Dissolving & Solutions

Activities to try

1. 1

   Dissolve, weigh, recover bench

   Why it works: When a soluble solid dissolves it is still present — spread through the liquid as bits too small to see — because the sealed system weighs the same before and after it dissolves; and it can be recovered by evaporating the liquid. Some solids (sand) do not dissolve in water and can be filtered, and there is a limit to how much will dissolve (saturation).

   1. Place a jar of water on a balance, add a measured spoon of sugar, and cover or seal the jar so no matter can escape or enter during the experiment.
   2. Record this starting reading while the sugar is still visible and has not yet dissolved.
   3. Predict whether the reading will go up, down, or stay the same as the sugar dissolves, and state WHY, before stirring.
   4. Stir and watch the grains break into finer and finer specks that spread out until the water looks clear.
   5. Read the balance again and compare with the starting reading: in a sealed jar it should be the same, apart from tiny flickers within the balance's precision. The dissolved sugar's weight is still there, so the sugar cannot have disappeared.
2. 2

   Zoom-in: where did the sugar go?

   Why it works: A dissolved solid becomes clear not because it vanished but because it has broken into pieces too small to see and spread evenly; the sequence can be reversed to recover the solid.

   1. Reveal a magnified view of a dissolving grain splitting into ever-smaller specks that spread through the liquid.
   2. Trace the sequence dissolving → solution → evaporate → recovered solid, lifting a 'the sugar is still here' flap at the solution stage.
   3. State in your own words why the clear solution still contains the solid.
3. 3

   Predict-Observe-Explain and check your reasoning

   Why it works: A full understanding says the solid is still there AND offers converging evidence — weight, taste, and recoverability by evaporation — not just a memorised 'still there'. It also transfers: sand won't dissolve, and solutions can saturate.

   1. Weigh a glass of plain water and record the reading. Add a spoon of salt without stirring, cover the glass if possible, and record the reading again while the salt is still visible. Before stirring, predict whether the balance will read more, less, or the same as this second reading once the salt has dissolved, and say why.
   2. Stir until the salt is invisible, read the balance, and compare it with both earlier readings (plain water, and water with undissolved salt) and with your prediction.
   3. Note the two distinct comparisons: the salty water weighs MORE than the plain water you started with (because you added salt), while the reading is UNCHANGED from just before to just after the salt dissolves (the conservation point).
   4. Answer probing questions: If all the water dried up, what would be left, and does that show the salt was there all along? Would salty water weigh more, less, or the same as the plain water you started with, and why? If it had been sand instead of salt, would stirring make it disappear — what's different? Is salty water one substance or a mixture of two, and how would you argue it?
   5. Give the most important part: how you KNOW the salt is still there — weight, evaporating it to find it left behind, or taste (only if the teacher says so, using clean food-safe cups, never science equipment).