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Transcript
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 H20 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 (H2O) and nonpolar (H2) 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, H3O+ H+ + H2O H3O+ • 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+ H3O+ + OH- 2H2O VI What is pH? • pH is related directly to its concentrations of hydronium ions (H3O+) and hydroxide ions (OH-) • Acidic solutions (pH 0-6) have more hydronium ions (H3O+) • Basic solutions (pH 8-14) have more hydroxide ions (OH-) • Neutral solutions (pH 7) have equal numbers of hydronium (H3O+) 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 10n where n = the difference between pHs pH3 – pH1 = 2 102 = 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 H2O + Na+Cl-