Friends often tell me to add salt to make water boil faster.
As explained above,
- salt has a very low specific heat capacity (amount of energy needed to raise temperature by 1°)
- water has a much higher one (so salt's is insignificant)
- If you have 1L of pure water and 1L of 20% salt water (20% salt, 80% water), then yes, the salt water will boil in 4/5 the time.
- You are not cooking with 20% salt water (which would probably be a bad idea) if you're sprinkling salt from a shaker in. Perhaps at best you have 1% salt water.
- Amusingly, though, salt water has a higher boiling temperature, at 102°C instead of 100°C (for pure water), so 1% salt water won't speed you to boiling at all
- Also, if you take 1L of pure water versus 1L of pure water where you've sprinkled salt, it will definitely not go faster, because even though you just added salt, you still have that 1L of water that needs to boil through.