Short Answer

Temperature affects viscosity by changing the internal motion of molecules in a fluid. For liquids, when temperature increases, viscosity decreases because the molecules move faster and slide more easily. When temperature decreases, viscosity increases and the liquid becomes thicker.

For gases, the effect is opposite. When temperature increases, viscosity increases because gas molecules move faster and collide more often. When temperature decreases, viscosity decreases. Thus, temperature plays a major role in determining how easily a fluid flows.

Detailed Explanation :

Temperature affects viscosity

Viscosity is the property of a fluid that tells us how easily it can flow. It depends on the internal friction between layers of the fluid. Temperature has a very strong influence on viscosity because it changes the energy and movement of the molecules inside the fluid. Both liquids and gases show clear changes in viscosity when temperature changes, but they behave in opposite ways.

Understanding how temperature affects viscosity is useful in daily life, industry, engineering, medicine, and natural processes. It helps us predict how fluids will behave under different conditions and why their flow changes with heating or cooling.

Effect of temperature on liquids

In liquids, molecules are closely packed. They attract each other with moderate forces, and viscosity mainly arises from these attractions. Temperature changes this attraction.

1. When temperature increases → viscosity decreases

When a liquid is heated:

• Its molecules gain more energy.
• They start moving faster.
• The attraction between the molecules becomes weaker.
• The layers of the liquid slide more easily over each other.

As a result, the liquid becomes thinner and its viscosity decreases.

Examples:

• Warm honey flows easily compared to cold honey.
• Hot oil spreads more smoothly than cold oil.
• Heated syrup becomes less sticky.

This is why heating is used in industries to pump or mix liquids more easily.

2. When temperature decreases → viscosity increases

When a liquid is cooled:

• Molecules lose energy and move slowly.
• They attract each other more strongly.
• Movement becomes difficult.

So, the liquid becomes thicker, and viscosity increases.

Examples:

• Engine oil becomes thick in winter.
• Coconut oil solidifies in cold weather.
• Shampoo becomes thicker in winter.

Thus, temperature and viscosity in liquids are inversely related.

Effect of temperature on gases

Gases behave differently from liquids because their molecules are far apart and have very weak attraction. Viscosity in gases is mainly caused by collisions between molecules.

1. When temperature increases → viscosity increases

When a gas is heated:

• Molecules move faster.
• They collide more frequently.
• More collisions create more internal friction.

Therefore, viscosity of gases increases with temperature.

Examples:

• Hot air has slightly higher viscosity than cold air.
• Gas flow becomes slower through narrow gaps at higher temperatures.

This increase is the opposite of what happens in liquids.

2. When temperature decreases → viscosity decreases

Cooling a gas:

• Slows down its molecules
• Reduces the number of collisions
• Causes the viscosity to decrease

Thus, viscosity of gases decreases with temperature.

Why liquids and gases behave differently

The reason for their opposite behaviour is based on molecular structure:

• Liquids: Their viscosity comes from attraction between molecules. Heating weakens these attractions → viscosity decreases.
• Gases: Their viscosity comes from collisions between molecules. Heating increases collisions → viscosity increases.

Therefore, the effect of temperature on viscosity is opposite for liquids and gases.

Practical examples in daily life

1. Cooking oils become thin when heated
   This makes frying easier.
2. Cold cream becomes thick in winter
   Due to increase in viscosity.
3. Hot tar flows quickly on roads in summer
   Because its viscosity drops.
4. Car engines need low-viscosity oil in winter
   To ensure smooth starting.
5. Airflow changes with temperature
   Hot air becomes more viscous and affects aerodynamics.
6. Honey and ghee flow differently in summer and winter
   Because of temperature differences.

Applications of temperature-viscosity relationship

Understanding this relationship is important in:

• Food industry (syrups, oils, chocolates)
• Petroleum industry (crude oil transportation)
• Lubrication of machines
• Chemical processing
• Paint and cosmetic production
• Medical science (blood viscosity and temperature changes)

Engineers adjust temperature to control the flow of fluids during manufacturing.

Graphical idea (no graph needed)

If we imagine a simple graph:

• For liquids: viscosity goes down as temperature goes up
• For gases: viscosity goes up as temperature goes up

This idea shows the opposite behaviour clearly.

Conclusion

Temperature strongly affects the viscosity of fluids. In liquids, viscosity decreases as temperature increases because molecules move more freely. In gases, viscosity increases with temperature because faster-moving molecules collide more often. This difference arises from the molecular structure and behaviour of liquids and gases. Understanding how temperature affects viscosity is important in daily life, industry, engineering, and natural processes, as it helps control and predict the flow of various fluids.