Lesson A7: Viscosity

Key Ideas     Unit Checklist

  Video Lesson

Why does molasses pour slowly, while water pours quickly? Watch this video to learn more about the viscosity of fluids.

 
 

  Lesson A7: Viscosity

Figure A.3.7.1 – Lava flows from erupting volcanoes.
Figure A.3.7.2 – Pahoehoe lava can flow quickly.

Volcano Viscosity
Lava is molten rock that flows from volcanoes. Some volcanoes explode lava violently into the air, while other volcanoes produce a steady stream of flowing lava. Volcanoes in Hawaii erupt slowly and steadily. The Hawaiian volcano Kilauea has been continuously erupting since 1983!

Flowing lava is not all identical. Hawaiians observed and named two main kinds of lava. Pahoehoe lava moves quickly. It has a low viscosity because it contains dissolved volcanic gases. When pahoehoe lava cools, it has a smooth rope-like texture and is easy to walk on.

Aa lava flows slowly. It has a high viscosity because dissolved gases have escaped from the lava. Aa lava has a chunky appearance and cools into sharp, jagged rocks that are difficult to walk on without getting hurt.
Reading and Materials for This Lesson

Science in Action 8
Reading: Pages 39–41

Materials:
Old newspapers, large bowl, cornstarch (1 box), water, measuring cup, food colouring (optional), maple syrup, oil, honey, water, dish detergent, cookie sheets (2), old books or wood blocks, timer, thermometer, cups, measuring spoons.

Figure A.3.7.3 – Pahoehoe lava cools in smooth rope-like patterns.
Figure A.3.7.4 – Chunky aa lava flows slowly.

 Watch More

Pahoehoe and Aa Lava

Watch this video to see the fast movement of pahoehoe lava.

 
 
 
 
This video shows a volcanic eruption of slow-moving aa lava.

 
 
Figure A.3.7.5 – Water is a Newtonian fluid because its viscosity stays constant.
Figure A.3.7.6 – Wet cement is a non-Newtonian fluid. Its viscosity changes as it flows.


Figure A.3.7.7 – Ketchup is a non-Newtonian fluid because its viscosity can change when force is applied.
Non-Newtonian Fluids

Some fluids show strange behaviour related to viscosity. We describe these fluids as non-Newtonian.

The viscosity of non-Newtonian fluids changes under force or stress. Some non-Newtonian fluids decrease in viscosity when force is applied, and others increase in viscosity. An example of a non-Newtonian fluid is ketchup. If you turn a glass bottle of ketchup upside down, it is very viscous and doesn’t flow out of the bottle. However, you don’t have to heat up the ketchup to make it flow. If you apply force by hitting the bottom of the bottle, the ketchup will suddenly become less viscous and flows quickly out of the bottle.

In contrast, Newtonian fluids have a viscosity that changes predictably. Newtonian fluids have a higher viscosity at lower temperatures, and a lower viscosity at higher temperatures. In short, they flow more easily when they are warmer. Two examples of Newtonian fluids are water and air.

 Watch More

Fluids That Don't Always Go With the Flow

This video explains some examples of both Newtonian and non-Newtonian fluids.

 
 
 
 
Ketchup is a non-Newtonian fluid that is difficult to pour. It flows more easily when force is applied. Watch this video to learn more about how particles behave in ketchup.

 
 
 
 
A cornstarch and water mixture called oobleck is a non-Newtonian fluid. Watch this video to see a person attempt to bicycle across a trough of cornstarch!

 
 

Figure A.3.7.8 – Quicksand is sometimes found on beaches.
Figure A.3.7.9 – At low tide, this French island is surrounded by quicksand.

Quicksand

Quicksand is another example of a non-Newtonian fluid. Quicksand is a mixture of sand or silt, and water. Quicksand forms in areas where water can’t drain away easily, such as on beaches at low tide, or in a marsh. If a person sinks into quicksand, the high viscosity of the quicksand makes it hard to escape. Struggling to get out just makes the person sink deeper, because applying force makes the quicksand more viscous. Slow careful movements make the quicksand less viscous, which can sometimes help a person to wiggle their way out.

Quicksand is usually not that deep, so people don’t usually sink in over their heads, as shown in movies. The danger of quicksand is being stuck in it at low tide and becoming covered with water when the tide rises.

 Watch More

Slowly Perish in Quicksand

Is it possible to escape from quicksand? Watch this video to find out.

 
 
 
 
This video explains how to escape from quicksand.

 
 

  Try It! 

Figure A.3.7.10 – It sometimes takes help to escape from quicksand.. Photo by Pierce Martin. 
Making Quicksand

Try this simple experiment to create a non-Newtonian fluid similar to quicksand. 

Materials: 
  • Old newspapers
  • Large bowl
  • Cornstarch (1 box)
  • Water
  • Measuring cup
  • Food colouring (optional) 


Instructions:

  1. Cover your working surface with old newspapers to protect the surface from a mess.

  2. Pour the box of cornstarch into the large bowl.

  3. Add 2 cups of water to the cornstarch.

  4. Add a few drops of food colouring to the cornstarch.

  5. Mix the cornstarch, water, and food colouring together thoroughly with your hands. Your hands will get messy, so take off any jewelry or watches beforehand and make sure there’s a sink nearby.

  6. Play with your “quicksand”. What happens when you let it drip through your fingers? What happens when you punch the bowl? Can you trap your fingers in the fluid?

  7. To see this experiment in action, and more ideas of how to test your “quicksand”, watch this video:


 
 

Lesson Activity


Viscosity Ramp Test

 Problem:

Out of five fluids, which one has the highest viscosity? Which one has the lowest viscosity? Design and test a ramp test experiment to find the answer.

Hypothesis:

A hypothesis is a testable answer to a scientific question. In other words, what do you think the result of this activity will be?

Hint: In this case, think about how you would state a prediction of how fast and slow different viscosity fluids will flow.

Before you make your own hypothesis, you should think about the manipulated, responding, and controlled variables in this activity. Review the science skills video on variables if you need to.

Have you thought carefully about the experiment variables? If you have, then click for the answers below.
This experiment is interested in how fast fluids flow. The time it takes for a fluid to travel from the top to the bottom of the ramp is the responding variable.
One thing has been manipulated or changed in this experiment, which is the type of fluid used. That is the manipulated variable.
We keep controlled variables the same to be sure the experiment is fair. This experiment has several controlled variables including the angle of the ramp, the length of the ramp, the temperature of the fluids, the amount of each fluid placed on the ramp, and the position on the ramp where the fluids are placed.
Download:

DOWNLOAD this document. It provides a space for you to write your own hypothesis. This document also includes a sample hypothesis you can compare to your own. Also use the document to write your procedure and analysis questions that come later in this activity.

Materials:

  • Maple syrup
  • Oil
  • Honey
  • Water
  • Dish detergent
  • Cookie sheets (2)
  • Old books or wood block
  • Timer
  • Thermometer
  • Cups
  • Measuring spoons
  • Any other materials you think are useful
 
Instructions:

  1. Design and write a procedure for this experiment. Write your procedure as numbered steps with enough detail so someone else could easily do the experiment.

    Before you start writing your procedure, consider these questions:

    • How are you going to measure the responding variable?
    • What are all the specific things you need to keep the same in order to make it a fair test?
--------------------
Use the document you downloaded to write your procedure.
 

  1. Test your procedure and record your results in a table like this:

 
 Fluid  Time
 maple syrup
 
 oil  
 honey  
 water
 
 detergent
 
 

Analysis Questions:

Think about the following questions very carefully. Refer to your observation table. Then, type or write your answers. When you have your answers, click the questions for feedback.

Honey had the highest viscosity because it took the longest amount of time to flow down the ramp. The particles in honey are very attracted to each other, which means they bump into each other more often. These collisions create more friction between the honey particles, slowing them down.
Water had the lowest viscosity because it took the shortest amount of time to flow down the ramp. The particles in water are less attracted to each other, which means they bump into each other less often. Less collisions create less friction between the water particles, so they don’t slow down.
The fluids in order from highest viscosity to lowest viscosity are honey, dish detergent, maple syrup, olive oil, and water.
 Sharing:

Congratulations on completing this activity! Consider sharing your completed work (or any thoughts and experiences you might have regarding this activity) in the course Sharing Forum. You can also email your completed work to your teacher and ask for feedback.



  Make sure you have understood everything in this lesson. Use the Self-Check below, and the Self-Check & Lesson Review Tips  to guide your learning.

Unit A Lesson 7 Self-Check

Instructions


Complete the following 6 steps. Don't skip steps – if you do them in order, you will confirm your understanding of this lesson and create a study bank for the future.

  1. DOWNLOAD the self-check quiz by clicking here.

  2. ANSWER all the questions on the downloaded quiz in the spaces provided. Think carefully before typing your answers. Review this lesson if you need to. Save your quiz when you are done.

  3. COMPARE your answers with the suggested "Self-Check Quiz Answers" below. WAIT! You didn't skip step 2, did you? It's very important to carefully write out your own answers before checking the suggested answers.

  4. REVISE your quiz answers if you need to. If you answered all the questions correctly, you can skip this step. Revise means to change, fix, and add extra notes if you need to. This quiz is NOT FOR MARKS, so it is perfectly OK to correct any mistakes you made. This will make your self-check quiz an excellent study tool you can use later.

  5. SAVE your quiz to a folder on your computer, or to your Private Files. That way you will know where it is for later studying.

  6. CHECK with your teacher if you need to. If after completing all these steps you are still not sure about the questions or your answers, you should ask for more feedback from your teacher. To do this, post in the Course Questions Forum, or send your teacher an email. In either case, attach your completed quiz and ask; "Can you look at this quiz and give me some feedback please?" They will be happy to help you!

Be a Self-Check

Superhero!



Self-Check Quiz Answers

Click each of the suggested answers below, and carefully compare your answers to the suggested answers.

If you have not done the quiz yet – STOP – and go back to step 1 above. Do not look at the answers without first trying the questions.

 
Olive oil would become more viscous if you put it into the fridge. The lower temperature of the fridge would cause the olive oil particles to move and flow slower.
Because the honey doesn’t flow very fast, it has a high viscosity. To make the honey flow faster, you could increase its temperature by running the bottle under hot water.
Shampoo is a viscous fluid, which means it flows slowly. If all the shampoo is at the bottom of the bottle, it takes a long time for it to flow to the opening, making it difficult to get out of the bottle. Storing the shampoo upside down provides more time for the shampoo to flow to the opening. When you open the bottle, the shampoo doesn’t have far to travel, so it flows out right away.
At low temperatures, motor oil becomes more viscous. When motor oil flows slowly over engine parts, the engine parts have trouble moving. This makes the car hard to start.
Viscous cough syrup flows slowly. It travels down the throat slowly, which coats the throat and soothes it for a longer period of time.