酸 (suān) is an adjective for the sour taste (like a lemon) and for physical soreness (like aching muscles after exercise). For physical pain, 疼 (téng) and 痛 (tòng) are more general and common; 酸 specifically means a dull, tired ache. For emotional sadness, 酸 is a brief, poignant feeling (a 'pang' of grief), while 伤心 (shāng xīn) is deeper and longer-lasting heartbreak.