L38. Solutions and Solubility

States, Solutions, and Acids

R-report

L38. Solutions and Solubility

Why does sugar seem to vanish in water while sand stays as grit?

Central Idea:

Solutions are uniform mixtures where one substance (the solute) is dispersed among particles of another (the solvent). In sugar water, sugar is the solute and water is the solvent; sugar molecules separate into small particles and spread between water molecules, so the sugar is present but no longer visible as separate grains. Solubility is the maximum amount of solute that will dissolve in a given amount of solvent under stated conditions (for example, grams solute per 100 g water at a specified temperature). Temperature, stirring, and particle size affect rate and amount of dissolving.

How to investigate:

1. Measure 100 g of water into two clear beakers and label one 'sugar' and one 'sand'. 2. Add 10 g sugar to the sugar beaker and stir 20 s; note whether the sugar disappears and whether the liquid stays clear. 3. Add 10 g sand to the sand beaker and stir 20 s; observe whether particles settle and whether filtering recovers the sand. 4. Warm the sugar beaker to about 40 °C and add sugar in small steps, recording when no more dissolves. 5. Evaporate or cool the sugar solution to recover crystals and confirm the solute is present.

Application & Practice:

Compare sugar and sand by asking whether the solid spreads at the particle level and whether you can get the original material back by physical means. To test this, filter and evaporate samples: if filtering removes the solid (as with sand) the solid did not dissolve, while if evaporation leaves dissolved material as crystals (as with sugar) the solute was dispersed at the molecular level. A worked example: if a stated solubility is 60 g sugar per 100 g water at 25 °C, then 200 g water can dissolve 120 g sugar (60 g/100 g × 200 g = 120 g), so adding 150 g would leave 30 g undissolved. Short checks: look for clarity, try filtration, try evaporation, check for color, bubbles, or temperature change suggesting a chemical reaction rather than a physical change. Quick-check: if a clear solution becomes cloudy after cooling, what did that indicate?

Synthesis:

Solutions are uniform mixtures; call the dissolved part the solute and the dissolving medium the solvent. Use words like soluble, insoluble, saturated, and concentrate to be precise. Tiny symbolic check: 60 g sugar / 100 g H2O × 200 g H2O = 120 g dissolved, so mass added − mass dissolved = undissolved mass (150 g − 120 g = 30 g). Always keep particle-level reasoning when you explain results.

Final checks: verify units and amounts in any calculation (include g and °C), and make sure your mass accounting closes (mass of solution = mass solvent + mass solute recovered or dissolved). Tell the micro-level story: show where solute particles are among solvent particles. Ask whether a filter, evaporation, or a chemical test can falsify your idea. Record conditions (temperature and stirring) because solubility depends on them. Note time too.