The mechanochemical effects observed in the processes of dissolution of bent glass and silicon plates in aqueous solutions of hydrogen fluoride and potassium hydroxide are investigated. Special attention is focused on the strain sign effect caused by the surface stresses in solids. Direct visual observations of the displacement of the boundaries of solids on the concave and convex sides of the plates confirm the existence of a mechanochemical effect of the strain sign. The dominance of dissolution on the concave side of the plates indicates the negative sign of the surface stress in the glass and in silicon at the interface with the solutions. This can be explained by the formation of a double electric layer.