If you drink soda after eating spicy food , what happens is the burning from the spicy food intensifies.
The capsaicin chemical in spicy foods is fat soluble and the carbonation of soda and lack of fat in soda can create a carbonic acid, which, when combined with the capsaicins pain receptors, also intensifies the spicy sensation, instead of providing relief from the spicy foods.
When eating spicy food what you should not drink is water, coffee, other hot beverages, carbonated drinks like soda as these drinks can actually spread the heat causing capsaicin oil and make the burning sensation worse instead of neutralizing it.
Instead you should drink things like milk or eat yogurt, starchy foods like rice or bread or even drink acidic drinks like some lemonade.
Coffee not only can make the burning sensation worse when eating spicy foods but the coffee when consumed with the spicy foods actually tastes awful.
The reason why you should not drink water after eating spicy foods is because the water can make the spiciness worse.
The capsaicin which is the active compound in peppers is what is responsible for the spiciness, is not soluble in water.
Capsaicin is a hydrophobic molecule that hates water and binds to pain receptors in your mouth which causes the burning sensation from spicy foods.
When you drink water after eating spicy foods, it does not dissolve or neutralize the capsaicin and instead water can spread the capsaicin and make the spiciness worse.
Instead of drinking water after eating spicy foods to cool down the heat you should drink liquids that can dissolve the capsaicin.
Drinks and foods that can dissolve the capsaicin are cold drinks other than water, bread, yogurt and milk.
The starch in bread can absorb the capsaicin and reduce the burning sensation.
Yogurt, similar to milk contains fat and proteins that can also help with the spiciness.
And milk has fat and proteins which can bind to the capsaicin and neutralize the effect of the capsaicin.