# What are the system and surroundings when dissolving sodium nitrate in water?

When sodium nitrate ($$\ce{NaNO3}$$) dissolves in water at room temperature, the solution cools down. $$\Delta H$$ is positive because heat flows into the system during the process.

I thought though, that if the water cools down, the water would be the surroundings as heat is being taken from the water in solution to dissolve the $$\ce{NaNO3}$$ however my professor said:

The system is the calorimeter and its contents, including all of the water/ $$\ce{NaNO3}$$. The surroundings are everything outside of the calorimeter (room etc.)

I'm slightly confused here.

Why does heat flow into the system if water forms part of the system in the first place?

• $\Delta H$ represents the amount of heat that would have to flow into the system of water and NaNO3 (from the surroundings) if the final temperature was to be held constant at the initial temperature. If that amount of heat is not supplied to the system from the surroundings, the system will cool down. – Chet Miller Jun 23 '19 at 1:20
• So the solution initially cools down and then there is heat flow from the surroundings in order for the solution and the surroundings to reach the same temperature again, correct? @ChetMiller – vik1245 Jun 23 '19 at 1:24
• If the solution is insulated from the surroundings, no heat will flow and the solution will cool down and just stay there. If it is contact with surroundings that are at the initial temperature, heat will be flow from the surroundings to the solution, and, when the final state is reached, the system and surroundings will be in thermodynamic equilibrium and back at the original temperature. – Chet Miller Jun 23 '19 at 1:29
• "System" and "surroundings" are labels for regions in space, and do not define the direction of heat flow. In general you define the system to be the part of interest (where a reaction occurs for instance) and the surroundings are everything else. – Buck Thorn Jun 23 '19 at 8:56
• see for instance chemistry.stackexchange.com/questions/117115/… – Buck Thorn Jun 23 '19 at 8:57