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Water – The Universal Solvent •Many solutions have water as the solvent. •These solutions are called an aqueous solution. •Water dissolves many solutes and so scientists call it the universal solvent. •Why is water such a great solvent? Let’s ﬁnd out.
The positive area of the water molecules surround the negative Chloride ions. 6
The negative area of the water molecules surround the positive sodium ions.
As the attractions from the water molecules and their motion pulls the ions apart, the sodium chloride crystal dissolves.
Dissolving a Molecular Compound
Dissolving a Molecular Compound example: sugar
Water molecules arrange themselves around thesucrose molecules according to opposite polar areas. 11
The attraction of the water molecules and theirmotion overcome the attraction between sucrose molecules. 12
The sucrose molecules dissolve as they areseparated from the other molecules and mix into the water. 13
The sucrose molecules dissolve as they areseparated from the other molecules and mix into the water.
What willdissolve?The spoon or the sugar??
Like Dissolves Like•Polar solvents will dissolve polar solutes.•Nonpolar solvents will dissolve nonpolarsolutes.•Polar will not dissolves nonpolar and vice versa.•Think about oil and water. Is the oil polar ornonpolar?
How Much Will Dissolve?
How Much Will Dissolve?•If you tried to dissolve a lot of sugar in a small glassof water, not all of the sugar would dissolve.•Solubility is the measurement of how muchsolute dissolves in a given amount of solvent.•Solubility is usually describes as how much candissolve in 100g of solvent at a given temperature.
Solubility in aLiquid-Solid Solution Temperature is included here because the solubility of a solute changes asthe temperature of the solvent changes.
Solubility in aLiquid-Solid Solution As you heat water, you can dissolve sugar in water faster rate, but also more sugar will dissolve.
Solubility in a Liquid-Gas SolutionIn a liquid-gas solution, the opposite is true. As the temperatureincreases, the solubility decreases.
Solubility in a Liquid-Gas Solution This is because the water molecules will be moving faster at higher temperatures. This movement will bump the gas out ofthe solution, therefore decreases solubility.
Saturated Solutions•If you add calciumcarbonate to 100g of waterat 25˚C, only 0.0014g willdissolve. No more candissolve.•This solution is called asaturated solution.•If the solution was a liquid-solid solution, the extra solidwould settle on the bottomof the container.
Saturated Solutions•The solubility of sugar in 100g of water at25˚C is 204g.•If you had a solution with 50g of sugar in 100gof water, the solution is called unsaturated.More solute could dissolve.
Saturated Solutions•A hot solvent canusually hold moresolute then acooler one.•If a saturatedsolution cools, someof the solute can fallout of the solution.
Saturated Solutions•A hot solvent can usually hold more solute thena cooler one.•If a saturated solution cools, some of the solutecan fall out of the solution.• But if the solution is cooled slowly,sometimes the excess solute remains dissolved fora period of time. This solution is calledsupersaturated.
Rate of Dissolving•Solubility does not tell you how fast a solute willdissolve.•Some solutes will dissolve quickly, others take a long timeto dissolve.•A solution dissolves faster when: ✴it is stirred ✴it is shaken ✴the temperature is increased•How exactly do these factors help? ✴They increase the rate at which the surfaces of the solute come into contact with the solvent. ✴Increasing the area of contact between the solute and the solvent can also increase the rate of dissolving
Concentrated!•The concentration of a solution tells you howmuch solute is present compared to the amount ofsolvent.•When you are comparing the concentrations oftwo solutions with the same type of solute andsolvent, you can use a simple description such as:dilute or concentrated.
Concentrated!•The Dead Sea is5+ times saltier thanEarths oceans. Aswater evaporates,salt is left behind.When thesaturation point isreached, the saltforms these pillars.
Measuring Concentrations•One way to give theconcentration is by using thepercentage of the volumeof the solution that is madeup of the solute.
Effects on Solute Particles•Solute particles affect the physical propertiesof the solvent, such as the boiling point or thefreezing point.
Effects on Solute Particles•Adding a solute, such as NaCl, changes the waythe water molecules can arrange themselves. Thesolute gets in the way.
Effects on Solute Particles•To overcome this interference of the solute, alower temperature is needed for the solvent tofreeze.
Effects on Solute Particles •When a solvent begins to boil, the solvent molecules are gaining enough energy to move from the liquid state to the gaseous state.
Effects on Solute Particles •When a solute is introduced, its particles interfere with the evaporation of the solvent particles.
Effects on Solute Particles •For water and NaCl, the NaCl will block the H2O from escaping the liquid and becoming a gas.
Effects on Solute Particles •More energy is needed for the solvent particles to escape from the liquid, and the boiling point of the solution will be higher.