In the 1st law of Thermodynamics $\Delta U=q+w$, $w=-p\Delta v$ why there is a minus sign before $p$?
In very simple terms, when considering work you're talking about the energetic interactions within the system. You'll notice that, when an expansion takes place, ΔV has a positive sign as the final volume is greater than the initial volume. With the outer minus sign, the whole value adopts becomes negative. Negative values of energetic magnitudes are associated with the liberation of energy, considering it a loss of it. The work was done by the system. In the inverse case, during a compression, the final volume is smaller than the initial volume, therefore the ΔV is negative and the outcome of W is positive. This informs us of an absorption of energy, caused by the pressure the system was subject of. Now, the work was done on the system. Hope this helped.