Chapter 8 Substances, Mixtures, and Solubility

I. Substances A. Atoms and Elements A substance is matter that has the same fixed composition and properties; i.e. an element A substance can not be broken down into simple parts by physical processes Only a chemical process can change a substance into one or more new substances

B. Compounds A compound is made of two or more elements that are chemically combined in fixed compositions - The ratio of compounds is always the same; i.e. water H 2 0

II. Solutions A solution is a homogeneous mixture – two or more substances that are evenly mixed but are still not bonded together; i.e. sugar water

Solutions are formed by dissolving - The substance that dissolves – or seems to disappear – is called the solute; i.e. the sugar - The substance that dissolves the solute is called the solvent and is usually present in the greatest quantity; i.e. the water

A. Forming Solids from Solutions A solute can come back out of its solution and form a solid - Crystallization - Result of a physical change

A solid can form as a result of a chemical reaction when some solutions are mixed - Precipitation - Result of a chemical change

B. Liquid Solutions 1. Liquid-Gas Solutions Carbonated beverages Solvent – water, other liquid Solute – carbon dioxide (gas)

2. Liquid-Liquid Solutions A smaller amount of one liquid (solute) is dissolved in a larger amount of another liquid (solvent) Vinegar Solvent – water Solute – acetic acid

C. Gaseous Solutions A smaller amount of one gas (solute) is dissolved in a larger amount of another gas (solvent) The air you breathe Solvent – Nitrogen (78%) Solute – Other gases i.e. oxygen, carbon dioxide

D. Solid Solutions A solid can be mixed with another solid, liquid, or gas Solid-Solid solutions are called alloys i.e. steel and brass

III. Solubility Water can dissolve many different solutes, thus chemists call it the universal solvent A solution in which water is the solvent is called an aqueous solution

A. Molecular Compounds In a covalent bond, electrons are shared Compounds that contain covalent bonds are called molecular compounds, or molecules Include polar (H 2 O) and nonpolar (H 2 ) molecules

B. Ionic Bonds In an ionic bond, electrons are gained or lost The compound formed is called an ionic compound Na + Cl -

C. How water dissolves ionic compounds Water is polar and has positive and negative ends - The positive parts of water (H + ) are attracted to negative ions (Cl - ) - The negative part of water (O-) is attracted to positive ions (Na+) - Water can thus pull apart the different ions of an ionic compound

D. How water dissolves molecular compounds Water does not break molecular compounds (sugar) apart Instead, water moves between different molecules of sugar, separating them

E. What will dissolve? “Like dissolves Like” Polar solvents dissolve polar solutes Nonpolar solvents dissolve nonpolar solutes If a solvent and a solute are not similar, the solute will not dissolve (oil and water)

F. How much will dissolve? Solubility is a measurement that describes how much solute dissolves in a given amount of solvent

1. Solubility in Liquid-Solid Solutions Increased temperature of solvent may or may not also increase - The rate at which the solute dissolves - The amount of solute that dissolves

2. Solubility in Liquid-Gas Solutions Increased pressure can increase the amount of solute (gas) in a solvent (liquid)

3. Saturated Solutions A solution is said to be saturated when the solvent can not hold any additional solute In a liquid-solid solution, the extra solid solute will settle to the bottom of the container A hot solvent can hold more solute than a cold solvent can - When the hot solvent cools, the solute usually falls out of solution

- If solute stays in the solvent for a period of time after the solvent cools, it is supersaturated because it contains more than the normal amount of solute

G. Rate of Dissolving A solute dissolves faster when the solution is stirred or shaken This method increases the rate at which the surfaces of the solute come into contact with the solvent Increasing the collisions between the solvent and solute can also increase the rate of dissolving

H. Concentration Tells you how much solute is present compared to the amount of solvent In a concentrated solution, there is more solute dissolved in a solvent compared to another solution In a dilute solution, there is less solute dissolved in a solvent compared to another solution

1. Measuring Concentration Described as a percentage of the volume (liquid) or mass (solid) of the solution that is made up of the solute

2. Effects of Solute Particles Adding solutes to a solvent, decreases the freezing point of the solute i.e. Antifreeze Adding solutes to a solvent, increases the boiling point of the solute

IV. Acids Release positively charged hydrogen ions, H +, in water HCl  H + + Cl - Hydrogen ions, H +, combine with water, to form hydronium ions, H 3 O + H + + H 2 O  H 3 O + Hydronium ions in acidic solutions can carry the electric charges in a current - Therefore acids can conduct electricity

V. Bases Bases are substances that release negatively charged hydroxide ions, OH -, in water NaOH  Na + + OH - Bases can thus accept hydrogen ions, H + H 3 O + + OH -  2H 2 O

VI What is pH? pH is related directly to its concentrations of hydronium ions (H 3 O + ) and hydroxide ions (OH - ) Acidic solutions (pH 0-6) have more hydronium ions (H 3 O + ) Basic solutions (pH 8-14) have more hydroxide ions (OH - ) Neutral solutions (pH 7) have equal numbers of hydronium (H 3 O + ) and hydroxide (OH - ) ions

A. pH Scale 1 pH unit represents a tenfold change in the acidity of a solution Example, if one solution has a pH of 1 and a second solution has a pH of 2, the first solution is ten times more acidic than the second

To find the difference in pH strength, use the following calculation 10 n where n = the difference between pHs pH3 – pH1 = = 100 x more acidic

B. Strengths of Acids and Bases The strength of an acid is related to how easily hydrogen ions, H +, are released - A strong acid (pH = 1) releases more hydrogen ions than a weak acid (pH = 6)

The strength of a base is related to how easily, hydroxide ions, OH -, are released - A strong base (pH = 14) releases more hydroxide ions than a weak base (pH = 8)

C. Indicators Compounds that react with acidic and basic solutions and produce certain colors, depending on the solution’s pH

D. Neutralization A reaction of an acid with a base The properties of both the acid and base are diminished, or neutralized Produces a water and a salt in most cases HCl + NaOH  H 2 O + Na + Cl -