Hi - This is an easy one. Here's the background: The process of dissolving can be endothermic (temperature goes down) or exothermic (temperature goes up). When water dissolves a substance, the water molecules attract and “bond” to the particles (molecules or ions) of the substance causing the particles to separate from each other. The “bond” that a water molecule makes is not a covalent or ionic bond but a strong attraction caused by water’s polarity. It takes energy to break the bonds between the molecules or ions of the solute. Energy is released when water molecules bond to the solute molecules or ions. If it takes more energy to separate the particles of the solute than is released when the water molecules bond to the particles, then the temperature goes down (endothermic). If it takes less energy to separate the particles of the solute than is released when the water molecules bond to the particles, then the temperature goes up (exothermic).
So, all you need to find out which it is with baking soda is to put a thermometer in half a glass of water and record the temperature. Then add a teaspoon of baking soda and stir, put the thermometer back in and read the temperature. Voila! Your answer!