Flow Rate and Viscosity

Students explore viscosity and rate of flow of fluids.


In this lesson we will be exploring how to separate mixtures created by the Earth. Previous information from lessons 1 and 2 will be incorporated into this lesson to help push and connect your understanding. In many cases when separating mixtures we take advantage of the different properties of the elements, for example, in a salt-water solution we can evaporate the water to extract the salt because the boiling point of water is far lower than the boiling point of salt.

Viscosity and Flow Rate

Viscosity is the term used to describe the thickness of a fluid, remember that fluids include gas and liquids. When measuring viscosity we often observe the fluid flow rate. Flow rate is how fast a fluid moves from one point to another. Measuring flow rate of gases can be a little difficult, for now we will focus on liquids.

The Importance of Viscosity

Viscosity can be a very important property of everyday products. Products like lipstick, nail polish, salad dressings, and lubricating oils all require specific viscosities to meet consumer demand or perform as needed.

Lipstick is composed of multiple substances, different colours will have slightly different formulas. Yet, the final product of most lipsticks are all applied the same way. This is because lipstick is engineered to a specific viscosity to allow for easy application, yet lasting power Рa term used to determine how long the lipstick will stay on someone’s lips. Other beauty products have similar engineering processes. Think of nail polish or mascara, the application process of both products is very important to their consumers.

Another industry that values the property of viscosity is the food industry. Your mouth is incredibly sensitive, and it can determine consistency or thickness of the foods you consume. In many cases you may not lick a food because it feels weird in your mouth. This feel is the viscosity of the food. Think of salad dressings, sauces, soups, gravies, ice creams, and shakes. The viscosity of all of these foods are very important.

Friction, Viscosity, Liquids

One way of viewing viscosity is the resistance to flow. Think of the particles of the liquid as it flows. The thicker the liquid (higher viscosity) the stronger the bonds between the particles would be, and they would need to exert more force to flow by each other. Likewise, a thinner liquid (lower viscosity) would have weaker bonds between the particles and individual particles would move more freely. This form of internal friction determines how fast the liquid flows.

One interesting effect of this process is that as you add heat the bonds between particles weaken and the viscosity decreases because it can move more freely. The reverse is true, as you cool down a liquid the viscosity increases because the bonds between particles become stronger. Therefore, the viscosity of a liquid DECREASES as it is HEATED, and INCREASES as it is COOLED.

Viscosity of Gases

Gases are a fluid as well and they too are affected by the properties of viscosity. One thing to remember about gases is that they have very weak bonds between particles and individual particles can easily flow between other particles. One of the limiting factors of flow for gases is the tendency for particles to collide with each other. These colliding particles slow down the flow of particles and thus decrease the viscosity of the gas. What is interesting about gases, opposed to liquids, is that as you heat up a gas’s particles they become more active and have more collisions. Likewise as you cool down a gas its particles slow down and have less collisions. Therefore, the viscosity of a gas INCREASES as it is HEATED, and DECREASES as it is COOLED. Temperature has the opposite effect on the viscosity of gases as on liquids.

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