Solutions

A solution is a homogenous mixture of 2 or more substances The solute is(are) the substance(s) present in the smaller amount(s) The solvent is the substance present in the larger amount 12.1

Solutions Solvent: greater quantity (water) Solute: smaller quantity (sugar) Immiscible: two liquids do not mix miscible: two liquids can mix alcohol in water (in any quantities)

Parts of a Solution Solute Solvent Example solid liquid gas

A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature. An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature. A supersaturated solution contains more solute than is present in a saturated solution at a specific temperature. Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate. 12.1

Three types of interactions in the solution process: solvent-solvent interaction solute-solute interaction solvent-solute interaction DHsoln = DH1 + DH2 + DH3 12.2

“like dissolves like” Two substances with similar intermolecular forces are likely to be soluble in each other. non-polar molecules are soluble in non-polar solvents CCl4 in C6H6 polar molecules are soluble in polar solvents C2H5OH in H2O ionic compounds are more soluble in polar solvents NaCl in H2O or NH3 (l) 12.2

Concentration Units Molarity (M) moles of solute M = liters of solution Molality (m) m = moles of solute mass of solvent (kg) 12.3

Temperature and Solubility Solid solubility and temperature solubility decreases with increasing temperature solubility increases with increasing temperature 12.4

Temperature and Solubility Gas solubility and temperature solubility usually decreases with increasing temperature 12.4

Pressure and Solutions Henry’s law P  Solubility  (gas in liquid)

Pressure and Solubility of Gases The solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (Henry’s law). c is the concentration (M) of the dissolved gas c = kP P is the pressure of the gas over the solution k is a constant (mol/L•atm) that depends only on temperature low P high P low c high c 12.5

Calculating the solubility If we want to make an accurate prediction about whether something will dissolve in water or not, we need to know four key factors: 1 the type of solute 3 the volume of solvent 2 the mass of solute 4 the temperature We can say, for example, that the solubility of salt in water at 20 °C is 35 grams per 100 cm3 of water. This means that at 20 °C you can completely dissolve 35g of salt in 100cm3 of water. In general:  Solubility = mass of solute / volume of solvent

Solubility of potassium nitrate How Much Will Dissolve? The table shows how much potassium nitrate (a type of fertiliser) and potassium chloride (the main ingredient in ‘Lo-Salt’) dissolve in water at different temperatures. The numbers tell you how many grams of solute dissolve in 100 cm3 water. Temperature (°C) Solubility of potassium nitrate Solubility of potassium chloride 10 20 32 30 35 44 36 40 60 38 50 78 100 42

Graph showing the solubility of potassium nitrate and potassium chloride.

Questions How does the solubility of the substances change as the temperature changes? Which substance is the most soluble at 10°C? Which substance is the most soluble at 40°C? Copy and complete this sentence: ‘As the temperature rises, the solubility …’ Which of the two solids dissolves best a at 10 °C b at 40 °C? Use your graph to estimate the solubility of potassium chloride at 55 °C

Colloid versus solution A colloid is a dispersion of particles of one substance throughout a dispersing medium of another substance. Colloid versus solution collodial particles are much larger than solute molecules collodial suspension is not as homogeneous as a solution 12.8

Suspensions These are mixed, but not dissolved in each other Will settle over time Particles are small Mayo