Unit A: Chemical Change

Lesson 1.4: Solutions and Concentrations

How many products in the form of a solution (solute dissolved in a solvent) do you use over the course of a day? If you drink fruit juice, use household cleaners, or use hair-care products, you are using a solution. These products require care in manufacture so the solute to solvent ratio is appropriate for the application.

This is a quick lesson that introduces the effects of solutions with little or a lots of solute.  It covers the properties of these solutions.  You know that solutions are important for chemical reactions, so the Collision - Reaction Theory is introduced.

  1. define concentration.
  2. identify solutions a concentrated or diluted.
  3. give examples of when an awareness of concentration is useful.
  4. give 5 observations that might help determine the amount of solute in a solution
  5. give two ways to determine the concentration of a solution
  6. use the Collision - Reaction Theory to explain how concentration affects the rate of chemical reactions

Concentration describes the amount of solute, compared to the amount of solution.   

A solution with a lot solute is called a concentrate.  Concentrated laundry detergent is a good example. 

A solution with little solute is called diluted.  The vinegar you use in the kitchen is likely 95% water.  An it is diluted further when used in foods.
  • Read page 35, with targets .  Answer the practice questions 25 and 26 and check your answers below.

 

Science 20 Textbook, Courtesy of Alberta Education



 

You may want to explore concentrations and dilutions using this gizmo.  It may help you with some assignment questions.
1.  Drain the water to the 400 mL mark (2  graduations).  Add solute to make a solution.  Measure the concentration and record.
2.  Double the water and measure the concentrations.  Represent new concentration / initial concentration as a fraction.
3. Double the water again and find the new fraction  (compare to the very first concentration.
4.  play with the gizmo



Figure 1:  Floating in the Dead Sea by Pete.  Courtesy Wikipedia

Solutions with different concentrations have different properties.  You local swimming pool may be a dilute salt solution that controls harmful bacteria, but its doubtful you have noticed a difference in how well you float.  Figure 1 shows someone floating in the Dead Sea without a flotation device.  The dead sea is a concentrated salt solution that is very dense and produces a strong buoyant force.
There are five properties a person can sense that indicate the relative concentration of a solution.
  • colour
  • taste
  • odour
  • colour intensity
  • transparency

There are two ways someone might determine the actual concentration of a solution.
  • Do a conductivity test (only if the solute is an electrolyte)
  • Test the rate of reaction.  More concentrated solutions react more quickly
Read and complete the Investigation below.  
  • Read the entire investigation on pages 36 and 37 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 investigation, and do questions 1, 2, 4 and 5ab, then check the answers below.   

 

Science 20 Textbook, Courtesy of Alberta Education



For this path you can use grape drink crystals and water to produce a brightly coloured solution. (Any brightly coloured drink crystals can be used.) Use nine plastic glasses in place of the 100-mL beakers. Start with a full glass of water and add about 20 mL (2 teaspoons) of drink crystals. Follow steps 3 to 5 of the procedure on page 36 of the textbook to dilute this solution into the remaining glasses. Compare the colour of the solution in each glass. This will give you an idea of how dilution affects the concentration of a solution.

  • Answer questions 1, 2, 4 and 5ab of “Analysis” as if you had performed the original investigation with the sodium hydroxide and phenolphthalein.

    Check your answers in "Path 1" above.
Read 'Did you Know?'  on page 38, and page 39 to 1.4 Summary.

Recall that for chemical reactions to happen two things need to occur;  bonds need to break, then bonds need to form again with the atoms in a new combination.  We know that atoms are always moving.   They move much more in a liquid or gas than in a solid.  When molecules collide they may be broken into individual atoms.  If they recombine in a different arrangement of atoms then new molecules have been formed.   How could someone increase the rate of reaction?
If there are more molecules  (a higher concentration) then there will be more collisions, so there will be more molecules that break and recombine.  In other words, increasing the concentration of molecules will speed up chemical reactions.

  • Read “1.4 Summary” on page 39 of the textbook. Then, complete “1.4 Questions” # 1 - 6.  check your answers below.

 

Science 20 Textbook, Courtesy of Alberta Education



Go to Assignment 1.4: Solutions and Concentrations.