L7. Density: A Useful Property

Atoms and the Periodic Table

R-report

L7. Density: A Useful Property

How can mass and volume together tell us what a material is and whether it will float?

Central Idea:

Density is the amount of mass packed into a certain volume. We write it as d = m / V, where d is density, m is mass, and V is volume. Common units are grams per milliliter (g/mL) or grams per cubic centimetre (g/cm³) — note that 1 mL equals 1 cm³. Mass measures how much matter (use a balance), volume measures space (use a graduated cylinder or ruler and formulas). Example calculation: a sample with mass 20 g and volume 5 mL has density d = 20 g / 5 mL = 4 g/mL. That value lets you compare materials even when sizes differ.

Investigation Procedure:

1. Gather samples and tools: weigh each object on a balance and record masses (for example, 20 g) and check balance calibration. 2. Measure volume by filling a graduated cylinder and reading displaced liquid for irregular shapes, or use ruler measurements and geometry for regular solids (show units mL or cm³). 3. Calculate density for each sample using d = m / V and include units with the result carefully. 4. Compare densities to a reference liquid (for example, water at about 1.00 g/mL) and predict float or sink. 5. Note observations as factual entries (mass, volume, density) and write a short explanation linking density to the float/sink result.

Core Diagram:

After you measure densities, compare samples by asking what stayed the same: same material, same mass, or same volume. Propose a simple hypothesis: “Sample A is denser than Sample B.” To test that hypothesis, suggest one falsifying observation: if cutting A into smaller pieces changes density then the hypothesis is wrong (density should not change when shape or size changes). Another falsifier is an unexplained mass change — if mass is added or lost without a chemical reaction, check errors. Also watch for gas or a new solid forming during handling; that would indicate a chemical change, not a density difference. Quick checklist: remeasure mass, remeasure volume, repeat calculation, and compare to known values like 1.00 g/mL for pure water.

Everyday Application:

Density connects mass and volume so you can compare materials: two objects can have different sizes but the same density, or similar sizes but different densities. Use density to identify unknown samples and to predict whether an object floats in a liquid by comparing its density to the liquid’s density. Key terms: mass (g), volume (mL or cm³), density (g/mL or g/cm³). Notations and examples: d = m / V; 20 g / 5 mL = 4 g/mL; Fe + S → FeS (atoms conserved).

Check units on mass and volume (g, mL or cm³) and ensure density unit matches (g/mL or g/cm³). Confirm the micro-level explanation: particle spacing or arrangement must support why one sample has higher or lower density. Compare your numeric density to known references (e.g., water 1.00 g/mL, aluminum ≈2.70 g/cm³) for plausibility. Ensure no chemical change happened (no gas, precipitate, mass loss) that would invalidate density comparison. Repeat measurements at least once and average results before accepting a final conclusion about identity or float/sink behavior.