Effect of Temperature on the Thickness of Thixotropic Liquids

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Author: Janice VanCleave

What Effect Does Temperature Have on the Thickness of Ketchup?

Category: Chemistry—Physical Changes

Project Idea by: Lacey Russell

Ketchup is an example of a heterogeneous mixture, which means that it is not the same throughout. Some of the particles in ketchup are dissolved and are spread throughout the mixture, but some particles remain suspended in the liquid.

Ketchup is a type of mixture called a thixotropic liquid, which increases in thickness when allowed to stand and decreases in thickness when shaken or stirred. This happens because when the substance is still, weak bonds form between the particles, linking them and forming a support frame. Since the support frame is weak, it is easily broken if the liquid is shaken, squeezed, or stirred; thus, the thickness decreases. But when the motion stops, the particles reconnect again, forming the support structure and thickening the mixture.

Toothpaste is another example of a thixotropic liquid. When left alone, it acts more like a solid. If you turn an open toothpaste tube upside down, the paste will not flow out. But if you squeeze the tube, the paste, even though thick, flows out much like a liquid would. A project question might be, "What effect does temperature have on the thickness of ketchup?"

What Effect Does Temperature Have on the Thickness of Ketchup?

Clues for Your Investigation

Design a way to test the thickness of different samples of ketchup at different temperatures. One way is to compare their thickness by measuring how easily they flow. You could place equal-sized blobs of ketchup at one end of a baking sheet. Then raise the end of the baking sheet with the ketchup samples to create an inclined surface. In advance, determine that one ketchup sample will be the control, such as the one at room temperature. Arbitrarily assign the control a thickness value of 5. Compare how fast the other samples of ketchup, which are at different temperatures, flow in comparison to the control. Rate each sample with a thickness number from 1 to 10, with 1 flowing the slowest and 10 the fastest. You may use fractions.

Independent Variable: Temperature

Dependent Variable: Flow rate

Controlled Variables: Thixotropic liquid (one brand of ketchup), measuring device for flow rate, measuring device for temperature, environmental factors

Control: Median temperature (room temperature can arbitrarily be used)

Other Questions to Explore

  1. Would changing the concentration of the thixotropic liquid affect the results?
  2. What is a dilatant and how is its thickness by temperature?

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