When air is blown from mouth into a test tube containing lime water, the lime water turned milky due to the presence of:

We exhale carbon dioxide, so when air is blown from mouth, i.e., carbon dioxide, into a test tube that contains lime water, the lime water turns milky due to presence of carbon dioxide. Carbon dioxide turns lime water milky. This is due to the formation of calcium carbonate when carbon dioxide combines with calcium hydroxide (limewater).

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