Comparing the ph papers, we observe that the content of our stomach is normally acidic. Our stomach normally produces acid to help with the digestion of food and to kill germs (bacteria). This acidity increases even more when we have excessive spicy food or when there is excess secretion of acids in the gastric glands of the stomach.
Antacids work by counteracting (neutralizing) the acid in our stomach. They do this because the chemicals in antacids are bases (alkalis) which are the opposite of acids. A reaction between an acid and base is called a neutralization reaction. This neutralization makes the stomach contents less acidic and thus helps to relieve the burning sensation of acid reflux.