See CLEAPSS Recipe Book RB000, which also covers bicarbonate indicator solution.Įventually sufficient carbon dioxide from the students’ breath dissolves and produces enough acid in the solution to change the colour of the indicator:ĬO 2(aq) + H 2O(l) ⇌ H +(aq) + HCO 3 –(aq)ĬO 2 also reacts with NaOH.
Thymolphthalein (the alternative bromothymol blue could also be used) changes from blue (alkaline) to colourless (acid) over the pH range 9.3–10.5. Phenol red indicator changes from yellow to red over the pH range 6.8–8.4. Straws are not necessary for blowing exhaled air into the flask simply breathing or speaking into the flask is sufficient to cause the indicator to change colour. To ensure the solution is slightly alkaline at the beginning and to neutralise any CO 2 or any other acid initially present.The amount of carbon dioxide in each breath is small, so it takes a lot of breaths to react with the alkali.What is the reason for adding a few drops of sodium hydroxide solution (NaOH) before each experiment?.Why does the colour change not occur instantly?.Talk or blow gently into the flask – ie add carbon dioxide.Add two drops of sodium hydroxide solution to produce a red solution.Add one or two drops of phenol red to the water.Continue adding the carbon dioxide until a colour change is observed.Talk or blow gently into the flask – ie add the carbon dioxide.Add just enough sodium hydroxide solution (about two or three drops) to produce a blue colour.Add five or six drops of thymolphthalein indicator to the water.Place about 125 cm 3 of water in a 250 cm 3 conical flask.Source: Royal Society of Chemistry Activity 1 Sodium hydroxide solution, NaOH(aq), (IRRITANT at concentration used) – see CLEAPSS Hazcard HC091a and CLEAPSS Recipe Book RB085.Ethanol (IDA – Industrial Denatured Alcohol), CH 3CH 2OH(l), (HIGHLY FLAMMABLE, HARMFUL) – see CLEAPSS Hazcard HC040A.While thymolphthalein itself is not flammable, its solution in ethanol is highly flammable. If 30 cm 3 or 60 cm 3 dropping bottles with integral dropping pipettes are available, these are ideal for dispensing the indicator solutions. The solution may be made from the solid reagents by preparing a 5% w/v solution in ethanol (IDA). The indicator may be purchased as a solid reagent or as a ready-made solution in ethanol. Thymolphthalein indicator – see CLEAPSS Hazcard HC032.While phenol red itself is not flammable, its solution in ethanol is highly flammable. Phenol red indicator – see CLEAPSS Hazcard HC032.Read our standard health and safety guidance.Distilled (or deionised) water, 125 cm 3, x2.Sodium hydroxide solution, 0.4 M (IRRITANT), small bottle with dropper.Phenol red indicator solution (HIGHLY FLAMMABLE), access to small bottle with dropper.Thymolphthalein indicator solution (HIGHLY FLAMMABLE), access to small bottle with dropper.Ethanol (IDA – Industrial Denatured Alcohol) (HIGHLY FLAMMABLE, HARMFUL).Indicator bottles with dropping pipettes, x3.The equation for the reaction between carbon dioxide and water may be introduced for appropriate students. If students have not yet met the compositions of inhaled and exhaled air, this experiment can serve as part of the learning sequence for the topic of breathing and respiration in an introductory science course, using an appropriately elementary approach to the chemistry involved.įor students who have already covered the topic of breathing and respiration, and know that carbon dioxide is a significant component of exhaled air, the focus in this experiment can be transferred to the nature of the chemical reaction (other related topics could be acid rain, gas liquid reactions or indicators). Carbon dioxide present in exhaled air is blown into a flask containing an indicator sensitive to small changes of pH in the appropriate region of the pH scale, and the consequent colour changes observed and recorded. When carbon dioxide reacts with water a weak acid is formed. This is a relatively brief and straightforward exploration of the reaction of carbon dioxide and water at a simple level, which should take no more than 15 minutes. RSC Yusuf Hamied Inspirational Science Programme.Introductory maths for higher education.
The physics of restoration and conservation.
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