Melting vs Dissolving

Compare a phase change (melting) to forming a solution (dissolving) and use simple particle models to show how the particles behave differently in each case. Students melt common solids and then dissolve sugar in water to observe that dissolving is not melting.

1. Gather materials for melting: equal masses of ice, butter, and chocolate in separate microwave-safe cups; balance; microwave; timer; stir sticks.
2. Gather materials for dissolving: clear jar with lid, room-temperature water, colored sugar or table sugar, spoon.
3. For melting, record the initial mass of each solid and note appearance. Place all three in the microwave.
4. Heat in 20 second intervals, checking after each interval. Stir only if needed to distribute heat. Record which samples become liquid first and the total time for each.
5. Discuss using a particle model: draw closely packed particles for the solid and more spaced, mobile particles for the liquid. Emphasize that only state changes, not identity.
6. For dissolving, pour water into the jar and add a measured spoon of sugar.
7. Stir or cap and shake to mix. Observe color and clarity changes and note that the sugar does not disappear but disperses between water particles.
8. Compare the two processes in a short table or notes: melting needs heat to change state of one substance; dissolving forms a mixture of solute and solvent without changing chemical identity.

Observing & Modeling Melting vs Dissolving Lab Review - RheaultScience:

• Repeat dissolving with salt instead of sugar or with warm vs cold water to test temperature effects.
• Time to dissolve with and without stirring to quantify the effect of agitation and surface area.

• Use microwave-safe containers and heat in short intervals to prevent overheating or splattering.
• Handle hot containers and melted liquids with oven mitts. Allow samples to cool before moving.
• Do not overfill closed jars when shaking to avoid pressure buildup and leaks.
• Keep food items used for demonstrations separate from classroom food and do not consume melted butter or chocolate used as lab samples.
• Clean spills promptly to prevent slips. Wash hands after the activity.

• What changes during melting at the particle level? (Particles gain energy and move farther apart, changing from an ordered solid to a mobile liquid without changing identity.)
• What changes during dissolving at the particle level? (Solvent particles surround and separate solute particles, dispersing them throughout the liquid to form a mixture.)
• Why is dissolving not the same as melting? (Melting is a state change of a single substance; dissolving mixes two substances to form a solution.)
• How do stirring and temperature affect dissolving rate? (They increase collisions between solvent and solute particles, speeding dispersion.)
• Why might ice melt more slowly than butter in the microwave? (Different intermolecular forces and specific heat capacities lead to different energy needs for phase change.)
• If the solution looks clear, where did the solute go? (It is present as particles between water particles; it can be detected by taste or by evaporating the water to recover the solute.)