Title: Hotter Boiling Water
Objective: To demonstrate to students that a beaker of boiling water initially has a
temperature of 100° C, depending on elevation, but when salt or sugar is added to the
water, the temperature rises well above 100° C.
Purpose: To show that adding a water-soluble solid to water raises its boiling
temperature.
Standards: EALR 4 Physical Science: Big Idea: Force and Motion
Materials: one 250 ml beaker, water, salt or sugar, one thermometer, one thermometer
support, one spoon, one beaker support, one Bunsen burner, matches.
Procedure: Place the beaker on the support and fill it with water. Carefully insert the
thermometer in it and support the thermometer so that it doesn't touch the walls of the
beaker. Light the burner and gently heat the beaker. Be careful not to heat the beaker too
quickly or it may break. Soon the water will boil and the thermometer will read about
100° C. Now add a large spoonful of salt or sugar to the boiling water. Stir the mixture.
When the mixture again begins to boil, the thermometer will read well above 100° C.
Student Questions for Inquiry:
1. Explain what a water-soluble solid is.
2. Discuss what happens when the water-soluble solid is added to the water.
Science Behind It: Adding a water soluble solid to water interferes with its ability to
evaporate. With many of the water molecules involved in stabilizing the dissolved solid,
there are fewer water molecules evaporating at any given temperature. The water
temperature must exceed 100° C before evaporation is fast enough for evaporation
bubbles to become stable within the body of the water so that boiling can occur.