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Chapter 10.2: Properties of Solutions


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Grade 8 Integrated Science Chapter 10 Lesson 2 on properties of solution, solubility, concentration, solvents, and solutes. Understanding how to change solubility of a solute in a solvent.

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Chapter 10.2: Properties of Solutions

  1. 1. Properties of Solution Ch 10.2 (p342 – 350)
  2. 2. 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
  3. 3. • Air – What is the solvent? – What is the solute?
  4. 4. 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
  5. 5. 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?
  6. 6. 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
  7. 7. The Polarity of Water • A polar molecule is a molecule with a slightly negative end and a slightly positive end
  8. 8. 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
  9. 9. 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
  10. 10. 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.
  11. 11. 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
  12. 12. 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
  13. 13. 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
  14. 14. 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.
  15. 15. 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.
  16. 16. • What is the percent by volume? – If there are 1000mL of solution, how much isopropyl alcohol and water are in the solution?
  17. 17. 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
  18. 18. 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
  19. 19. • 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
  20. 20. 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
  21. 21. 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
  22. 22. 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
  23. 23. 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