Hope's Experiment
About the Scientist
Dr. Thomas Charles Hope (1766–1844) was a Scottish chemist and physicist. He is best known for his famous experiment which proved that water has maximum density at 4°C. This discovery helps us understand why ice floats on water and why aquatic life can survive in frozen lakes.
About the Experiment
The experiment uses an apparatus with a cylindrical jar filled with water and a central trough for a freezing mixture. Two thermometers, one at the top (T2) and one at the bottom (T1), measure the temperature. As the freezing mixture cools the water, the denser water sinks and collects at the bottom, causing the lower thermometer (T1) to show a rapid drop to 4°C. The water above this 4°C layer cools, becomes less dense, and rises, causing the upper thermometer (T2) to then drop to 0°C. This video explains the setup and process of Hope's experiment: 50s Aasoka YouTube · 1 Jun 2015 Apparatus Setup A tall, cylindrical jar is filled with water. A central trough, containing a mixture of ice and salt, is placed around the middle of the jar. Two thermometers, T1 and T2, are fitted into openings, with T1 near the bottom of the jar and T2 near the top. Procedure The jar is filled with water, and both thermometers show the same initial temperature. A freezing mixture (ice and salt) is placed in the central trough. The freezing mixture cools the water in the central portion of the jar. Observations and Explanation 1. Initial Cooling: The water at the central part cools. 2. Denser Water Sinks: As the water cools from room temperature down to 4°C, it becomes denser and sinks, while the lighter, warmer water rises. This convection current causes the lower thermometer (T1) to show a rapid fall in temperature, which eventually stabilizes at 4°C. 3. Upper Water Cools: While the water at the bottom remains at 4°C, the water in the upper part of the cylinder continues to cool below 4°C. This colder water becomes lighter and rises to the surface. 4. Upper Water Freezes: Eventually, the upper thermometer (T2) shows a drop to 0°C and the water at the top begins to freeze. The lower portion, at 4°C, remains liquid because water at this temperature has maximum density and does not sink. Conclusion Hope's experiment demonstrates that water's density is maximum at 4°C, and that water expands when cooled from 4°C to 0°C, a behavior known as the anomalous expansion of water. This property is why ice floats and why bodies of water freeze from the surface downwards. .
🧩 Puzzles on Hope's Experiment
1. Who performed the famous experiment on water’s density?
2. What property of water was proved by Hope’s experiment?
3. At what temperature is water densest?
4. Why does ice float on water?
5. What apparatus did Hope use in his experiment?
6. What happened to the thermometer reading when the water cooled to 4°C?
7. Why can fish survive in frozen lakes during winter?
8. What happens to the density of water when it cools below 4°C?
9. Which natural phenomenon is explained by Hope’s experiment?
10. Write the temperature range between which water shows anomalous behavior.
