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www.GOALias.blogspot.comtest-tube. Insert a plastic straw throughthe hole in the lid in such a way that itdips in lime water. Now blow gentlythrough the straw a few times (Fig.10.8). Is there a change in theappearance of lime water? Can youexplain this change on the basis of whatyou learnt in Chapter 6?You are aware that air weinhale or exhale is a mixture ofgases. What do we exhale? Do weexhale only carbon dioxide or a mixtureof gases along with it? You must havealso observed that if you exhale on amirror, a film of moisture appears on itssurface. From where do these dropletscome?Fig. 10.6 Measuring chest sizeTo understand the expansion of thelungs, pull the rubber sheet from thebase downwards and watch the balloons.Next, push the rubber/plastic sheet upand observe the balloons. Did you seeany changes in the balloons?What do the balloons in this modelrepresent? What does the rubber sheetrepresent?Now, you should be able to explainthe mechanism of breathing.10.4 WHATDO WE BREATHEOUT ?Activity 10.6Take a slender, clean test tube or aglass/plastic bottle. Make a hole in itslid and fix it on the bottle. Pour somefreshly prepared lime water in theBoojho wants to know howmuch air a person can hold inthe lungs.PlasticbottleBalloonsRubersheetFig 10.7 Model to show mechanism ofbreathing114SCIENCE