Investigating respiration

Respiration by living organisms can be investigated by carrying out experiments to show the production of carbon dioxide and heat.

Limewater can be used to detect carbon dioxide.

A respiration experiment using limewater in a test tube. Step 1: A pipe labelled carbon dioxide gas enters the test tube, there are bubbles in the limewater. Step 2: the limewater turns milky.

If carbon dioxide is bubbled through limewater then it turns from clear to cloudy/milky in colour. This is why limewater used in a simple respirometer can show that more carbon dioxide is present in exhaled air compared to inhaled air.

2 test tubes, both containing clear limewater, labelled A and B. A system of pipes labelled Mouthpiece connects the test tubes.

Breathing in through the mouthpiece draws air from the atmosphere in through tube A. Breathing out through the mouthpiece bubbles exhaled air through tube B. After a few breaths the limewater in tube B will turn cloudy as the exhaled air contains more carbon dioxide.


If inhaled air contains 0.04% carbon dioxide and exhaled air 4%, how many times more carbon dioxide do you breathe out than you breathe in?

100 times more.

Investigating heat production

The release of heat can be shown by carrying out an experiment using germinating seeds. Two vacuum flasks are used.

Aerobic respiration is investigated through an experiment which looks at heat loss in germinating peas as they respire.