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Let us examine experiments to prove that \(CO_2\) is released during respiration in human beings and during fermentation in yeast.
 
Aim:
 
To show that \(CO_2\) is produced during respiration. The composition of \(CO_2\) in the air we breathe out is more than that of the normal air.
 
Materials required:
  1. Two test tubes
  2. Freshly prepared lime water
  3. Syringe or pichkari
  4. Glass tube
  5. Rubber tube
Procedure:
 
1. Pour the freshly prepared lime water into two test tubes labelled \(A \)and \(B\).
 
2. With the help of a glass tube, blow air in the test tube labelled \(B\).
 
3. Using a syringe or pichkari fitted with a rubber tube, pass air through the lime water in the test tube labelled \(A\).
 
4. Note down the difference in the time it takes for the lime water to turn milky.
 
CO2 present.svg
Experimental setup
 
Observation:
 
The lime water in test tube \(B \)turns milky in no time showing that we exhale \(CO_2\). However, in test tube \(A\), the lime water takes a long time to turn milky.
 
Inference:
 
Lime water turns milky when \(CO_2\) is mixed with it. Air blown from the mouth contains \(CO_2\) - approximately \(4\%\). The composition of \(CO_2\) in atmosphere is \(0.04 \%\). This proves that the composition of \(CO_2\) in the air during exhalation is higher than the \(CO_2\) in the atmosphere.  
 
Aim:
 
To show that \(CO_2\) is produced during yeast respiration through the process of fermentation.
 
Materials required:
 
Beaker, \(2\) test tube fitted with a one-holed cork, bent glass tube, lime water, fruit juice or sugar solution, yeast.
 
Procedure
 
1. Take some sugar solution or fruit juice in a beaker.
 
2. Add yeast to the beaker containing the juice or sugar solution.
 
3. Pour the mixture (sugar solution containing yeast) into the test tube fitted with the one-holed cork.
 
4. Take a test tube and pour in some freshly prepared lime water.
 
5. Fit the cork with a bent glass tube. One end of the glass tube is in the mixture, while the other end of the glass tube is dipped in the test tube containing lime water.
 
6. Observe the changes that take place in the lime water.
 
Observation:
 
The lime water turns milky.
 
Inference:
 
Lime water turns milky when \(CO_2\) is mixed with it. Yeast, when added to the sugar solution or fruit juice that contains sugar, the yeast converts the sugar or glucose to produce ethyl alcohol and \(CO_2\).
 
Glucose (\(C_6H_{12}O_6\))  Pyruvate  InabsenceofO2  Ethanol or ethyl alcohol (\(C_2H_5OH\)) + Carbon dioxide (\(CO_2\))
 
The \(CO_2\) produced turns the lime water milky.
 
The following video demonstrates the experiment:
 
Reference:
https://www.youtube.com/watch?v=3ndVcEMj_V8