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When bacteria starts to grow in our mouth it starts to convert sugar into lactic acids which destroys our enamel and downs the pH value of the mouth. Hence, toothpaste helps in killing those bacteria and prevents our teeth from decay.
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When we eat something after a while the bacteria present in our mouth reacts with the food and releases lactic acid , resulting as lowering the PH of the mouth .
The formation of this acidic mouth breaks our enamel slowly ,but the toothpastes are basic in nature and decreases the action of acids and neutralize the PH value of the mouth  and prevents it from decaying.

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