Slide 1
Dissolved salt becomes invisible, not nonexistent
When salt dissolves in water, its particles become dispersed on a scale too small to see. Their disappearance from view is not disappearance of matter.
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In a closed container, total mass remains the same before and after dissolving. The solution can still be tested chemically, and evaporation can recover dissolved material under suitable conditions.
Slide 2
Water surrounds and separates charged ions
Water molecules have an uneven charge distribution. They can surround positive and negative ions and help separate them from an ionic crystal.
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The oxygen side of a water molecule is relatively negative, while its hydrogen side is relatively positive. Their orientations around an ion reflect electrostatic attraction rather than a random arrangement of solvent neighbours.
Slide 3
Sugar instead dissolves mainly as whole molecules
Sugar dissolves mainly as whole molecules rather than separating into positive and negative ions. Different solutes therefore create different kinds of solutions.
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This distinction affects electrical conductivity. Mobile ions in a salt solution can carry current, while ordinary sugar molecules do not provide a comparable population of charged carriers.
Slide 4
Saturation limits how much can dissolve
At given conditions, a solvent can dissolve only a limited amount of a particular solute before reaching saturation. Extra solid may remain.
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Stirring often speeds the approach to that limit by moving solution away from the surface. It does not necessarily change the final equilibrium amount that can dissolve at the same temperature.
Slide 5
Changing interactions can absorb or release heat
Separating and surrounding solute particles changes interactions. The balance can absorb energy from the surroundings or release energy to them.
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A temperature change during dissolution does not by itself prove a new chemical substance formed. Physical mixing and solvation can have substantial energy effects, so chemical identity needs separate evidence.
Sources
- Why Does Water Dissolve Salt?
American Chemical Society
- Why Does Water Dissolve Sugar?
American Chemical Society