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How does raising temperature affect the viscosity of a liquid?

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differential manometer, Torricellian barometer, and siphon — fluid mechanics Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.

In the study of fluid mechanics, the viscosity of a liquid—which represents its internal friction or resistance to flow—decreases rapidly as its temperature increases.12

Because viscosity acts as an opposition to flow, this reduction means that liquids flow more easily when heated. This behavior is a critical consideration in practical applications such as lubrication, where engineers often use a "viscosity index" to measure how much a fluid's viscosity changes with temperature fluctuations; a higher index indicates greater stability, which is generally desirable for maintaining consistent performance.123

It is important to note that this relationship is specific to liquids. In contrast, the viscosity of gases actually increases as temperature rises, causing them to flow more sluggishly.12