Lesson 3

What are acids and bases?

 

National Standards Addressed: NS912.2

 

Overarching Goal: The overarching goal of this lesson is to understand the differences between acids and bases.

 

Learning Objectives:

Name the properties of an acid and a base.

 

Lesson:

As a class, we will try to come up with the following list of the physical properties of acids and bases.

 

Properties of an Acid:

 

Properties of a Base

 

But instead of just telling the students these properties, the students will perform some simple experiments or tasks to discover some of these properties themselves.  The class will be divided into groups of two.  The class will be instructed to note all observations in their groups and of teachers demonstrations.  Each group will receive a small amount of a pH indicator (try to use cabbage juice indicator prepared from lesson on pH scale), blue and red litmus paper, beakers or cups (because this is a qualitative test, exact measurements are not necessary), and an acidic solution (prepared solution of vinegar/acetic acid) and a basic solution (prepared solution of baking soda/sodium bicarbonate).  The class will then use their indicators to test the pH of the solutions.  They will also note the effect of each solution on litmus paper.  I will then perform a couple of demonstrations.  The first demonstration is on the conductivity of different solutions.  The apparatus is a complete circuit, with a battery and a light bulb, that comes attached with two terminals that can be placed in different solutions.  The three solutions tested are distilled water, acidic, and basic.  For the acidic and basic solutions we will continue to use the same vinegar and baking soda solutions.  The next demonstration involves the reaction of magnesium metal and hydrochloric acid.  From these demonstrations, the class is instructed to come up with a chart of the properties of an acid and the properties of a base.  Give the class a few minutes to complete these themselves, answering any questions they may have.  In the end, ask students to present their findings.  Discuss and in the end design a class chart on the board and make sure all properties you want students to know are on it.  There are two properties that may have to be told to the students, because there is no safe way of demonstrating this in class.  The first is that certain acids have a sour taste, and bases a bitter taste.  The second is that acids react with bases to form salt an water, and vice-versa.  This lesson was designed to engage the students, and give them a chance to write down and interpret their observations.

 

 

After the class has completed, the class will be given the following activity:

 

Given the following chart of household items, indicate whether you think they are acids or bases. Then write down some properties or characteristics you would associate with these substances. What similarities or differences do you see between the ones you indicated acids and bases. Create a venn diagram of two of the materials listed to emphasize the properties if needed.

 

Material

Characteristics

Acid or Base

Coco-cola/Soda

 

 

Detergent

 

 

Orange juice

 

 

Milk

 

 

Salt water

 

 

 

*Depending on time and length of class, the class can be divided into groups and test each of these substances to see if they are an acid or a base.

 

Question for thought: Why is coco-cola an acid? (Besides the fact that it has a pH that is lower than 7), Hint: What makes a soda a soda?

 

Assessment: In the last 5-10 minutes of class (this assignment can also be given as a homework assignment in the event that there is a shortage of time), students will be working on determining if the following are acids or bases:

Milk, pH of 6.5

Tap water, pH 6.5-7.5

Hydrogen bromide, HBr,-conducts electricity, reacts with sodium metal to form hydrogen gas.
H2SO4-reacts wit base to form a salt and water, conducts electricity.

HClO4-reacts with baking soda to form water and a salt.

NaOH-conducts electricity, pH>than water’s pH.

KOH-conducts electricity, reacts with acid to form a salt and water.

LiOH

HF

 

The last two have no properties assigned to them. It is just there to get the students thinking. From the previous examples, students should begin to see a trend, the acids all have an H in the beginning and the bases, have OH in them. This will be explained in another lesson, which is part of this unit. While the students are working on these examples, the teacher will be walking around and asking each student to name a property of an acid, and to see if the students are having any problems with the questions or the material.