Surroundings

Energy can move between the reacting substances and their surroundings: for example, the liquid around them, the container or your hand. If energy is transferred from a reaction to its surroundings, the products have less energy than the reactants by the amount transferred. If energy is taken in, the products have more energy than the reactants.

A stoppered flask is a suitable example: energy may pass between the flask and its surroundings, but all the reacting substances and products remain inside.

Think of the reacting substances as one part of a system and everything around them as the surroundings. Energy is not created or used up: it moves between these parts. The direction of that transfer determines whether a reaction is exothermic or endothermic.

Everything outside that boundary is the surroundings. In a reaction carried out in a cup, the immediate surroundings include the rest of the solution, the cup and the nearby air.