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Viscosity and Density

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Viscosity

Viscosity is the resistance to flow of a liquid. Think how easy it is to pour water compared to syrup or honey. The water flows much faster than the syrup or honey.

Definition: Viscosity

Viscosity is a measure of how much a liquid resists flowing. i.e. The higher the viscosity, the more viscous a substance is.
Viscosity and Density

Pouring water versus pouring syrup.

You can see this if you take a cylinder filled with water and a cylinder filled with glycerol (propane-1,2,3-triol). Drop a small metal ball into each cylinder and note how easy it is for the ball to fall to the bottom (see figure below). In the glycerol the ball falls slowly, while in the water it falls faster.

Viscosity and Density

The higher the viscosity (red) the slower the ball moves through the liquid.

As implied by the definition, substances with stronger intermolecular forces are more viscous than substances with weaker intermolecular forces. The stronger the intermolecular forces the more the substance will resist flowing. The greater the internal friction the more a substance will slow down an object moving through it.

If the glass sheet is tilted to either side during the resistance to flow activity the drops will run into each other and the activity will not work. The drops should reach the finish line in the order of: alcohol (least viscous), water, oil, syrup (most viscous).

Optional Activity: Resistance to flow

Take a sheet of glass at least \(\text{10}\) by \(\text{15}\) \(\text{cm}\) in size. Using a water-proof marker draw a straight line across the width of the glass about \(\text{2}\) \(\text{cm}\) from each end.

Place the glass flat on top of two pencils, then carefully put a drop of water on one end of the line. Leave at least \(\text{2}\) \(\text{cm}\) space next to the water drop, and carefully place a drop of alcohol. Repeat this with a drop of oil and a drop of syrup.

Slowly and carefully remove the pencil from the end opposite the drops (make sure you don’t tilt the glass to either side in the process).

Viscosity and Density

  • Which drop moves fastest and reaches the end line first?

  • Which drop moves slowest?

The fastest moving substance has the least resistance to flow, and therefore has the least viscosity (is the least viscous). The slowest moving substance has the most resistance to flow, and therefore has the most viscosity (is the most viscous).

Density

Definition: Density

Density is a measure of the mass per unit of volume.

The solid phase is often the most dense phase (water is one noteworthy exception to this). This can be explained by the strong intermolecular forces found in a solid. These forces pull the molecules together, which results in more molecules in one unit of volume than in the liquid or gas phases. The more molecules in a unit volume the denser, and heavier that volume of the substance will be.

Density can be used to separate different liquids, with the more dense liquid settling to the bottom of the container, while the less dense liquid floats on top. If you throw a leaf into a river or pond the leaf will float. If you instead throw a rock (with the same surface area and volume) into the river or pond the rock will sink. This is due to the different densities of the two substances: rocks are more dense than water while leaves are less dense than water.

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