Viscosity Measurement Discussion Examples | Relationship Between Temperature Changes and Intermolecular Interactions

What Is Viscosity?

Viscosity is a physical quantity that represents the resistance of a liquid to flow.
Liquids with high viscosity flow with difficulty, while liquids with low viscosity flow more easily.
For example, glycerin and oil flow more slowly than water because they have higher viscosity.

When a liquid flows, molecules within the liquid move relative to one another.
At this time, attractive forces between molecules, molecular entanglement, molecular size, and similar factors act as resistance.
This internal resistance to flow appears as viscosity.

Main Items to Include in the Results

In the results of viscosity measurements, organize the measurement temperature, flow time, density, relative viscosity, specific viscosity, and calculated viscosity values.
When measurements are performed at different temperatures, organizing the relationship between temperature and viscosity in a table or graph makes the discussion easier.

Main Items to Include in the Results

• Name of the sample used
• Type of viscometer used
• Measurement temperature
• Equilibration time
• Flow time
• Mean value when multiple measurements were performed
• Density
• Flow time of the reference liquid
• Relative viscosity
• Viscosity or kinematic viscosity
• Temperature-viscosity graph
• Comparison with literature values
• Error rate
• Presence or absence of bubbles or contamination during measurement

Reference Experimental Values and Analysis Examples for Viscosity Measurement

Here, reference experimental values are organized for measuring liquid viscosity and discussing its relationship with temperature, concentration, and intermolecular interactions.
Measurements using flow time with an Ostwald viscometer and similar devices, relative viscosity, viscosity coefficients, temperature dependence, and differences among types of liquids are covered.

Viscosity is a physical property representing how difficult it is for a liquid to flow.
In liquids with strong intermolecular interactions or large molecules, the molecules are less able to move relative to one another, so viscosity becomes higher.
In addition, as temperature rises, molecular motion becomes more active and liquid molecules can slide past one another more easily, so the viscosity of many liquids decreases.

Reference Experimental Conditions

Basic Concept of Viscosity

Liquids with high viscosity flow with difficulty, while liquids with low viscosity flow easily.
For example, water is relatively fluid, whereas glycerin and concentrated sugar solutions flow slowly.

Example Measurement Using an Ostwald Viscometer

In an Ostwald viscometer, the time required for a fixed volume of liquid to flow between two marked lines is measured.
When the same viscometer is used, the viscosity of a sample liquid can be determined from the ratio of density and flow time relative to water.

ηx = ηw × (ρx tx) ÷ (ρw tw)

Here, η represents viscosity, ρ represents density, t represents flow time, w represents water, and x represents the sample.

Liquids with longer flow times tend to have higher viscosity.
However, when calculating viscosity, not only flow time but also density must be taken into account.

Example Calculation of Viscosity

Let the viscosity of water be 0.890 mPa·s, the density of water 0.997 g/mL, and the flow time of water 60.0 s.
If the density of a 20% aqueous glycerin solution is 1.050 g/mL and its flow time is 118.0 s,

ηx = 0.890 × (1.050 × 118.0) ÷ (0.997 × 60.0)

ηx = 0.890 × 123.9 ÷ 59.8 = 1.84 mPa·s

Therefore, the viscosity of the 20% aqueous glycerin solution is determined to be approximately 1.84 mPa·s.

Example Calculation of Relative Viscosity

Relative viscosity represents how viscous a sample is compared with a reference liquid.

Relative viscosity = Viscosity of sample ÷ Viscosity of reference liquid

Relative viscosity makes it easier to intuitively compare how difficult different samples are to flow.

Changes in the Viscosity of Water With Temperature

The viscosity of a liquid is strongly affected by temperature.
In the case of water, viscosity decreases as the temperature increases.

As temperature increases, the thermal motion of water molecules becomes greater and the molecules can move relative to one another more easily, so viscosity decreases.

Glycerin Concentration and Viscosity in Aqueous Solution

Glycerin has multiple hydroxyl groups and forms strong hydrogen bonds with water.
Therefore, as the concentration increases, the viscosity of the aqueous solution becomes greater.

As the glycerin concentration increases, both flow time and viscosity increase greatly.
This is considered to occur because many hydrogen bonds are formed between glycerin molecules and between glycerin and water molecules, making molecular movement more difficult.

Sucrose Concentration and Viscosity in Aqueous Solution

Sucrose also has many hydroxyl groups and interacts with water, so viscosity increases as the concentration becomes higher.

In high-concentration sucrose solutions, many solute molecules are present and the free movement of water molecules is hindered, so viscosity increases greatly.

Differences in Viscosity Among Types of Liquids

Molecules with many hydroxyl groups readily form hydrogen bonds and tend to have high viscosity.
Glycerin shows extremely high viscosity because many hydrogen bonds make it difficult for the molecules to move relative to one another.

Example of Viewing Temperature Dependence as an Exponential Relationship

Liquid viscosity does not necessarily change linearly with temperature and may decrease rapidly as the temperature increases.
The temperature dependence of viscosity may also be treated in relation to the concept of activation energy.

lnη decreases as the temperature increases, showing that viscosity decreases greatly.
When discussing temperature dependence in detail, the relationship between lnη and 1/T may be used.

Example of Viscosity Measurement by the Falling-Ball Method

In the falling-ball method, viscosity is determined from the motion of a sphere falling at a constant velocity through a liquid.
The more viscous the liquid, the slower the sphere falls.

The slower the sphere falls, the greater the viscous resistance exerted by the liquid, indicating a liquid with higher viscosity.

Example of Variation in Measured Values

In viscosity measurements, temperature, bubbles, the cleaning condition of the viscometer, sample volume, and timing of readings affect the results.

If only the fourth flow time is long, possible causes include a bubble remaining inside the viscometer, misreading the time at which the liquid passed the marked line, or contamination or blockage inside the tube.

Sources of Error in Viscosity Measurement

Example of How to Write the Results

Using an Ostwald viscometer, the flow times of water, ethanol, and aqueous glycerin solutions were measured.
At 25°C, the flow time of water was 60.0 s, while that of the 20% aqueous glycerin solution was 118.0 s.
Using water as the reference and calculating from density and flow time, the viscosity of the 20% aqueous glycerin solution was 1.84 mPa·s.

As the glycerin concentration increased, the flow time and viscosity increased greatly.
This is because glycerin molecules contain multiple hydroxyl groups and therefore readily form hydrogen bonds with water molecules and with other glycerin molecules.
As the concentration increases, intermolecular interactions in the solution increase and molecular movement is hindered.
As a result, the liquid as a whole becomes more difficult to flow and the viscosity increases.

As the temperature increased, the viscosity of water decreased.
It was 1.31 mPa·s at 10°C but decreased to 0.466 mPa·s at 60°C.
This is considered to occur because rising temperature increases the thermal motion of molecules and allows them to move more easily by overcoming intermolecular interactions.

Points for Connecting the Results to the Discussion

In a discussion of viscosity measurement, it is important to explain not only differences in flow time but also density correction, intermolecular interactions, temperature, concentration, and measurement errors in relation to one another.

• Can you explain the tendency for viscosity to increase as flow time becomes longer?
• Can you take both density and flow time into account in calculations using a viscometer?
• Can you use relative viscosity to compare how difficult different samples are to flow?
• Can you explain why viscosity decreases as temperature rises in relation to thermal motion?
• Can you discuss why glycerin and sucrose solutions have high viscosity in relation to hydrogen bonding and molecular size?
• Can you explain differences in viscosity among types of liquids from molecular structure and intermolecular interactions?
• Can you explain that the temperature dependence of viscosity may not be linear?
• Can you discuss bubbles, contamination inside the tube, temperature changes, and reading errors as sources of error?
• Can you explain that highly viscous samples require longer measurement times and are more likely to produce outliers?

Example Discussion

In this experiment, the flow time of liquids was measured using an Ostwald viscometer, and viscosity was determined using water as the reference.
Compared with the flow time of water, the flow times of aqueous glycerin and sucrose solutions were longer.
This is because these solutions had higher viscosity than water, and viscous resistance made the liquids more difficult to flow.

In aqueous glycerin solutions, viscosity increased rapidly as the concentration increased.
Glycerin has multiple hydroxyl groups and therefore readily forms hydrogen bonds with water molecules and with other glycerin molecules.
As the concentration becomes higher, intermolecular interactions in the solution increase and molecular movement is hindered.
Therefore, the liquid as a whole becomes more difficult to flow and the viscosity is considered to have increased.

Regarding temperature dependence, viscosity decreased as the temperature increased.
In liquids, rising temperature increases molecular thermal motion and reduces the resistance caused by intermolecular interactions.
Therefore, molecules can slide past one another more easily and fluidity increases.
This indicates that temperature control is extremely important in viscosity measurements.

Looking at differences among types of liquids, glycerin showed much higher viscosity than water or ethanol.
This is because glycerin molecules have three hydroxyl groups and can form many hydrogen bonds.
In contrast, liquids such as acetone, which do not readily act as hydrogen-bond donors, tend to have lower viscosity.
Therefore, differences in viscosity are affected not only by molecular size but also by the type and strength of intermolecular interactions.

Possible sources of error include temperature changes, bubbles inside the viscometer, contamination inside the tube, delayed reading of the marked line, and errors in concentration preparation.
Viscosity is particularly sensitive to temperature, and if the temperature rises during measurement, the viscosity may be underestimated.
In addition, bubbles or contamination can hinder the flow and may lengthen the flow time.
Therefore, it is important to thoroughly clean the viscometer before measurement and perform multiple measurements only after the temperature has stabilized.

Summary

Viscosity represents the resistance of a liquid to flow and is affected by intermolecular interactions, molecular size, concentration, and temperature.
With an Ostwald viscometer, the viscosity of a sample relative to water can be determined from flow time and density.

This reference example covered flow time, relative viscosity, temperature dependence, aqueous glycerin solutions, aqueous sucrose solutions, differences among types of liquids, the falling-ball method, variation in measured values, and sources of error such as bubbles and temperature changes.
In a report, rather than simply comparing measured values, it is useful to discuss them in relation to intermolecular interactions and thermal motion.

Principle of Viscosity Measurement

In student experiments, an Ostwald viscometer, Ubbelohde viscometer, or similar device may be used to measure the time required for a liquid to flow through a fixed section.
When the same viscometer is used, a liquid with a longer flow time is considered more difficult to flow and to have higher viscosity.

In a capillary viscometer, the time required for a liquid to flow through a narrow tube is measured and compared with a reference liquid to determine relative viscosity.
Viscosity depends not only on flow time but also on the density of the liquid.
Therefore, accurate viscosity calculations take both flow time and density into account.

How to Determine Relative Viscosity

Relative viscosity is a value indicating how much greater the viscosity of a sample is than that of a reference liquid.
When water is used as the reference liquid, the relative viscosity is determined by comparing the flow time and density of water with those of the sample.

Relative viscosity = Viscosity of sample ÷ Viscosity of reference liquid

When measurements are performed using the same capillary viscometer, viscosity may be treated as proportional to the product of density and flow time.
Therefore, relative viscosity may be determined in the following form.

η / η₀ = ρt / ρ₀t₀

Here, η is the viscosity of the sample, η₀ is the viscosity of the reference liquid, ρ is the density of the sample, ρ₀ is the density of the reference liquid, t is the flow time of the sample, and t₀ is the flow time of the reference liquid.

Relationship Between Flow Time and Viscosity

In a capillary viscometer, a longer flow time indicates that the liquid has greater difficulty flowing through the capillary.
In other words, a sample with a longer flow time is considered to have higher viscosity.
However, comparing viscosity based only on flow time may ignore differences in density.

It is important to measure under the same temperature, using the same viscometer, and under the same operating conditions.
Because viscosity changes greatly with temperature, flow-time comparisons must be made under the same temperature conditions.

Effect of Temperature on Viscosity

In many liquids, viscosity decreases as temperature rises.
As the temperature increases, the thermal motion of molecules becomes more active and the constraints caused by intermolecular interactions become relatively weaker.
As a result, molecules can move relative to one another more easily and the liquid flows more readily.

Conversely, at low temperatures, molecular motion becomes smaller and the effects of intermolecular interactions become more pronounced, so viscosity increases.
Temperature control is extremely important in viscosity measurements.
Even a slight difference in temperature may change the viscosity.

Discussion of a Temperature-Viscosity Graph

When viscosity is measured at different temperatures, a graph with temperature on the horizontal axis and viscosity on the vertical axis makes it possible to visually examine the temperature dependence.
In many liquids, a curved relationship is observed in which viscosity decreases as temperature rises.

If the temperature-viscosity graph decreases smoothly, the reduction in viscosity with increasing temperature can be considered to have been observed consistently.
If some points deviate greatly, possible sources of error include inadequate temperature control, errors in reading the flow time, bubbles, and contamination of the viscometer.

Relationship Between Intermolecular Interactions and Viscosity

Viscosity is closely related to intermolecular interactions within a liquid.
The stronger the intermolecular interactions, the more strongly molecules attract one another and the more difficult it becomes for them to move relative to one another, so viscosity tends to increase.
Representative intermolecular interactions include dispersion forces, dipole-dipole interactions, and hydrogen bonding.

Liquids capable of forming hydrogen bonds may have particularly strong attractions between molecules and may therefore have high viscosity.
In addition, liquids with large molecular weight or long molecular shapes that readily become entangled also tend to have high viscosity.

Effect of Hydrogen Bonding on Viscosity

Hydrogen bonding is one type of intermolecular interaction that greatly affects viscosity.
Molecules containing hydroxyl groups, amino groups, and similar functional groups readily form hydrogen bonds and may attract one another strongly.
As a result, liquid flow may be hindered and viscosity may increase.

For example, molecules such as polyhydric alcohols that contain multiple hydroxyl groups readily form networks of hydrogen bonds and tend to have high viscosity.
As the temperature rises, the influence of hydrogen bonding weakens and viscosity decreases.

Relationship Between Molecular Weight and Viscosity

In general, liquids with higher molecular weight tend to have higher viscosity.
This is because larger molecules have greater contact area with one another and stronger dispersion forces tend to act between them.
In addition, if molecules have long chain-like structures, they may become entangled and flow less easily.

However, viscosity is not determined by molecular weight alone.
Molecular shape, polarity, the presence or absence of hydrogen bonding, temperature, and other factors also have major effects.
In a report, it is useful to discuss molecular weight together with intermolecular interactions.

Effect of Concentration on Viscosity

The viscosity of a solution is also affected by solute concentration.
In general, as solute concentration increases, the number of molecules or ions in the solution increases and resistance to flow becomes greater.
In polymer solutions, a higher concentration may greatly increase viscosity because molecular-chain entanglement becomes more extensive.

In experiments measuring viscosity at different concentrations, a concentration-viscosity graph is prepared and the change in viscosity with increasing concentration is discussed.
Errors in concentration preparation directly affect the viscosity results.

Effect of Density on Calculations

In relative-viscosity calculations using a capillary viscometer, density as well as flow time may be required.
If density is not measured correctly, the calculated viscosity will also deviate.
Because density also changes with temperature, it may be necessary to take the density at each temperature into account.

Error Caused by Temperature Control

Viscosity is a physical quantity that is extremely sensitive to temperature.
If the temperature is not constant during measurement, the flow time changes and large errors occur in the viscosity value.
Even when a thermostatic bath is used, correct viscosity cannot be obtained if measurement begins before the sample has fully reached the measurement temperature.

In addition, if the room temperature or thermostatic-bath temperature changes during measurement, flow times may vary even for the same sample.
In a report, inadequate temperature control can be discussed as a major source of error.

Error Caused by Bubbles

If bubbles enter the viscometer, the liquid flow may be disturbed and the actual flowing volume or flow velocity may change.
If a bubble is present in the capillary section, a large error may occur in measurement of the flow time.
In addition, bubbles may make it difficult to judge the moment when the liquid passes the marked line.

Error Caused by Contamination of the Viscometer

If contamination or residue from a previous sample remains on the inner wall of the viscometer, the way the liquid flows changes.
Contamination inside the capillary may narrow the flow path or alter interactions between the liquid and the wall surface, affecting the flow time.
It is important to thoroughly clean the viscometer before measurement and dry it when necessary.

Error in Reading the Flow Time

In a capillary viscometer, the time at which the liquid surface passes a marked line may be judged visually.
As with reaction-time measurements, an error may occur in the timing of starting and stopping the stopwatch.
Particularly for samples with short flow times, even a slight timing difference becomes a relatively large error.

Error Caused by Differences in Sample Volume

A viscometer may require an appropriate sample volume.
If the amount of sample is insufficient or excessive, the liquid level and pressure difference may change and affect the flow time.
When comparing measurements using the same viscometer, it is important to introduce the same amount of sample each time.

When the Measured Value Is Larger Than the Literature Value

If the experimentally determined viscosity is larger than the literature value, possible causes include a measurement temperature lower than the literature condition, contamination in the viscometer, flow being hindered by bubbles or foreign matter, or reading the flow time as too long.
Because viscosity tends to increase at lower temperatures, differences in temperature are particularly important.

When the Measured Value Is Smaller Than the Literature Value

If the experimentally determined viscosity is smaller than the literature value, possible causes include a higher measurement temperature, reading the flow time as too short, contamination of the sample with volatile components or low-viscosity impurities, or underestimating the density.
Because even a slight increase in temperature can reduce viscosity, confirmation of thermal equilibrium is important.

Calculation of Error Rate

When comparing experimentally determined viscosity with a literature value, the error rate can be calculated to express the difference quantitatively.
The error rate is obtained by dividing the difference between the experimental and literature values by the literature value and expressing it as a percentage.

Error rate (%) = |Experimental value − Literature value| ÷ Literature value × 100

After showing the error rate, discuss why the difference occurred in relation to temperature control, reading of the flow time, density measurement, bubbles, contamination of the viscometer, and similar factors.

When the Results Can Be Considered Good

Viscosity measurement results can be considered good when flow times measured multiple times under the same conditions agree well and theoretically reasonable trends, such as decreasing viscosity with increasing temperature, are observed.
It is also important that the results do not greatly contradict literature values and that temperature conditions and density correction are handled appropriately.

Example Discussion When the Experiment Did Not Go Well

If viscosity measurement does not go well, possible causes are considered from results such as variation in flow times, unnatural temperature dependence, large deviation from literature values, bubbles entering the viscometer, contamination of the viscometer, or unstable temperature.
Organizing temperature control, sample condition, viscometer cleaning, flow-time reading, and density measurement separately makes the discussion easier to write.

How to Write Points for Improvement

In a discussion of viscosity measurement, including points for improvement as well as sources of error makes the report easier to organize.
Improvements are easier to organize when divided into temperature control, viscometer handling, flow-time measurement, and sample preparation.

Improvements to Temperature Control

• Allow the temperature to stabilize sufficiently in a thermostatic bath
• Measure only after the sample has reached the measurement temperature
• Record the temperature during measurement
• Match the temperature conditions when comparing with literature values
• Measure quickly within a range where temperature changes are small

Improvements to Viscometer Operation

• Clean the viscometer thoroughly
• Do not leave residue from the previous sample or cleaning solution
• Introduce the sample without allowing bubbles to enter
• Measure using the same sample volume
• Keep the viscometer vertical

Improvements to Measurement and Calculation

• Measure the flow time multiple times and use the mean
• Keep the reading position at the marked line consistent
• Measure density accurately
• Take density at each temperature into account
• Check the causes of outliers
• Use consistent units in calculations

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of viscosity measurement, simply writing that “viscosity decreased as temperature increased” or “the flow time was long” results in a superficial discussion.
Relating temperature, molecular motion, intermolecular interactions, flow time, density, and sources of error produces a more persuasive discussion.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of viscosity measurements.
Adjust the necessary parts according to your own experimental results.

• Viscosity is a physical quantity representing the internal resistance of a liquid when it flows.
• The longer the flow time, the more difficult the liquid is to flow and the higher the viscosity is considered to be.
• As the temperature increased, molecular motion became more active and the resistance caused by intermolecular interactions decreased, so viscosity decreased.
• Liquids with stronger intermolecular interactions tend to have higher viscosity because molecular movement is hindered.
• In molecules capable of forming hydrogen bonds, strong intermolecular attraction may result in high viscosity.
• As molecular weight increases, dispersion forces and molecular entanglement increase, and viscosity tends to become higher.
• Because viscosity is sensitive to temperature, thermal equilibrium must be maintained during measurement.
• Bubbles inside the viscometer disturb liquid flow and lead to errors in flow-time measurement.
• Contamination of the viscometer or residue from the previous sample can change the flow time.
• Errors in density affect the calculated relative viscosity and viscosity values.

Points to Check When Discussing Viscosity Measurements

Checking the following points before writing the report makes the discussion easier to write.

• Have you stated the measurement temperature?
• Have you measured the flow time multiple times?
• Have you used the mean flow time?
• Have you compared the reference liquid and sample under the same conditions?
• Have you taken density into account in the calculation?
• Have you explained the decrease in viscosity with increasing temperature in terms of molecular motion?
• Have you related intermolecular interactions and hydrogen bonding to viscosity?
• Have you considered bubbles and contamination of the viscometer as sources of error?
• Have you considered inadequate temperature control?
• Have you considered errors in reading the flow time?
• Have you compared the result with literature values?
• Do the points for improvement correspond to the sources of error?

Summary

Viscosity is a physical quantity representing the internal resistance of a liquid when it flows.
In experiments using a capillary viscometer, the time required for a liquid to flow through a fixed section is measured and compared with a reference liquid to determine viscosity or relative viscosity.
The longer the flow time, the more difficult the liquid is to flow and the higher the viscosity is considered to be.

In many liquids, viscosity decreases as temperature rises.
This is because increasing temperature increases the thermal motion of molecules and reduces the resistance to flow caused by intermolecular interactions.
Hydrogen bonding, dipole-dipole interactions, dispersion forces, molecular weight, and molecular shape also affect viscosity.

In a report, do not simply write that “viscosity decreased as temperature increased.”
Discuss molecular motion, intermolecular interactions, flow time, density, temperature control, bubbles, and contamination of the viscometer in relation to one another.
Because viscosity is extremely sensitive to temperature, it is important to maintain constant-temperature conditions and confirm reproducibility through multiple measurements.