Engine cylinder has an area of 0.01 m2. The gas exerts a constant pressure of 7.5 * 105 Pa on the piston and moves the piston to a distance of 0.04 m. Is work done on the gas or by the gas?

The positive sign indicates that work is done by the gas.

Explanation:

Let consider that gas inside the cylinder behaves ideally and process is isothermal, so that change in internal energy can be neglected. The engine cylinder is modelled after the First Law of Thermodynamics:

The positive sign indicates that work is done by the gas.

freezing

deposition

condensation

explanation;

melting also called fusion is the process where solid becomes liquid, sublimation is the process where solid becomes gaseous without first changing to liquid, deposition involves change from gaseous to solid state, freezing involves change from liquid to solid, boiling or evaporation involves change of liquid to gaseous state, while condensation is the change from gaseous state to liquid state.

endothermic reactions involves absorption of heat from the surroundings or the environment while exothermic reactions involves loss of heat energy to the surroundings.

in this case, sublimation, evaporation and melting are endothermic reactions (gain energy from the environment) while, freezing, deposition and condensation are exothermic (loss of energy to the surroundings) reactions.