L36. Solids, Liquids, and Gases

States, Solutions, and Acids

R-report

L36. Solids, Liquids, and Gases

How does particle motion and spacing determine whether a substance is a solid, liquid, or gas?

Central idea:

States of matter depend on particle arrangement and how freely particles move. In a solid the particles are closely packed and vibrate in fixed positions; in a liquid they remain close but slide past one another allowing flow; in a gas particles are far apart and move independently to fill the container. Adding heat increases particle energy and can cause melting or boiling; removing heat reduces energy and causes freezing or condensation. These are physical changes: the chemical identity (for example H2O) stays the same even though H2O(s) → H2O(l) → H2O(g).

How to observe states:

1. Gather materials: clear container, ice (or sample), heat source, thermometer (°C), scale (g), stopwatch, notebook. 2. Observe: record temperature, appearance, shape, and any bubbles or vapor every 30 s while heating or cooling. 3. Separate observations from explanations: write one line for what you saw and another for why you think it happened. 4. Test reversibility: cool a heated sample to see if it returns to the original state (melting ↔ freezing; condensing ↔ boiling). 5. Check mass: weigh sample plus container before and after to look for mass change if gas escapes. Note the melting point (0 °C for pure water) and boiling point (100 °C at 1 atm) as reference.

Particle comparison:

Compare what stays the same versus what changes when you watch a sample. If the chemical identity stays the same (same atoms and formula) but spacing and motion change, that's a physical change; if a new substance forms (different formula, new solid or gas produced), it's chemical. Decision rule: Do the atoms and bonds of the sample match the original? Yes → physical; No → chemical. Ways to falsify a physical-change claim: Reverse the change: can the original form be recovered by simple heating or cooling? Look for new substances: is a new solid, color change, or gas produced that was not present before? Measure mass and composition: does mass change after allowing gases to escape or after filtering (conserved mass if contained)? Quick-check: You boil water and collect the vapor which recondenses to liquid — does this produce a new substance? Explain briefly.

Checklist for explanations:

Always check units and measurement precision (°C, s, g) and report them. Tell the micro-level story: describe how atoms are arranged and whether bonds change; this helps decide physical vs chemical change. Make sure your observations (color, mass, state, bubbles) match the explanation you propose. Reasoning steps: name materials, record clear observations, propose a particle-level explanation, then design a test to confirm or falsify that explanation with simple controls.

Check calculations and units in any reported numbers (use °C for temperature, g for mass). When writing particle stories, conserve atoms and charge; draw before-and-after sketches for H2O as needed. Confirm that observations match predictions: can you reverse the change or isolate a new substance? Repeat the test; compare results. Restate the steps: list materials, observe, explain using particle motion, then test by reversing or measuring mass and gases.