Chapter 10.2: Properties of Solutions

Properties of Solution

Ch 10.2 (p342 – 350)

Parts of Solution

• Dissolving is the process of mixing one substance into another to form a solution.

• There are two terms to refer to the substances that make up a solution

• Solvent is the substance that exists in the greatest quantity in a solution

• All other substances in a solution are solutes

• Air– What is the solvent?– What is the solute?

Types of Solutions

• Solutions can exist is all three states of matter – solid, liquid, and gas

• The state of the solvent determines the state of the solution because it exists in the greatest quantity

Water as a Solvent

• Water is one of the few substances on Earth that exists naturally in all three states – solid, liquid, gas

• Much of the water found on Earth is not pure

• Water almost always exists as a solution

• Why is this?

The Polarity of Water

• A water molecule is a covalent compound held together by the sharing a electrons between the atoms

• However the electrons are not equally shared• Electrons in the hydrogen-oxygen bond are

pulled more closely to the oxygen• This unequal sharing gives the oxygen end of

the water molecule a slight negative charge• It also gives the hydrogen end a slight positive

charge• Thus, the water molecule is polar

The Polarity of Water

• A polar molecule is a molecule with a slightly negative end and a slightly positive end

Like Dissolves Like

• Water is often called the universal solvent because it dissolves many different substances

• However, water cannot dissolve everything• Polar solvents dissolve polar solutes easily• Nonpolar solvent dissolve nonpolar solutes

easily– This is summarized by the phrase “like dissolves like”

• Because water is a polar solvent, it dissolves most polar and ionic solutes

Polar Solvents and Polar Molecules

• Rubbing alcohol, or isopropyl alcohol, is a substance that is used as a disinfectant

• Molecules of rubbing alcohol are also polar• Therefore, when rubbing alcohol and water mix, the

positive ends of the water molecules are attracted to the negative ends of the alcohol molecules.

• Similarly the negative ends of the water molecules are attracted to the positive ends of the alcohol molecules• In this way, alcohol molecules dissolve in the solvent

Polar Solvents and Ionic Compounds

• Many ionic compounds are also soluble in water.• Recall that ionic compounds are composed of

alternating positive and negative ions– Sodium Chloride (NaCl) is an ionic compound

composed of sodium ions (Na+) and chloride ions (Cl-)

• When sodium chloride dissolves these ions are pulled apart by the water molecules– The negative ends of the water molecules attract the

positive sodium ion– The positive ends of the water molecule attract the

negative chloride ions.

Concentration – How much is dissolved

• Have you ever tasted soup and wished it has more salt in it?– In a way your taste buds were measuring the

amount, or concentration, of salt in the soup.

• Concentration is the amount of a particular solute in a given amount of solution– In soup, salt is a solute– Soup with less salt has a lower concentration

of salt

Concentration and Dilute Solutions

• One way to describe the saltier soup id to say that it is more concentrated

• The less saltier soup is more dilute• Concentrated and dilute are one way to describe

how much solute is dissolved in a solution– However, these terms don’t tell us the exact amount

of solute dissolved

Describing Concentration Using Quantity

• A more precise way to describe concentration is to state the quantity of solute in a given quantity of solution.

• When a solution is made of a solid dissolved in a liquid, such as salt in water, concentration is the mass of solute in a given volume of solution.

• Mass is usually stated in grams and volume is usually stated in liters.– For example, concentration cam be stated as grams

of solute per 1 L of solution– However, concentration can be stated using any unit

of mass or volume

Calculating Concentration – Mass per Volume

• One way that concentration can be calculated is by the following equation– Concentration (C) = mass of solute (m)

volume of solution (v)– To calculate, you must know the mass of

solute and the volume of solution that contains the mass.

Concentration – Percent by Volume

• Not all solutions are made of a solid dissolved in a liquid.• If a solution contains only liquids or gases, its

concentration is stated as the volume of solute in a given volume of solution

• In this case, the units of volume must be the same – usually mL or L.

• Because the units match, the concentration can be stated as a percentage.

• Percent by volume is calculated by dividing the volume of the solute by the total volume of the solution and then multiplying the quotient by 100.

• What is the percent by volume?– If there are 1000mL of solution, how much

isopropyl alcohol and water are in the solution?

Solubility – How much can dissolve?

• Have you ever put too much sugar into a glass of iced tea?– Not all the sugar dissolves– This is because these is a limit to how much solute

(sugar) can be dissolved in a solvent (water)

• Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a given temperature and pressure– If a substance has a high solubility, more of it can

dissolve in a given solvent

Saturated and Unsaturated Solutions

• If you add water to a dry sponge, the sponge absorbs the water.– If you keep adding water, the sponge becomes

saturated. It can’t hold anymore water

• This is analogous, or similar, to what happens when you stir too much sugar into ice tea

• Some sugar dissolves, but the excess sugar does not dissolve– The solution is saturated

• A saturated solution is a solution that contains the maximum amount of solute the solution can hold at a given temperature and pressure

• An unsaturated solution is a solution that can still dissolve more solute at a given temperature and pressure

Factors that Affect How Much Can Dissolve

• Can you change the amount of a particular solute that can dissolve in a solvent? Yes

• Recall the definition of solubility – – The maximum amount of solute that can

dissolve in a given amount of solvent at a given temperature and pressure

• Changing either temperature or pressure changes how much solute can dissolve in a solvent

Effect of Temperature

• The solubility of most solid substances increases with temperature– However, some can become less soluble as

temperatures increase• How does temperature affect the solubility of a

gas in a liquid?– Recall soda, or soft drinks, contain carbon dioxide, a

gas solute dissolved in liquid water– The warmer the soda the more carbon dioxide

bubbles out than when it is cold– This is because the solubility of a gas in a liquid

decreases when the temperature of the solution increases

Effect of Pressure

• What keeps carbon dioxide in an unopened can of soda?

• In a can, the carbon dioxide in the space above the liquid soda is under pressure

• This causes the gas to move to an area of lower pressure – the solvent

• The gass moves into the solvent, and a solution is formed– When the can is opened, this pressure is released

and the carbon dioxide gas leaves the solution.– *Pressure does not affect the solubility of a solid

solute in a liquid

How Fast a Solute Dissolves

• Temperature and pressure can affect how much solute dissolves

• If solute and solvent particles come into contact more often, the solute dissolves faster

• There are three ways to increase how often solute particles contact solvent particles– Stirring– Crushing

• *Stirring and crushing will not make more solute dissolve; They’ll only increase how fast it dissolves

– Increasing temperature