Summer Research Program for Science Teachers


Seltzer Breath

Jeanette Kim, Asst. Director of Education

NY Academy of Sciences

Summer 2004


This activity will teach you about using and “indicator” (Bromothymol Blue or BTB) to test for the presence of certain chemicals in liquids and gases.  The use of indicators and titrations is a useful technique for many activities. 

SAFETY: Before distributing any chemicals, talk about safety, and in particular how to deal with an unknown chemical: basically, always assume that it is dangerous, and NEVER EAT, DRINK, TOUCH, or SMELL a chemical in the laboratory unless your teacher tells you that you can.  Also remind students that the lab (or classroom, when it is being used as a lab) is never a place for eating or drinking.

Remind students that if they ever have any questions about what they are doing, ASK FIRST—DO LATER.  This is usually a good rule of thumb.  Always err on the side of caution when it comes to safety.  Always try the activity yourself first to make sure you won’t get any unexpected surprises.

BTB is not considered hazardous as defined by OSHA Hazard Communication Standard.  However, as with all chemicals, general safety procedures should be followed. 

Part I:  Set-up

Put one dropper-full of BTB in each of two plastic cubs. At each lab station, put one-half cup of distilled water* (labeled “A”) and one-half cup of seltzer water (labeled “B”), and paper towels.  (See Tricks of the Trade section for a note about distilled water).

Distribute to each team:

            Seltzer Breath Worksheet

            1 pan or paper plate

            2 clear plastic cups with about 5mL of BTB

            1 straw

            1 sheet of plastic wrap

            NOTE: the straw and plastic wrap can be distributed later

Part 2:  Activity

1)       Explain that the dark liquid at the bottom of each cup is identical

2)       Explain that the students will try to solve a mystery today, and that it will take close observation and teamwork 

3)       Ask the students to observe the dark liquid (NO TASTING, NO SMELLING, NO TOUCHING, NO SPILLING!) and record their observations (eg. “it is very dark, but slightly blue, and I can see through it”.) 

4)       Ask the students to observe Liquid A (NO TASTING, NO SMELLING, NO TOUCHING, NO SPILLING!) and record their observations (eg. “it’s clear and colorless, looks like water).  Ask them to predict what will happen if you pour some of Liquid A (distilled water) into the dark liquid (and record the prediction).   

5)       Now ask students to pour some of Liquid A into the dark liquid and write down their observations.   

6)       Do the same with Liquid B (seltzer water). 

7)       Something different, and probably unexpected, happens.  What is different about the two trials?  Why did such different things happen?  Ask them to discuss this in their groups.   

8)       Obviously, there is something different about the two liquids.  What is it?  Look at your observation sheets to see if they help.  Does anyone have any ideas?  What do you think the two liquids are?  Reveal what the two liquids are, and explain that what makes the bubbles in seltzer water is carbon dioxide.  The blue liquid must be sensitive to carbon dioxide, then. 

9)       Do you know of any other sources of carbon dioxide?  If you are studying the body or the carbon cycle, your students should have some ideas. 

10)   Either as demo or in the teams, insert straw into a cup with the blue liquid, cover with plastic wrap (you can also put a rubber band around the cup to hold the plastic wrap on), and blow gently into the straw.  DO NOT DRINK OR BREATHE IN!  The liquid will turn yellow.   

Part 3:  Examples of Extensions

§         Adding baking soda will turn a yellow mixture back to blue. 

§         You can also use a water plant, like Elodea, to use up the carbon dioxide in a yellow mixture to make it blue again. 

§         Lung capacity test: starting with equal volumes, equal strength BTB solutions, have several people do the breath test using only one breath.  Line up the cups in order of how “yellow” they became to compare lung capacity.

§         Indicators in general are very handy for activities, experiments, and long term investigations.  What are some other indicators you can use and what do they test for?


Part 4:  Final notes

Where to get BTB:

We used BTB purchased from Carolina Biological Supply <>.  A 120mL bottle costs about $6 + shipping, and a 500mL bottle about $12 + shipping.  In this activity, the set-up for each team uses about 10mL of BTB.  You can also speak to your science coordinator or chair about how to order some.   

Tricks of the Trade:

BTB is usually used as an indicator for carbon dioxide.  It works because CO2 makes water slightly acidic, which turns the BTB yellow.  Some tap or distilled water can also be slightly acidic, which means that it will turn the BTB yellow instead of blue.

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