LabLearner—The Science of Learning
LabLearner—The Science of Learning

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Acids and Bases

Investigation 2 – Concept Day

 

 

 

 

 

 

 

Back to Investigation 2

ZERO-IN

Italicized font represents information to be shared orally or physically completed with the students at this time.

The non-italicized font represents additional information included to support the teacher’s understanding of the content being introduced within the CELL.

ASK WHY

Remind students that acids and bases are all around us. They play an important role in the digestion of the foods we eat, the medicine we take, and even the cleaning products we use. Understanding these two classes of compounds can help us stay safe, healthy, and be informed consumers.

BRANCH OUT

Explain to students that 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.

PRINT IT

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NAVIGATE IT

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SLIDE ACIDS-2-1

  • In this Investigation, we wish to focus on the concentration of hydrogen ions in solutions of varying pHs.

Note: Since students are typically familiar with concentration as the mass of a compound in a given volume, we will begin with a quick review of this topic. Students will also be introduced to the logarithmic nature of pH concentrations. Most students will have very little previous exposure to logarithms at this point. Finally, we will introduce students to the concept and methods of serial dilutions.

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SLIDE ACIDS-2-2

  • This slide reviews “traditional” concentrations.
  • Concentration = Mass/Volume is reviewed.
Note: As students are familiar with this concept of concentration and have prepared many solutions in LabLearner using this strategy, it is important to have students certain of this concept prior to moving into a discussion of logarithmic concentrations and pH.

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SLIDE ACIDS-2-4

  • The logarithmic nature of the pH scale is discussed on this slide.
  • Unfortunately, the pH scale may look a lot like a simple number line.
  • It may be initially thought that going from pH 1 to pH 2 would result in a solution half as acidic, as would going from pH 2 to pH 4. However, since the pH scale is logarithmic, going from pH 1 to pH 2 actually results in a solution that is ten-times less acidic. Going from pH 2 to pH 4 would be 100-times less acidic.
Note: The teacher may wish to go over several examples like those described above to further familiarize students of the concept of logarithms and pH values. Additional examples are presented beneath the graphic in this slide. Finally, notice the use of the terms acidic and basic in this slide. A solution of pH 8 is ten times more basic than a solution of pH 7, while at the same time the solution at pH 7 is ten times less acidic than the solution at pH 8.

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SLIDE ACIDS-2-5

  • This slide introduces the concept of serial dilution. A serial dilution is a stepwise dilution of a substance in solution and allows for the accurate preparation of extremely dilute solutions.
  • Serial dilutions are also used to determine the number of bacterial cells in a sample by serially diluting the sample until a reasonable number of cells can be counted from a specific volume of a diluted sample.
  • This “counted” number can then be multiplied by the “dilution factor” to arrive at a remarkably accurate approximation of the number of bacterial in the original, undiluted sample.

Note: The most common point of confusion regarding serial dilution is that the addition of 1 part (ml in this case) of a sample to 9 parts (mls in this case) solvent, results in a ten-fold dilution or 1/10 concentration, NOT a 1/9 dilution or concentration.

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SLIDE ACIDS-2-6

  • This slide gives an illustration of another system in which the value is increased from one step to the next by a factor of ten.
  • Notice that there are no five dollar or twenty dollar bills in this series. They would have disrupted the progressive, geometric increase in value between the copper penny and the 1,000 dollar bill.

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SLIDE ACIDS-2-7

  • Remember to use gloves, goggles, and lab coats when working in the lab with strong acids and bases.