What hypothesis can I use for salt dissolving faster in water than baking soda?
salt won't dissolving quicker than baking soda because baking soda is a power, and salt will be a little slower because the size is less than a gram.
Baking powder is Sodium Bicarbonate a polar compound that is particularly amenable to water (a polar solvent). So Baking powder will dissolve faster than a non-polar powder like Talcum powder. Also baking power is very finely ground and will present a large surface area to the water which hastens dissolving. Salt in comparison although a strongly polar compound is course ground and will take longer to dissolve.