Benchmark Lesson 2

 

AUTHOR: Sumana Islam

 

GRADE LEVEL: High School

 

TIME: 50 minutes.

 

TEKS:

Environmental Science and Chemistry.

 

Knowledge and skills.

students demonstrate safe practices during field and laboratory investigations; and

make wise choices in the use and conservation of resources and the disposal or recycling of materials.

Scientific processes. The student uses scientific methods during field and laboratory investigations. The student is expected to:

collect data and make measurements with precision;

organize, analyze, evaluate, make inferences, and predict trends from data; and

communicate valid conclusions.

 

Science concept.

Scientific processes. The student uses critical thinking and scientific problem solving to make  informed decisions. The student is expected to:

analyze, review, and critique scientific explanations, including hypotheses and theories, as to their strengths and weaknesses using scientific evidence and information;

research how regional changes in the environment may have a global effect;

 

CONCEPT(S): Students will estimate the amount of carbon dioxide present in normal, combusted, and exhaled air by monitoring the change in acidity caused by CO2 absorption in a solution. The change in acidity will be tested by color change of an indicator which is bromthymol blue.

 

OBJECTIVES (LEARNER OUTCOMES):

 

Students will be able to:

i) understand that the rate of combustion is much greater than the rate of respiration.

 

ii) know that both combustion and respiration produces CO2 in the air or atmosphere.

 

iii) make a connection of human activities (which leads to increasing CO2 level in

atmosphere) and increasing temperature change on earth.

           

MATERIALS LIST (for a class of 24 working in pairs):

12 plastic glass funnels (7-cm diameter)

12 250-ml filter flasks, wrapped with duct tape or towels

12 1-hole stoppers to fit filter flasks

12 aspirator adapters for faucets

12 small (30-cm) pieces of rubber aspirator hose

24 straight glass tubes

24 glass bends

24 ring stands and clamps

36 rubber connectors

36 test tubes (18x150 mm)

12 large candles

12 straws

bromthymol blue solution

12 250-ml Erlenmeyer flasks

0.1 M NaOH (4.0g NaOH in 1000 mL solution, placed in each of six 15-20 mL dropper bottles)

 

ADVANCED PREPARATIONS: Bromthymol blue can be purchased as either a 0.04% solution or as a powder that can be used to prepare a solution.

 

SAFETY: students should wear safety goggles and aprons throughout the laboratory activity. Glass tubes should be inserted by the teacher to prevent student injury.

 

SUPPLEMENTARY MATERIALS: diagram of apparatus for collecting air and description of the procedure of how it works.

 

RESOURCES: Chemistry in the community, 2002, fourth edition, by W.H. Freeman and Company.

 

 

ENGAGEMENT

 

Time: ___7 minutes__
 
 

 

 

 

 


What the Teacher Will Do

Eliciting Questions

Student Responses

Show a topographic map of a low-lying coastal region of the United States, such as Florida.

What do you think will happen if the polar icecaps start melting?

Some part of land in Florida will be under water.

 

What might make the polar icecaps melt?

Increasing the temperature of the Earth.

In the last class we talked about atmospheric gases.

Can you remember (from previous classes) the name of an atmospheric gas that can act as greenhouse gas also?

Not sure.

 

Hint: which gas we breathe out?

CO2

 

It is!! Can you think about any other sources of CO2?

Cars need gases and release CO2 while moving.

 

What about burning candles?

Yes.

Tell them that air usually has a low concentration of CO2. However, the CO2 level can be increased in a closed space by burning coal or petroleum.

 

 

Today we are doing an activity in order to estimate and compare the amounts of CO2 in several air samples.

 

 

 

To do this, the air will be bubbled through water that contains an indicator, bromthymol blue. CO2 reacts with water to form carbonic acid:

 

CO2 (g) + H2O (l) à H2CO3 (aq)

 

As the concentration of carbonic acid in the bromthymol blue solution increases, the indicator changes from blue to green and finally to yellow.

 

EXPLORATION

 

Time: _30 minutes____

 
 
 

 

 

 

 

 


What the Teacher Will Do

Eliciting Questions

Student Responses

Describe the main procedure and go through with the experiments as below.

 

 

 

Part 1

  1. Pour distilled water (125 ml) into a filter flask (250 ml) and add 10 ml of bromthymol blue. The solution should look blue.
  2. Pour 10 ml solution prepared in step1 and label ‘control’.
  3. Make arrangement of apparatus as shown in figure 2.8, page 288 of the book (Chemistry in the community), which is basically joining the flask is attached with aspirator which is connected to the faucet. The flask is attached with clamp and the opening has a hole stopper. Finally, the opening is connected to a glass funnel through glass bends (passed through the hole of the stopper) and a rubber connector.
  4. Record the time, then turn on the aspirator until the aspirator pulls air through the flask. (mark the position so that the same flow rate can be maintained later also).
  5. Let the aspirator run until the indicator turns yellow. Record the time taken for indicator color change.
  6. Pour 10 ml solution to a test tube and mark as ‘normal air’ and stopper the tube.

 

What the Teacher Will Do

Eliciting Questions

Student Responses

Compare the color of this sample with the control.

Do you see any color change between the samples?

Yes. The control solution and the normal air solution are yellow.

 

Part2

  1. Repeat step 1 with another flask and reassemble apparatus as in step3.
  2. Light a candle and position it so that the tip of the flame is just inside the base of the glass funnel attached to the flask.
  3. Record the starting time, and then repeat step4 and record the time it takes to change color. Pour 10 ml of solution in a test tube labeled ‘CO2 combustion’.

 

What the Teacher Will Do

Eliciting Questions

Student Responses

Compare the colors and the time taken to turn yellow by burning candle and normal air.

Which time is longer, normal air or burning candle?

The sample from burning candle took less time compared to the normal air.

 

Part3

  1. Make the same arrangement with another flask, record the starting time and exhale your breath through a clean straw into the solution until the indicator changes color.
  2. Record the time. Put 10 ml of this solution in a clean test tube labeled ‘CO2 breath’ and stopper the tube.

 

What the Teacher Will Do

Eliciting Questions

Student Responses

Compare the color change and burning times of sample labeled ‘breath’ with normal and combustion samples.

What difference do you observe?

The time taken for the sample ‘CO2 breath’ took longer time than ‘combusted air’ while took less time compared to ‘normal air’.

 

 

EXPLANATION

 

Time: _8 minutes___

 

What the Teacher Will Do

Eliciting Questions

Student Responses

Comparing the times it took for turning the indicator from blue to yellow….

Which sample took longest and which sample took shortest time to change color?

Normal air took longest, and then breath air and combusted air took smallest amount of time.

Showing the initial equation…

What are we trying to measure from indicator color change?

CO2 in the samples.

 

So, which sample do you think has more CO2?

Some said the right order, but some said the opposite.

Mentioning about the equation again…

The more CO2 in the air, the more or the less carbonic acid formation in the sample?

More.

 

And the more carbonic acid in the sample the more or less time it takes to change indicator color to yellow?

Less.

 

Very good!

 

Mention that this technique only gives a rough estimation of CO2, but can be done quantitatively to know the exact amounts.

If left exposed in a room indefinitely, do you think the indicator will change color?

Yes. No.

 

Why do you think is that?

Various responses.

Explain that in that case the indicator solution would absorb CO2 from the surroundings and could change color.

 

 

 

 

 

ELABORATION

 

Time: __5 minutes___

 

 

 
 
 


           

 

 

 

What the Teacher Will Do

Eliciting Questions

Student Responses

Mentioning about the experimental findings above..

If green plants use CO2 during photosynthesis, do you think in the experiment done above, the time to change color should increase or decrease?

Increase.

 

Right. What about the time change when people produce CO2 during respiration?

Time should decrease.

 

Or burning fuels?

Time should decrease.

 Elaborate on the sources of CO2. Bring up the idea that we need food to live and the food is converted to energy with the help of oxygen.

So, it is inevitable that CO2 will be produced when fuels, including foods, are combined with oxygen. How do you think this type of pollution can be decreased?

Reducing use of energy.

Using other source of energy, like solar energy..

 

 

 

 

 

EVALUATION

 

Time: ___

 
 
 


Evaluation will be mostly during and after doing the experiment. It will focus on how the students are thinking at each step, for example, what is the purpose of the experiment, what we learned from it and what we think ahead from the learning.