No, there is no specific chemical rule or principle that dictates pouring a heavier liquid into a lighter one. The choice of pouring one liquid into another can depend on various factors, including the specific application or desired outcome.
When two immiscible liquids with different densities are poured together, such as oil and water, the liquid with higher density (heavier) will tend to settle at the bottom while the one with lower density (lighter) will float on top. This behavior is due to the effect of gravity, where denser substances tend to sink below less dense substances.
However, there are situations where it may be desirable to pour a lighter liquid into a denser one. For example, in the process of layering cocktails, bartenders often pour liquids with different densities in a specific order to create distinct layers in the glass. This technique relies on understanding the relative densities of the liquids and carefully pouring them to achieve the desired visual effect.
In general, the choice of which liquid to pour into another depends on the specific context, application, or desired outcome rather than any rigid chemical rule.