Chapter 8Chapter 8

Section 2Section 2

Water—The Universal Solvent

• A solution in which water is the solvent is called an aqueous (A kwee us) solution.

• Because water can dissolve so many different solutes, chemists often call it the universal solvent.

SolubilitySolubility

22

Molecular Compounds

• When certain atoms form compounds, they share electrons. Sharing electrons is called covalent bonding.

• Compounds that contain covalent bonds are called molecular compounds, or molecules.

SolubilitySolubility

22

Molecular Compounds• If a molecule has an even distribution of

electrons it is called nonpolar.

• In a water molecule, the electrons spend more time around the oxygen atom than the hydrogen atoms.

SolubilitySolubility

22

• Such a molecule is polar.

Ionic Bonds

• Atoms with a charge are called ions.

• Bonds between ions that are formed by the transfer of electrons are called ionic bonds, and the compound that is formed is called and ionic compound.

SolubilitySolubility

22

• Table salt is an ionic compound that is made of sodium ions and chloride ions.

How Water Dissolves Ionic Compounds

• Because water molecules are polar, they attract positive and negative ions.

SolubilitySolubility

22

• The more positive part of a water molecule—where the hydrogen atoms are—is attracted to negatively charged ions.

How Water Dissolves Ionic Compounds

SolubilitySolubility

22

How Water Dissolves Ionic Compounds

• The more negative part of a water molecule—where the oxygen atom is—attracts positive ions.

SolubilitySolubility

22

• When an ionic compound is mixed with water, the different ions of the compound are pulled apart by the water molecules.

How Water Dissolves Molecular Compounds

• Water does dissolve molecular compounds, such as sugar, although it doesn’t break each sugar molecule apart.

SolubilitySolubility

22

• Water simply moves between different molecules of sugar, separating them.

What will dissolve?

• When you stir a spoonful of sugar into iced tea, all of the sugar dissolves but none of the metal in the spoon does.

SolubilitySolubility

22

• A substance that dissolves in another is said to be soluble in that substance.

• You would say that the sugar is soluble in water but the metal of the spoon is insoluble in water.

Like Dissolves Like

• When trying to predict which solvents can dissolve which solutes, chemists use the rule of “like dissolves like.”

SolubilitySolubility

22

• Polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes.

Like Dissolves Like

• On the other hand, if a solvent and a solute are not similar, the solute won’t dissolve.

SolubilitySolubility

22

• For example, oil and water do not mix.

• Oil molecules are nonpolar, so polar water molecules are not attracted to them.

How much will dissolve?• Solubility (sahl yuh BIH luh tee) is a

measurement that describes how much solute dissolves in a given amount of solvent.

SolubilitySolubility

22

• The solubility of a material has been described as the amount of the material that can dissolve in 100 g of solvent at a given temperature.

• When a substance has an extremely low solubility, it usually is considered insoluble.

Solubility in Liquid-Solid Solutions

• The solubility of many solutes changes if you change the temperature of the solvent.

SolubilitySolubility

22

• For example, if you heat water, not only does the sugar dissolve at a faster rate, but more sugar can dissolve in it.

Solubility in Liquid-Solid Solutions

SolubilitySolubility

22

• This graph shows how the temperature of the solvent affects the solubility of some solutes.

Solubility in Liquid-Gas Solutions• Unlike liquid-solid solutions, an increase

in temperature decreases the solubility of a gas in a liquid-gas solution.

SolubilitySolubility

22

• You might notice this if you have ever opened a warm carbonated beverage and it bubbled up out of control while a chilled one barely fizzed.

• Carbon dioxide is less soluble in a warm solution.

Saturated Solutions• A solution that contains all of the solute

that it can hold under the given conditions is called a saturated solution.

SolubilitySolubility

22

• If a solution is a liquid-solid solution, the extra solute that is added will settle to the bottom of the container.

• It’s possible to make solutions that have less solute than they would need to become saturated. Such solutions are unsaturated.

Saturated Solutions

• A hot solvent usually can hold more solute than a cool solvent can.

SolubilitySolubility

22

• If a saturated solution is cooled slowly, sometimes the excess solute remains dissolved for a period of time.

• Such a solution is said to be supersaturated, because it contains more than the normal amount of solute.

Rate of Dissolving

• Some solutes dissolve quickly, but others take a long time to dissolve.

SolubilitySolubility

22

• A solute dissolves faster when the solution is stirred or shaken or when the temperature of the solution is increased.

Rate of Dissolving

SolubilitySolubility

22

• These methods increase the rate at which the surfaces of the solute come into contact with the solvent.

Concentration• The concentration of a solution tells you

how much solute is present compared to the amount of solvent.

SolubilitySolubility

22

• You can give a simple description of a solution’s concentration by calling it either concentrated or dilute.

• A concentrated solution has more solute per given amount of solvent than a dilute solution.

Measuring Concentrations

• One way of giving the exact concentration is to state the percentage of the volume of the solution that is made up of solute.

SolubilitySolubility

22

Measuring Concentrations

SolubilitySolubility

22

• Labels on fruit drinks show their concentration.

• Another way to describe the concentration of a solution is to give a percentage of the total mass that is made up of solute.

Effects of Solute Particles

• The effect that a solute has on the freezing or boiling point of a solvent depends on the number of solute particles.

SolubilitySolubility

22

• When a solvent such as water begins to freeze, its molecules arrange themselves in a particular pattern.

Effects of Solute Particles

• Adding a solute such as sodium chloride to this solvent changes the way the molecules arrange themselves.

SolubilitySolubility

22

• To overcome this interference of the solute, a lower temperature is needed to freeze the solvent.

22Section CheckSection Check

Question 1

Bonds between ions formed by the transfer of electrons are known as _______.

The answer is ionic bonds. “Ions” are atoms with a net charge, either positive or negative.

Answer

22Section CheckSection Check

Question 2

How does water dissolve ionic compounds?

When an ionic compound is mixed with water, the different ions of the compound are pulled apart by the water molecules.

Answer

22Section CheckSection Check

Question 3

The measure of how much solute will dissolve in a given amount of solvent is its _______.

The answer is solubility. If you have ever stirred too much sugar into a glass of water and had some left on the bottom, you have observed solubility.

Answer