Solution Density Mass Volume

Solution density: ρ = m/V

A solution's density ties its mass to its volume through ρ = m/V, and you'll usually see it written in g/mL or kg/L. Dissolve 120 g into 100 mL and you get ρ = 1.20 g/mL. A few reference points at 20 °C help calibrate your intuition: pure water sits at 1.000 g/mL, 0.9 % saline at 1.005, concentrated HCl 37 % around 1.19, concentrated H₂SO₄ 98 % around 1.84, and mercury way out at 13.55. Temperature shifts all of this, since liquids expand and contract — water actually hits its maximum of 0.99997 g/mL at 4 °C. To measure density for real you reach for a hydrometer/densimeter (a glass tube that floats), a pycnometer (a calibrated bottle), or one of the modern oscillating U-tube densimeters (Anton Paar). Brix and API gravity are just other scales built on top of density.

Applications

It turns up in analytical chemistry, where you make up reagent solutions by mass and then convert to volume. Home brewers track OG and FG readings to estimate how much alcohol they've produced. Reef keepers watch specific gravity, aiming near 1.025 for a marine tank. The petroleum industry leans on API gravity, pharmacists rely on it when compounding, and food technologists read sugar content off the Brix scale.

FAQ

What's the difference between density and specific gravity? Specific gravity compares a substance's density to water's at a reference temperature, so the ratio comes out dimensionless. Density itself carries units, like g/mL or kg/m³.

Does temperature matter? It does. Heat a liquid and it expands, which drops the density. So note the temperature whenever you report a value; lab work usually pins it to 20 °C or 25 °C.

Why is mass preferred over volume in formulations? Mass doesn't budge with temperature or pressure, while volume does. That's why Pharmacopeia and ANVISA standards tend to spell out quantities by mass — it's what makes a formula reproducible.