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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?

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  • $\begingroup$ $\Delta H$ 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. $\endgroup$ – Chet Miller Jun 23 at 1:20
  • $\begingroup$ 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 $\endgroup$ – vik1245 Jun 23 at 1:24
  • $\begingroup$ 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. $\endgroup$ – Chet Miller Jun 23 at 1:29
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    $\begingroup$ "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. $\endgroup$ – Buck Thorn Jun 23 at 8:56
  • $\begingroup$ see for instance chemistry.stackexchange.com/questions/117115/… $\endgroup$ – Buck Thorn Jun 23 at 8:57
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This is the basic principle of cooling a system. for example when water cools in an earthen pitcher, the heat is taken from the water, pitcher and the atmosphere to convert the surface molecules into vapors. Yet water is considered as a system there and not surroundings.So in your question,during solvation of sodium nitrate, energy is absorbed even from water to dissociate the sodium nitrate into ions (as solvation energy).

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    $\begingroup$ Which one's the system and surroundings again? Please explicitly state that. $\endgroup$ – William R. Ebenezer Jun 23 at 12:34
  • $\begingroup$ Yes please do define the system and surroundings just for clarity! $\endgroup$ – vik1245 Jun 23 at 13:48
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The system and surroundings are designations to help simplify the universe. There are things to focus on and details to ignore. Those, respectively, are very rough definitions of what the system and surroundings are.

From a chemical standpoint, the system is everything of interest where the chemistry is happening. Dissolution is a process that occurs because a solute and solvent interact. Therefore, it would a mistake to look at the solute or solvent only when examining a dissolution process. Both of those components at least must be in the system.

The surroundings are generally understood to be large enough that they are more or less fixed or unchanging. Nevertheless, if there is heat transferred to or from the surroundings, even though this cannot change the temperature of the surroundings, it will change the entropy.

For calorimetry, you need to limit heat transfer so that you can measure accurately the enthalpy change of the system. A calorimeter helps to do this, but it is now a heat sink and will need to be accounted for in your calculations. Since its temperature is changing significantly, it should not be included in the surroundings, but rather as part of the system.

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