To convert $3.5$ litres to millilitres, you simply multiply $3.5$ by $1,000$, resulting in $3,500$ millilitres.

To elaborate, the conversion between litres and millilitres is grounded in the metric system, which defines $1$ litre as equivalent to $1,000$ millilitres. This conversion is quite straightforward due to the systematic nature of the metric system, which is designed for ease of use, particularly for calculations involving multiples of ten.

To perform the conversion, take the number of litres and multiply it by $1,000$. In this case, starting with $3.5$ litres, you compute:

Thus, $3.5$ litres is equal to $3,500$ millilitres.

This conversion method is applicable to any quantity of litres. For instance, if you have $2$ litres, you would calculate:

resulting in $2,000$ millilitres. Similarly, for $0.75$ litres, the calculation would be:

yielding $750$ millilitres.

Understanding how to convert between litres and millilitres is beneficial in numerous real-life scenarios, including cooking, conducting science experiments, and interpreting product labels. This knowledge enables you to switch effortlessly between different units of measurement, making it easier to compare quantities and carry out calculations.