In biology class we did a practical with lime water. Lime water is reagent which indicates the presence of CO2. To be able to test this, we put 4 test tubes in a rack, each one containing lime water. Then, with four straws (one test tube each) we expired carbon dioxide with our mouth into the test tube, making the CO2 be in contact with the reagent. Finally, after blowing for some minutes, we realised that the lime water had turned cloudy. So, finally, we concluded that when carbon dioxide is present the lime water turns cloudy.