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Aquatic Ecology - Pre-site Activities/Teacher led

Grade Level: 5
Content Area: Science
Time to Complete: 45 minutes
Title of Lesson: Using pH

South Carolina State Standards Addressed:


Identify questions suitable for generating a hypothesis.
5-1.2 Identify independent (manipulated), dependent (responding), and controlled variables in an experiment.
5-1.3 Plan and conduct controlled scientific investigations, manipulating one variable at a time.
5-1.5 Construct a line graph from the recorded data with correct placement of independent (manipulated) and dependent (responding) variable.
5-1.6 Evaluate results of an investigation to formulate a valid conclusion based on evidence and communicate the findings of the evaluation in oral or written form.

Lesson Description:

Students will conduct activities to learn how to determine pH of water, and conduct an experiment to determine the pH level of different solutions.

Focus Questions for Students:

1. What is pH?
2. What are the independent, dependent and controlled variables in an experiment comparing pH levels of liquids?
3. Why is it important to know if a substance is acidic (acid) or basic (alkaline)?
4. What are some factors that could affect the pH level of a lake or stream, or of soil?

Culminating Assessment:

Students will be able to draw and label the pH scale, and arrange sampled liquids based on resulting pH readings.


Provided Recommended liquids to get
pH paper strips (30 per class) tap water
50ml graduated beakers distilled water
50 ml cylinders milk
vocabulary vinegar
  Coke or other soft drink
  orange juice or other juices
  baking soda/distilled water mixture

Teacher Preparation:

1. Read background information.
2. Be prepared to introduce pH.
3. Collect and label different liquids for students to use to determine pH.

Background Information:

The pH is a measure of how much acid is in a liquid.  H refers to hydrogen ions.  Hydrogen is a common element that along with oxygen forms water (2 hydrogens + 1 oxygen = HO).Water molecules can  split, and at any instant in liquid water some water molecules break apart into positively charged hydrogen ions (H+) and negatively charged hydroxide ions (OH-). When they dissolve in water, acidic substances such as lemon juice release more H+ than OH-.  Substances categorized as basic release more OH- than H+ when they dissolve in water.  Thus, the more hydrogen ions that are formed in a solution, the more acidic it becomes.  Scientists use a number line called the pH scale to rank the acidity of a fluid or substance.  Pure water (not rain water or tap water) always contains the same number of H+ and OH- ions.  This condition may occur in other fluids, and signifies neutrality.  We assign neutrality a value of 7 at the midpoint of the pH scale, which ranges from 0 (most acidic, highest H+ concentration), to 14 (most basic or alkaline, lowest H+ concentration. Starting at neutrality, each change by one unit of the pH scale signifies a ten-fold increase or decrease in the H + concentration. The pH indicator strips change colors depending on the acidity or alkalinity of the substance being tested. 

0  7  14
Acid Neutral Basic

The pH level is an important abiotic (nonliving) factor in determining what can live in a place.  Animals have pH ranges within which they can live.  Some can live in fairly acidic water or soil with a pH of 4.  Others, like salamanders need a pH level closer to 6, or almost neutral.  Knowing the pH of water is important in determining the quality of the water. Factors that can affect the pH of lakes, streams and forest soils include the amount of rainfall, the amount and types of leaves that fall (leaves contain tannic acid), and the type of soil near the lake or stream.  Some minerals in soil can stabilize pH, acting as a buffer to counter shifts in acidity.  Pollution can change pH.   Knowing the pH is also important because very acidic solutions, like battery acid, and very basic solutions, like lye, are harmful to humans.


1. Explain that pH is a measure of the acidity level in a liquid, and explain how to use the pH scale to determine if a sample is an acid, base or neutral.
2. Demonstrate to students the correct and safe way to use equipment, avoiding spills, cross-contamination of samples, and not drinking anything in the lab.

Divide students into groups of 4 or 5.  There are enough pH strips for 6 groups, with each group using 5 strips, and 5 beakers (use a new pH strip for each sample).

4. After telling students that distilled water has a neutral pH (neither basic or acidic) of 7, have students predict the pH level of each liquid that they will be using.  Identify questions that could be used to generate a hypothesis.  For example, "Since fruit juices sometimes cause lips to sting, could that be because they are more acidic?" Come up with a possible hypothesis.  For example, "If fruit juices are more acidic than water, then they will have an acidic pH number (low)."
5. Students collect 30 ml samples of five liquids to use during the activity, and label samples on the worksheet, 1-5.
6. Students dip one strip of pH indicator paper into a liquid for at least 1 minute, and compare all the segments to the pH chart.  Try to match all segments to the chart. 

Students record the pH reading, and repeat for each liquid.

8. Students construct a line graph, with samples 1-5, and pH levels 1-14.

 Discuss why the independent (manipulated) variable in the experiment was the type of liquid, the dependent (responding) variable was the pH level, and the controlled variable was the amount of water (50 ml).    Students can label these on their line graph. 

Discuss the important role of pH level as an abiotic (non-living) factor in determining what plants and animals can live in a place, since different organisms have different tolerances for acidity.

Discuss factors that determine or change pH levels in water or soil (for example, tannic acid in leaves, baseness of minerals, pollution), and the importance of knowing pH levels when assessing water or soil quality

Discuss the results of the pH experiment.  For example, were solutions containing fruit juice acidic? 


Wash and dry the beakers.


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