Acids and Bases: Together again!
Wilma Amaro
Time:
One class period, Week 3
Preparation
Time:
Approximately 1hour
Materials:
Board, dry erase markers or chalk; 500ml-graduated
cylinder, approximately 500ml of 1M, NaOH and
500ml, 1M HCl, phenolphthalein and methyl red
indicators, distilled water, balance, beakers, funnels,
droppers, glass stirrers, funnels, wash bottles.
Abstract
Many chemical compounds are produced as industrial waste and discarded into the air and water.
Understanding the physical and chemical properties of compounds (i.e. pH) allows the
development of simple detection methods. This lesson is designed to engage students in the
topic of acid-base reactions. At this point, students should have an understanding of molecular
and structural formulas, atomic and molecular weights, the mole, and concentration (specifically in
terms of molarity.) They should also have been exposed to the concept of chemical reactions.
Objectives
Students will be able to:
1. Identify principles behind acid-base reactions
2. Predict factors that may affect an acid-base reaction
National Science Education Standards:



Content Standard A: Inquiry
o Identify questions and concepts that guide scientific investigations
o Formulate and revise scientific explanations and models using logic and
evidence
Content Standard B: Physical Science
o Structure and properties of matter reactions
o Chemical reactions
Content Standard F: Science in Personal and Social Perspectives
o Personal and community health
o Environmental quality
o Natural and human-induced hazards
Teacher Background:
Strong acids (having a pH of 2 or less) and strong bases (having a pH of 12.5 or greater) are
considered corrosive. Products with these properties are often generated in association with
businesses such as vehicle maintenance and dismantling, printing, photo finishing labs, surface
coating, metal finishing electronics, computers, and educational institutions.
The release of products with such properties can often result in a threat to human health and the
environment.
Preventing disease involves understanding the cause of an illness and changing the conditions
that permit it to occur. When chemicals are implicated, this is usually done by trying to minimize
or eliminate the chemical in the environment or by minimizing the adverse effects of chemicals
found in the environment and food supply (NIEHS, 2000). One of the environmental resources
that need to be monitored closely is water. Water can be analyzed for acid/base content by doing
titrations.
Related and Resource Websites
1. http://www.chemistrycoach.com/acids.htm
Review of fundamental concepts in acid/base reactions. Includes a discussion on
electrolytes.
2. http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch11/acidbaseframe.html
Review of acid/base definitions.
3. http://members.aol.com/logan20/ionic_eq.html
Acid/base equilibrium.
4. http://www.flinnsci.com/Documents/ProductPDFs/ChemTopicLabs/vol%2013/Demo_
Summaries_Concept_vol13.pdf
Acid/base demonstrations
5. http://www.dec.state.ny.us/website/ppu/ecppsqg.pdf
EPA regulations for small businesses
6. http://www.chemguide.co.uk/physical/acidbaseeqia/indicators.html
Theoretical background on indicators
7. http://qldscienceteachers.tripod.com/junior/chem/acid.html
Examples of bases and acids; includes examples of indicators and the pH range they
detect
8. http://classes.aces.uiuc.edu/ACES100/Mind/CMap.html
Information about concept maps. Includes different types
Activity:
Materials, for teacher:
Two beakers, any size
Triple-beam or any kind of balance
Glass stirrer
NaOH pellets
Two 1-L volumetric flasks
HCl, 15 M (molarity is commonly found on liquid reagent bottles. HCl is usually 15M when
purchased through chemical supply companies. Some local companies may be able to prepare
solutions of a specific molarity for a fee)
Funnel
Droppers
Wash bottle
Teacher prep:
Solution preparation:
CAUTION: Wear goggles, apron and rubber gloves when preparing these solutions. HCl
solution must be prepared under a hood.
NaOH
Weigh 40.0g of NaOH in a beaker, add enough distilled water to dissolve it; do not add more than
200 or 300 ml. Transfer to 1-L volumetric flask using a funnel and wash bottle. Bring water level
up to the 1-liter mark on the neck of the flask using a dropper. Label flasks clearly with NaOH,
1M, include date.
HCl
Measure 15ml of 15M HCl using a pipette and transfer into 1-L volumetric flask. Bring water level
up to the 1-liter mark on the neck of the flask using a dropper. Label flask clearly with HCl 1M,
include date.
Class Demo:
1. Ask each student to generate a list of 15-20 words/ideas they relate to chemical reactions.
Organize these into a concept or mind map.
2. In a 500mL-graduated cylinder, pour about 200mL of 1M NaOH. Add three or four drops
of phenolphthalein. Gradually add 1M HCl, until the pink color disappears. The color
change of phenolphthalein from pink to clear indicates an acidic solution. Use 1M NaOH
to bring the color back to pink and then add more 1M HCl to make a clear solution again.
Do this a few times. Write out the formula for HCl and NaOH, on the board.
3. Ask students to pair up and come up with an explanation for what they saw. Have them
speculate about the reactants, concentration and what it takes for the color to change.
4. Add three or four drops of methyl red indicator to the mixture used in steps 1 and 2
above, and gradually add 1M NaOH until the color of the solution changes from red to
yellow. The color change to yellow indicates that the solution now has a basic pH.
5. Repeat steps 1-3 a couple of times so students can see how the reaction goes back and
forth, noting the color change. Have students make predictions about the color change
(Will it turn pink, red, etc.?) as the demonstration progresses. Identify HCl and NaOH as
an acid and a base.
Class discussion
6. Start asking students: “What happens between HCl and NaOH?”, “Why do acids and
bases matter to us?”, “What are some acids and bases used at home?”, “How do we
discard them?”. Present facts about production of acids and bases by local businesses
using provided teacher background. Ask students if they live, or know anyone, who lives
near one of these businesses.
7. Have students add more terms on their concept maps this time using the demo and
including class discussion.
Closure:
Students will exchange their concept maps with a classmate. They will write down three main
ideas represented on their classmate’s concept map on a separate piece of paper. Teacher will
ask students to volunteer some of the ideas.
Embedded Assessment:
The lesson will be assessed using the concept map. Collect and save concept maps for future
reference.
Homework:
Find three examples of acids and three examples of bases commonly used at home. Determine
how each is discarded. Answer the question: “What actually happens to them?”