4.2: solutions and dilutions

solution basics

• solution: a homogenous mixture of two or more pure substances
  • the solvent is present in the greatest amount possible
  • all other substances are solutes
• suspension: a heterogeneous mixture in which solute-like particles settle out of a solvent-like phase after mixing
• colloid: a solution that has particles which remain evenly distributed throughout the solution; the particles do not settle to the bottom of the container, and they do not filter

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how solutions form

• “like dissolves like”—the more similar the solvent and solute molecules are, the more likely one substance is to be soluble in another
• polar water molecules are able to dissolve an ionic compound like NaCl
• polar water molecules are not able to dissolve a non-polar compound like propane

electrolytes

• electrolyte: a substance that forms ions (charged particles or species) when dissolved in water
  • ionic compounds are electrolytes
• strong electrolytes dissociate completely when dissolved in water (lots of ions)
• weak electrolytes only partially dissociate when dissolved in water (fewer ions)
• non-electrolyte: a substance that does not form ions when dissolved in water
  • covalent compounds are non-electrolytes
  • eg. sugar—dissolves in water but doesn’t form ions

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types of solutions

• solutions are categorized into one of three groups: saturated solutions, unsaturated solutions, and supersaturated solutions

  • these are based on the amount of solute dissolved within them

• unsaturated solution: a solution that has less than the maximum amount of solute dissolved in the solvent (for that temperature)

  • you can continue to add solute and it will continue to dissolve

• saturated solution: a solution in which the solvent holds as much solute as is possible at that temperature

  • saturation is dependent on temperature

• supersaturated solution: a solution in which the solvent holds more solute than is typically possible at a given temperature

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factors that affect how fast a substance dissolves

finding percent by mass and percent by volume of a solution

• percent by mass: ([solute mass/solution mass] x 100)
• percent by volume: ([solute volume/solution volume] x 100)

dilutions

• dilution: the process of adding more solvent to a solution without adding more solute, which decreases the concentration of a solution
  • eg. only adding water to the solution; that means that the number of moles of solute stays the same, only the volume of the solution changes
• used to weaken the properties of the solute
• “dilution” can also be used to describe the liquid itself that has been diluted

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