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Acids and Bases

Investigation 2 – PreLab









The health and success of plants depends on the soil. For this reason, soil scientists work closely with farmers to test the pH of their soil to see if it is within the desired range recommend for the field crops they are planting. If a desirable soil pH is not created and maintained, farmers will not be able to grow or harvest healthy crops.


This Investigation is designed to help you to answer the following Focus Question:

  • What is the relationship between the pH of a solution and the concentration of hydrogen ions (H+)? 

Note: This question is located in your SDR at the beginning and end of the Investigation.


As a class, read the Background(s) in the Investigation. When finished, discuss the following concepts as a class:

  • The concentration of a solute is described as the amount of solute that is dissolved in a certain volume of solvent to make a solution.
  • Dissolving a large amount of solute in a certain volume would result in a high concentration.
  • Dissolving a small amount of solute in the same volume would result in a low concentration.
  • A less concentrated solution can be made by adding less solute or by diluting a more concentrated solution by adding more solvent.
  • Using a serial dilution, a series of less and less concentrated solutions can be made.


The following list includes Key Terms that are introduced in the Investigation Background(s). They should be used, as appropriate, by teachers and students during everyday classroom discourse.

  • solute
  • solvent
  • solution
  • dilution

Note: Definitions of these terms can be found on the Introduction page to the CELL.

Note: Additional words may be bolded within the Background(s). These words are not Key Terms and are strictly emphasized for exposure at this time.


  • Complete the Recall section in your SDR. 
    • What is the definition of concentration?
    • Can you think of some examples of different solutions that have different concentrations of a solute?
  • Consider the example of the two different concentrations of cherry soft drink and how the more concentrated drink will be darker red and have a stronger cherry flavor than the more dilute drink.
  • The more concentrated drink has twice the dissolved solid as the more dilute drink.

The following is a demonstration that your teacher may perform in Prelab:.

  • To demonstrate the concept of concentration, fill a 1000ml graduated cylinder to the 1000ml mark. Add 1 drop of red food coloring, cover with a piece of plastic wrap and invert to mix.
  • Discuss the color of the solution describing its shade of red as pink or light red. The concentration of the food coloring is 1 drop in 1 liter of water.
  • Fill a second 1000ml graduated cylinder to the 1000ml mark. Add 10 drops of red food coloring, cover with a piece of plastic wrap and invert to mix.
  • Compare the two solutions side-by-side. Discuss the color of the two solutions comparing the color of each, describing the color of the second solution as dark red. The concentration of the food coloring in the second cylinder is 10 drops in 1 liter of water. This represents a more concentrated solution of the food coloring compared to the first solution.
  • In this Investigation, you will measure the pH of different concentrations of hydrochloric acid to demonstrate the relationship between pH and hydrogen ion concentration.
  • Play the video below. Remember to follow along with your SDR and make any notes that you think might be helpful in the lab.
  • After the video, divide into lab groups to discuss strategy for the lab. For example, you may assign certain group members to perform specific functions during the lab.

Note: The purpose of the video is to allow you to anticipate the laboratory experience you will soon encounter. You should leave this PreLab session with a firm idea of what to expect and how to perform in the lab.

Note: Homework is posted below the video.


You should review the Investigation and video in preparation for the Lab.