唔識呀!!!!Chem呀!!!有冇人幫我呀!!!!!!What is the fizzy drink about???

6.A fizzy drink tablet contains solid citric acid and a sodium-containing substance X. When it is added into a glass of water, effvescence occurs.

a). What may substance X be?

b). Write the ionic equation for the reaction.

c). Suggest ONE method to determine the pH of the resulting solution.

d). The fizzy drink tablets should be stored in dry places. Why?

e). Give ONE use of substance X in first aid process.