Surface tension and surface energy
Liquid State · Chemistry
Study notes
A needle floats on water but sinks in soapy water. Explain. Step 1: Clean water's high surface tension (0.072 N/m) acts like an elastic skin supporting the needle. Step 2: The needle doesn't break through if placed gently — surface tension balances its weight. Step 3: Soap molecules crowd the surface, cutting γ roughly in half. Step 4: Weakened skin can't support the needle → it sinks. Same physics: water striders walk on ponds; detergent lets water soak fabric.