning they are high-priority exam topics.
⚠️ Be prepared to calculate density given mass and volume, and to determine volume using water displacement.
⚠️ Know the Rf formula cold. Expect a question that gives you distances and asks you to compute Rf.
⚠️ Understand why a component with higher solubility in the mobile phase travels further.
⚠️ Remember which chemicals were used and why: 5% acetic acid (extraction), ammonia (dye release), 1% NaCl (mobile phase).
True or false: An object with a density of 0.8 g/mL will sink in water.
False. It will float, because its density is less than 1 g/mL.
Fill in the blank: The Rf value is calculated by dividing the distance travelled by the ______ by the distance travelled by the ______.
Spot; solvent front.
True or false: In chromatography, the mobile phase is the paper.
False. The paper is the stationary phase. The mobile phase is the solvent.
Fill in the blank: To measure the volume of an irregularly shaped solid, you use the ______ method.
Water displacement.
True or false: Capillary action is what draws solvent up the chromatography paper.
True.
Q: A metal cube has a mass of 27.0 g and sides measuring 1.5 cm each. What is its density?
A: Volume = 1.5 x 1.5 x 1.5 = 3.375 cm³. Density = 27.0 / 3.375 = 8.0 g/cm³. This is close to the density of iron.
Q: In a chromatography experiment, a dye spot travels 4.2 cm and the solvent front travels 7.0 cm. What is the Rf value?
A: Rf = 4.2 / 7.0 = 0.60.
Q: Why does the solvent rise up the chromatography paper without being pumped?
A: Capillary action draws the solvent upward through the narrow spaces in the paper fibres.
Q: You have an irregularly shaped rock. Describe how you would determine its volume in the lab.
A: Fill a graduated cylinder partway with water and record the initial volume. Submerge the rock and record the new volume. The difference between the two readings is the volume of the rock.
Q: A chromatography experiment produces two spots. Spot A has an Rf of 0.25 and spot B has an Rf of 0.75. Which spot has a greater affinity for the mobile phase?
A: Spot B. A higher Rf means it travelled further with the solvent, indicating greater solubility in (and affinity for) the mobile phase.
Density connects directly to the Avogadro's number lab, where you measure mass and use electrolysis to deposit known amounts of metal. If you can measure mass and volume accurately, you can verify molar relationships.
Chromatography connects to the fertiliser analysis lab: both involve separating a mixture into components and quantifying what you find. The analytical mindset (measure, separate, calculate) carries through every lab in this course.
Density, mass, volume, D = m/V, grams per millilitre, g/mL, g/cm³, sinking and floating, buoyancy, graduated cylinder, meniscus, water displacement, electronic balance, chromatography, Rf value, retention factor, thin-layer chromatography, TLC, column chromatography, gas chromatography, stationary phase, mobile phase, capillary action, solvent front, Tartrazine, Yellow No. 5, dye extraction, acetic acid, ammonia, NaCl, MSU chemistry lab midterm