The gram (g) quantifies mass, which is the amount of matter in an object. Conversely, the milliliter (mL) measures volume, the amount of three-dimensional space an object occupies. Because these two units measure fundamentally different physical properties, there is no single, fixed conversion factor that applies to all substances. Therefore, one milliliter of a substance will not always have the same mass as one milliliter of another material.
The Critical Link: Density
Density is the concept that links the mass of a substance to its volume, representing how tightly packed the matter is within a specific space. It is a characteristic physical property of a substance, meaning that pure gold will always have the same density as another sample of pure gold. The relationship is mathematically expressed by the formula: Density equals mass divided by volume (\(D = m/V\)). This formula reveals that density serves as the conversion factor between grams and milliliters for any given material. The density value, typically measured in grams per milliliter (\(\text{g/mL}\)), indicates the mass of a single milliliter of that substance.
The Standard Case: Water
The reason many people assume a simple conversion exists is due to the unique properties of water, the standard reference point for the metric system. For pure water, one milliliter is almost exactly equal to one gram, a convenient relationship resulting from how metric units were originally defined. The maximum density of pure water is approximately \(1.00 \text{ g/mL}\), occurring specifically at \(4.0\) degrees Celsius. For practical purposes, the conversion of \(1 \text{ mL}\) of water to \(1 \text{ g}\) is accurate enough. This \(1:1\) ratio applies only to water under specific conditions; any other liquid requires a different calculation.
Calculating Conversions for Other Substances
To accurately convert milliliters to grams for any substance other than pure water, you must know its density. By rearranging the density formula, you can calculate the mass by multiplying the substance’s density by its volume: Mass equals Density times Volume (\(m = D \times V\)). This calculation confirms that a milliliter of one substance will weigh differently than a milliliter of another.
For instance, ethyl alcohol has a density of about \(0.79 \text{ g/mL}\). This means that \(100 \text{ mL}\) of alcohol would only have a mass of \(79 \text{ grams}\) (\(100 \text{ mL} \times 0.79 \text{ g/mL}\)). Conversely, milk has a slightly higher density than water, typically ranging from \(1.028 \text{ g/mL}\) to \(1.035 \text{ g/mL}\) depending on its fat content. A \(100 \text{ mL}\) serving of milk would therefore weigh slightly more than \(100 \text{ grams}\).
Using the formula, if you have \(50 \text{ mL}\) of olive oil, which has a density of approximately \(0.9 \text{ g/mL}\), the calculation is \(50 \text{ mL} \times 0.9 \text{ g/mL}\), resulting in a mass of \(45 \text{ grams}\). A much denser substance, like liquid mercury, has an extreme density of about \(13.55 \text{ g/mL}\). Therefore, \(50 \text{ mL}\) of mercury would weigh \(677.5 \text{ grams}\), a mass nearly 15 times greater than the same volume of olive oil.
To convert a volume in milliliters to a mass in grams, first locate the density of the specific substance in a reliable reference table. Then, simply multiply the volume you have by the substance’s density value. This simple procedure is the only way to accurately convert a volume measurement to a mass measurement for any material.