{"id":63941,"date":"2026-05-29T04:29:38","date_gmt":"2026-05-28T19:29:38","guid":{"rendered":"https:\/\/phys-edu.net\/wp\/?p=63941"},"modified":"2026-05-29T04:30:03","modified_gmt":"2026-05-28T19:30:03","slug":"cloudy-day-no-problem-a-complete-guide-to-photosynthesis-experiments-with-elodea-brome-grass","status":"publish","type":"post","link":"https:\/\/phys-edu.net\/wp\/?p=63941&lang=en","title":{"rendered":"Cloudy Day? No Problem! A Complete Guide to Photosynthesis Experiments with Elodea & Brome Grass"},"content":{"rendered":"

I’m Ken Kuwako, your Science Trainer. Every day is an experiment.<\/span><\/p>\n

When the water’s color gradually shifts, students’ eyes light up. “Whoa, it changed!” That single moment of discovery is exactly why we do science experiments. Today, I’ve put together a guide on how to successfully run a middle school classic: observing photosynthesis using Elodea (waterweed) and BTB solution. While this experiment seems simple, the preparation makes all the difference between a “meh” result and a “wow” moment.
\nEquipment List
\n\u2022\tElodea (Make sure it\u2019s healthy and vibrant)
\n\u2022\tBTB Solution (Pre-diluted, ready-to-use versions are recommended)
\n\u2022\tBeakers
\n\u2022\t4 Test Tubes (Labeled A to D)
\n\u2022\tAluminum Foil
\n\u2022\tLED Grow Light (High output is best)
\n\u2022\tDarkroom or a light-proof box
\nPre-Experiment Prep
\nPlace the Elodea in a darkroom starting the day before. If you skip this, the plant might already have a “head start” on photosynthesis, which can muddy your results. Prepare your BTB solution and have droppers or pipettes ready for easy handling. Also, do a quick test run with your light source to check its strength\u2014some consumer lights take a surprisingly long time to trigger a reaction.
\nExperimental Procedure
\nAdd 2 to 3 drops of BTB solution to a beaker and add tap water. Use a straw to blow your breath into the water until the BTB solution turns yellow.
\n\"\"<\/p>\n

From Blue…<\/p>\n

\"\"<\/p>\n

…to Yellow!<\/p>\n

Exhaled breath contains carbon dioxide (\u202d$CO_2$\u202c). When it dissolves in water, it forms carbonic acid, making the liquid acidic and turning the BTB solution yellow. Now, pour this yellow BTB solution into four test tubes (A through D):
\n\u2022\tTest Tube A: BTB Solution + Elodea
\n\u2022\tTest Tube B: BTB Solution only (No plant)
\n\u2022\tTest Tube C: BTB Solution + Elodea + Wrapped in Aluminum Foil
\n\u2022\tTest Tube D: BTB Solution only + Wrapped in Aluminum Foil
\n\"\"<\/p>\n

Arranged from left to right: A, B, C, and D.<\/p>\n

Place the tubes under sunlight or a grow light for about 20 minutes. If it’s a cloudy day, use the grow light, but keep in mind you may need to leave them under the light longer if the output is weak.
\n\"\"
\nObserve the color changes after 2 hours.
\n\"\"
\nResults and Key Takeaways
\nIn my trial, there was almost no change in the first 30 minutes because the grow light was a bit weak, so I let it go for 2 full hours. Finally, Test Tube A (the one with the Elodea and light) shifted to a beautiful blue-green!
\nThe shift to blue-green is proof that the plant absorbed the dissolved \u202d$CO_2$\u202c through photosynthesis, causing the liquid to become more alkaline. BTB solution is a pH indicator that turns blue in alkaline conditions, green when neutral, and yellow when acidic. This color change essentially “visualizes” the plant breathing in carbon dioxide.
\n

\"\"

Left: Before \/ Right: After<\/p><\/div>
\nWe also saw tiny bubbles forming around the leaves of the Elodea.
\n\"\"
\nThese bubbles are pure oxygen produced by photosynthesis. Using light energy, plants convert \u202d$CO_2$\u202c and water into sugar and oxygen. Seeing this happen right before your eyes is incredible\u2014when you realize that the very oxygen we breathe was built by the world’s plants over millions of years, these tiny bubbles feel a lot more significant.
\nThe foil-wrapped tubes (C and D) showed no change. This confirms that without light, photosynthesis stops. This is the beauty of a “controlled experiment”\u2014by changing only one variable at a time, the visual evidence makes the science feel real.
\nPro-Tips for Success
\n\u2022\tUse a slightly more concentrated BTB solution than textbooks usually suggest to make the color change pop.
\n\u2022\tKeep a high-power grow light on standby for rainy or cloudy days.
\n\u2022\tAlways prep your Elodea in the dark for 24 hours beforehand.
\n\u2022\tGive yourself a 2-hour window for the reaction to fully develop.
\n\u2022\tHand out magnifying glasses! Watching the bubbles form is half the fun.
\nWhat if you use Limewater?
\nLet\u2019s look at what happens when we swap BTB for limewater.
\nI prepared four tubes by blowing into them for 20 seconds. I added Bromus catharticus (rescue grass) leaves to A and C, leaving B and D as controls. C and D were wrapped in foil. After 20 minutes in the sun, I checked the results. Since limewater turns cloudy when reacting with \u202d$CO_2$\u202c, we expect it to clear up if photosynthesis absorbs that \u202d$CO_2$\u202c.
\n\"\"
\nIt\u2019s hard to see through the leaves, so I pulled them out.
\n\"\"
\nTube A (left) is noticeably clearer, while the other three remain cloudy. While the photo makes it hard to distinguish A from C, the difference was striking in person. For reference, the textbook Science World 2 (Dainippon Tosho) shows the ideal result like this:<\/p>\n

\"\" Cited from Science World 2 (Dainippon Tosho, Feb 2021), P100<\/p>\n

In my test, the difference in Tube C wasn’t as dramatic as the textbook’s example\u2014leaving it outside longer likely would have cleared it up more. Comparing two different indicators (BTB and Limewater) is a fantastic way to verify the evidence from multiple angles. Here is why doing both is worth the effort:
\n\t1.\tVerifying the “Decrease” of \u202d$CO_2$\u202c in Two Ways
\nBTB shows it through color (yellow to blue-green), and limewater shows it through clarity (cloudy to clear). Confirming the same phenomenon with different tools teaches students the core scientific value of reproducibility and multi-angle verification.
\n\t2.\tDouble the Visual Impact
\nSeeing a liquid change color is one thing; seeing a cloudy liquid become crystal clear is another. Together, they create a stronger memory and spark deeper curiosity.
\n\t3.\tEasy to Expand into Comparative Studies
\nIn this post, I used Elodea for BTB and rescue grass for limewater. You can easily pivot this into a discussion or a new experiment: “Do different types of plants photosynthesize at different speeds?”
\nIn short, doing both experiments is a powerful way to make the science “stick.” If you have the class time, I highly recommend it. This is one of the most rewarding lessons in the curriculum, covering respiration, photosynthesis, and variable control all at once. Prepare well, and enjoy the “Aha!” moments with your students!
\nInquiries and Requests
\nWant to bring the wonders of science closer to home? I share tips and fun experiments you can try yourself. Feel free to explore!
\n\u2022\tLearn more about Ken Kuwako here<\/a>.
\n\u2022\tFor writing, speaking, or media requests, click here<\/a>.
\n\u2022\tGet the latest updates on X (formerly Twitter)<\/a>!<\/p>\n

\"\"<\/a> Check out my experiment videos on the Science Neta Channel<\/a>!<\/p>\n

\uff15\u6708\u306e\u30a4\u30c1\u30aa\u30b7\u5b9f\u9a13\uff01<\/h3>\r\n

\u30ad\u30fc\u30f3\u3068\u51b7\u3048\u308b\u30c9\u30e9\u30a4\u30a2\u30a4\u30b9\uff01\u6c17\u6e29\u304c\u4e0a\u304c\u3063\u3066\u304f\u308b\u3053\u306e\u6642\u671f\u30fb\u30c9\u30e9\u30a4\u30a2\u30a4\u30b9\u3092\u4f7f\u3063\u305f\u6607\u83ef\u30fb\u51dd\u7d50\u30fb\u7b49\u901f\u5ea6\u76f4\u7dda\u904b\u52d5\u306e\u5b9f\u9a13\u306f\u3044\u304b\u304c\uff1f<\/p>\r\n

\"\"<\/p>\r\n

\u6db2\u4f53\u30bc\u30ed\uff01\u30c9\u30e9\u30a4\u30a2\u30a4\u30b9\u304c\u6d88\u3048\u308b\u307e\u3067\u306e\uff13\u6642\u9593\u3092\u79d1\u5b66\u3059\u308b\uff08\u6607\u83ef\u30fb\u51dd\u7d50\u30fb\u7b49\u901f\u5ea6\u76f4\u7dda\u904b\u52d5\uff09<\/a><\/p>\r\n

\u30c6\u30ec\u30d3\u756a\u7d44\u76e3\u4fee\u30fb\u30a4\u30d9\u30f3\u30c8\u7b49\u306e\u304a\u77e5\u3089\u305b<\/strong><\/b><\/span><\/h3>\r\n