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Titanic's Lost Sister

Classroom Activity

Objective
To investigate how differences in temperature affect the density of water.

• copy of "Current Events" student handout (PDF or HTML)
• hot tap water (near boiling)
• cold water (from refrigerator or ice water)
• four unbreakable glass or transparent plastic cups
• food coloring (one color)
• plastic spoon
• sheet of paper

1. Robert Ballard used the "debris field theory" to answer questions about Britannic. This theory takes into account that materials sinking to the ocean floor settle at different rates, depending upon their weight, density, and shape, and the direction of the ocean current. Deep-water currents are caused by differences in the density of water due to variations in temperature and amount of salt.

2. In the "Current Events" activity, students can investigate how differences in temperature affect the density of water.

3. Gather materials, organize students into groups, and distribute copies of the "Current Events" student handout to each group.

4. Students will first predict which is more dense (or which will sink faster), cold water or hot water, and explain their reasoning. They will then compare the density of hot and cold water by investigating what happens when they add cold water to hot water, and vice versa.

Caution: students will be using hot water. Provide appropriate supervision.

Like air, water expands as it gets warmer and as a result becomes less dense. Water is most dense at temperatures near freezing. When water freezes, however, it expands, becoming less dense. If equal volumes of cold water (down to 4deg.C, 39.2deg.F) and hot water are compared, cold water weighs more than hot water.

Students should notice that cold water will sink when placed on top of hot water, and hot water will float on the surface of cold water. A slight amount of mixing will occur immediately, and over time, the initially separated strata of water will mix.

Differences in water density cause underwater currents. Cold, dense water sinks to deep levels in the ocean. Warm, less dense water moves in to replace it near the surface. Differences in density of water due to different amounts of salt also cause underwater currents. Students can repeat the activity, but this time substitute the comparison of cold and hot water with the comparison of salt and fresh water.