Problem 24 - Chemistry: The Central Science

Question

Three beakers contain clear, colorless liquids. One beaker contains pure water, another contains salt water, and another contains sugar water. How can you tell which beaker is which? (No tasting allowed!)

Step-by-Step Solution

Answer

Observe the physical properties of each liquid, and then perform conductivity tests, boiling point tests, and evaporation tests: 1. Check the conductivity - the liquid with the lowest conductivity is likely pure water, the highest is salt water, and the middle is sugar water. 2. Measure the boiling point - the liquid with the lowest boiling point is likely pure water, the highest is sugar water, and the middle is salt water. 3. Observe the residues after evaporation - no residue indicates pure water, white crystalline residue indicates salt water, and sticky whitish residue indicates sugar water.

Step 1: Observe the physical properties of each liquid

Start by observing each beaker carefully. It is possible that one of them might have an observable difference like particles settled at the bottom, which would indicate a salt or sugar solution. Keep in mind that sometimes salt and sugar dissolve completely, so the lack of particles is not a definitive answer.

Step 2: Check the conductivity of each liquid

Using an electrical conductivity meter, check the conductivity of each liquid. Pure water is a poor conductor of electricity, while saltwater is a good conductor. Sugar water, on the other hand, is less conductive than saltwater but more conductive than pure water. Measure the conductivity of each liquid and compare them: - The liquid with the lowest conductivity is likely to be the pure water. - The liquid with the highest conductivity is most likely the salt water. - The liquid with conductivity between the two extremes is likely to be the sugar water.

Step 3: Perform a boiling point test

Perform a boiling point test for each liquid using a heat source and a thermometer: - Heat each liquid while measuring and recording the temperature at which each starts to boil. - The liquid with the lowest boiling point is likely to be pure water (boiling point of \(100^{\circ}\)C at 1 atmospheric pressure). - The one with the highest boiling point is probably the sugar water, as sugar raises the boiling point more than salt does. - The liquid with a boiling point between those two is likely to be the salt water.

Step 4: Perform an evaporation test

Another way to identify the liquids is to observe them after evaporation: - Place a small amount of each liquid onto separate plates and let them evaporate (you can use a heat source to speed up the process if needed). - Observe the remaining residues after evaporation: - A plate with no residue or a very minimal residue suggests that it had pure water. - A plate with a white, crystalline residue indicates it had salt water. - A plate with a sticky, whitish residue means it contained sugar water. Following these steps, you will be able to deduce which liquid is in each beaker without tasting them.