Unit A: Chemical Change

Lesson 1.3: Breaking Bonds to Make Solutions

In ancient times, alchemists were unsuccessful in changing base metals, such as lead, into precious metals, like gold. Today, scientists—because of their knowledge of matter—are able to change natural resources into finished products, like plastic bags and clothing. These changes are the result of changing the chemical properties of the materials.

Most chemical reactions happen in solutions of liquids or gases.  There are two basic parts to a chemical reaction.  First compounds are broken apart, then they are put together in a new form.  When a sugar cube is dropped in hot water, the cube soon disappears.  In other words, the cube is being pulled apart into pieces so small that you cannot see them.  While dissolving is not chemical change, it is the first step.  That is why solutions are so important to chemistry.  They make the recombination of atoms into new compounds easier.


  1. recognize evidence of chemical change
  2. define endothermic and exothermic reaction
  3. know that most chemical reactions occur when solutions are mixed
  4. identify the solute and solvent in a solution
  5. define aqueous solution.
  6. explain how dissolving can be the first step in a chemical reaction.
  7. say how solutes of ionic and molecular compounds are different in solution.
  8. define polar and non-polar molecules
  9. sketch water as a polar molecule
  10. explain in detail how water molecules pull ions from a salt and hold them in solution
  11. describe why ionic compounds will conduct electricity, but molecular compounds will
  12. describe a test for conductivity
  13. define dissociation
  14. define electrolyte and non-electrolyte

    Recall: Evidence of Chemical Change


    Chemical change occurs when a new substance is produced.  This can be observed as..
    • a colour change
    • a change in state (gas or a precipitate is produced)
    • odour (gas is produced)
    • a change in energy.  Heat is given off or absorbed.
    An exothermic reaction gives off heat or light.  Candles and glow sticks are good examples An endothermic reaction absorbs heat.  The ice pack in your first aide kit, or cooking an egg are good examples

    • Read pages 24 and 25 of the textbook with a focus on targets 1 and 2

    Chemical change is one in which one or more new substances with different properties are formed when substances are mixed. The next activity will demonstrate how water affects chemical change. 
    • Read the entire activity on page 25 of the textbook.

      If you have access to a supervised laboratory, do Path 1. If you do not have access to a supervised laboratory, do Path 2.

    • Follow the directions of the activity, answer the questions and check your answers below. 
    For this path you will watch a teacher demonstration of this activity.

    • View the segment “Water Helps Break Chemical Bonds” in the “Multimedia Segments” in the online course. Record your observations when prompted.

    • Answer questions 1 to 4 of “Analysis” on page 25. Check the answers in "Path 1" above

    Most Chemical change happens when solutions are mixed.

    • Read pages 26 - 27 with a focus on targets 3 to 5.

    A lot of medicines and cosmetics are made from molecular compounds and are dissolved in glycerin to make creams or lotions.  The focus for this lesson is on aqueous solutions, or solutions with water as the solvent.   In general, molecular solvents dissolve molecular solutes, and ionic solvents dissolve ionic solutes, but water largely dissolves both, so it is called the "universal solvent".

    Why is Water a Good Solvent?


    You have observed dissolving as solids that are pulled apart until the pieces are too small to observe.  It turns out that good solvents not only pull solutes apart, but also hold them in the solutions so that they do not become solid again.

    Water is a molecular compound, but it does not share its electrons very evenly.  Oxygen has a stronger hold on the electrons, so the oxygen side of the molecules has a partial negative charge, and the hydrogen side has a slight positive charge.  Molecules with partial charges at their ends are called polar.  Molecules whose charges are balance are called non-polar.  It is this polar nature of water that makes it a solvent for so many compounds.

    Read page 28 - 30 and view the videos below, with a focus on targets 7 - 10  Answer questions 16, 19, 20 - 23 And check your answers below.

     

    Science 20 Textbook, Courtesy of Alberta Education



    The strong partial charge of water will dissolve most ionic compounds and polar molecular compounds.  It will not dissolve nonpolar molecular compounds.

    • Read page 31 and view the video below, with a focus on targets 11 - 14.  Do question 24 and check your answers below.

    • Read the entire investigation on page 32 of the textbook.

    In this investigation you will design an experiment.  This is an important part of the scientific process and is worth the time to do.  If you have access to a supervised laboratory, do Path 1. If you do not have access to a supervised laboratory, do Path 2.

    • Follow the directions of the investigation, and do analysis questions 1 - 3. Check your answers below.

     

    Science 20 Textbook, Courtesy of Alberta Education


    • Complete steps 1 and 2 of the procedure on page 32.

    • Use the sample data in "Path 1"  to answer questions 1 to 3 of “Analysis” on page 32. Check your answers above in "Path 1"

    Go to Assignment 1.3: Breaking Bonds.