What Is the Density of Urea in Solid and Solution?

Solid urea has a density of about 1.32 g/cm³ at room temperature, making it roughly a third denser than water. When dissolved in water, the density of the resulting solution climbs steadily with concentration, ranging from just above 1.00 g/cm³ for dilute mixtures up to around 1.15 g/cm³ near saturation at room temperature. These values shift with temperature, pressure, and what else is dissolved alongside the urea, which is why the question matters to everyone from fertilizer manufacturers to automotive engineers working with diesel exhaust fluid.

Density of Solid Urea

At ambient conditions, crystalline urea takes a tetragonal crystal structure known as Form I. In this arrangement, molecules pack into a well-ordered lattice stabilized by an extensive network of hydrogen bonds between neighboring amine and carbonyl groups. The result is a white, odorless solid with a density near 1.32 g/cm³ and a melting point around 133 °C. That density is modest compared to many inorganic salts but higher than most simple organic crystals, largely because urea molecules are small and pack efficiently.

Push the pressure high enough and the crystal rearranges. Quantum molecular dynamics simulations have shown that urea undergoes a polymorphic phase transition under high pressure, shifting from Form I to Form IV, which becomes the more stable arrangement at around 3.10 GPa. The two forms crystallize in different crystal systems with significantly different unit cell dimensions, meaning the density of the high-pressure form is higher than the familiar ambient-pressure crystal.¹ For most practical purposes, though, the 1.32 g/cm³ figure for Form I at room temperature and atmospheric pressure is the number you need.

How Concentration Changes Solution Density

Dissolve urea in water and the solution becomes denser than pure water. The relationship is close to linear at moderate concentrations: for every additional percentage of urea you dissolve, the density ticks up by a small, predictable amount. A 10% urea solution at 25 °C sits near 1.03 g/cm³, a 20% solution near 1.05 g/cm³, and a 32.5% solution, the concentration used in diesel exhaust fluid (commonly sold as AdBlue), comes in around 1.09 g/cm³. Near saturation, which at 25 °C is roughly 50–54% by weight, density approaches 1.15 g/cm³.

Detailed measurements across a wide temperature and pressure range confirm this near-linear behavior. Researchers measuring aqueous urea from 278 to 333 K and pressures up to 100 MPa found that the apparent molar volume of urea increases almost linearly with molality across the full range of conditions studied.² In practical terms, that means the density of the solution changes smoothly and predictably as you add more urea, with no sudden jumps or unusual kinks. Separate densimetric studies at temperatures from about 15 °C to 35 °C confirm this picture for the moderate concentrations most people encounter.³

Temperature and Pressure Effects

Warming a urea solution lowers its density, just as it does for most aqueous mixtures. At a fixed concentration, raising the temperature from about 5 °C to 60 °C can drop the density by a few hundredths of a gram per cubic centimeter. That sounds small, but for precision applications like pharmaceutical formulation or calibration of analytical instruments, the difference matters. A 32.5% urea solution that reads 1.090 g/cm³ at 20 °C will read a bit lower at 30 °C and higher if cooled to 10 °C.

Pressure pushes the density in the opposite direction. Compressing a urea solution to tens of megapascals squeezes the molecules closer together, raising the measured density. The volumetric data collected at pressures up to 100 MPa show that these changes are smooth and well-behaved, which is useful for engineers modeling urea behavior in high-pressure industrial systems like fertilizer reactors.² Thermodynamic modeling tools, including equation-of-state approaches, can accurately reproduce these experimental densities across wide temperature and pressure ranges, which makes it straightforward to look up or calculate the density for a specific set of conditions you care about.⁴

Why These Numbers Matter in Practice

Knowing the density of urea in solid form and in solution is not just academic bookkeeping. Several industries rely on these values daily.

Diesel exhaust fluid: The 32.5% urea solution used in selective catalytic reduction systems to cut nitrogen oxide emissions from diesel engines has a tightly specified density. Manufacturers and distributors use density as a quick quality-control check to verify the concentration is correct.
Fertilizer production: Urea is the most widely used nitrogen fertilizer in the world. Handling it in bulk, whether in storage silos, during transport, or in dissolution tanks, requires knowing how dense the product is and how its solutions behave at different temperatures.
Pharmaceutical and laboratory work: Urea solutions serve as protein denaturants, osmolytes in cell biology experiments, and components of skin-care products. Getting the concentration right means knowing the density-concentration relationship for the temperature at hand.
Clinical diagnostics: Urea is the dominant solute in urine by weight, and urine density is a routine clinical measurement.

Prilled Versus Granulated Urea

When you buy urea as a fertilizer or industrial chemical, it typically arrives in one of two physical forms: prills (small, roughly spherical beads formed by spraying molten urea into a cooling tower) or granules (larger, slightly irregular particles formed by layering urea onto seed particles in a rotating drum or fluidized bed). Both are the same chemical compound with the same crystallographic density, but their bulk behavior differs because of how the particles pack together.

Prills tend to be more uniform in shape and size, which lets them stack into a denser packed bed with smaller voids between particles. Granules, being less uniformly shaped, leave larger gaps. Comparative analysis of physical properties shows that prilled samples exhibit higher porosity in a packed bed and tighter packing with more contact points between particles, which promotes crystal bridge formation and caking during storage.⁵ From a density standpoint, the true particle density of both forms hovers around the same 1.32 g/cm³, but the bulk density you measure by pouring either form into a container will differ. Granules typically give a lower bulk density (more air space) while prills pack tighter. If you are sizing a storage bin or calculating how much product fits in a railcar, bulk density is what you care about, not crystallographic density.

Urea’s Role in Urine Density

Urine specific gravity, the ratio of urine density to the density of pure water, is one of the simplest and oldest clinical measurements in medicine. A normal urine specific gravity ranges from about 1.003 to 1.030, depending on hydration status. Urea is the single largest contributor to that density, accounting for roughly 73% of the total solute weight in urine. The remaining fraction comes from a mix of salts and metabolites: chloride makes up about 5.4%, sodium about 5.1%, potassium about 2.4%, phosphate about 2.0%, uric acid about 1.7%, and sulfate about 1.3%.⁶

Because urea dominates the solute composition so heavily, conditions that alter urea excretion can shift urine density noticeably. High protein intake raises urea production and tends to increase urine specific gravity, while kidney diseases that impair the ability to concentrate urine push it closer to the density of water. In clinical practice, a refractometer or dipstick measurement of urine specific gravity is really, in large part, a rough proxy for how much urea the kidneys are excreting.

Urea in Non-Aqueous Solvents

Urea does not dissolve only in water. One of the more commercially interesting non-aqueous contexts is the deep eutectic solvent formed by mixing choline chloride with urea in a 1:2 molar ratio, a mixture commonly called reline. This liquid has attracted attention as a greener alternative to conventional organic solvents for applications like metal electrodeposition, biomass processing, and drug delivery.

Reline is denser than most organic solvents, with a density around 1.20 g/cm³ near room temperature. Like aqueous urea solutions, its density decreases as temperature rises, following a smooth curve that has been measured from about 20 °C to 90 °C. Adding water to reline drops the density progressively, transitioning smoothly from the pure deep eutectic solvent value toward a range closer to concentrated aqueous urea.⁷ The viscosity of reline is much higher than water at the same temperature, which is one of the practical challenges of working with it, but the density data at least behave in a straightforward way.

When Salt Is Also Present

Real-world urea solutions rarely contain urea alone. Fertilizer solutions often dissolve urea alongside ammonium nitrate or other salts. Biological fluids like urine mix urea with sodium chloride, potassium, and a host of organic metabolites. In industrial settings, urea may be combined with sodium or calcium salts in process streams.

Adding salt to an aqueous urea solution raises the density beyond what the urea alone would produce, but the interaction is not always a simple sum of the individual effects. Thermodynamic models designed for these multi-solute mixtures have been shown to accurately predict experimental densities, along with other solution properties, for systems containing urea plus various inorganic salts.⁸ What this means practically is that if you need to know the density of a urea solution that also contains salt, you cannot simply look it up on a chart for pure aqueous urea. You either need data specific to that mixed system or a reliable model that accounts for the interactions between urea, salt, and water.

Does Urea Change the Structure of Water?

There is a long-running debate about whether urea disrupts the hydrogen bond network that gives liquid water its distinctive properties. Early thinking classified urea as a “structure breaker” because it is such an effective protein denaturant, and the assumption was that it worked by tearing apart water’s organized hydrogen bonding. If that were true, it could change how we interpret density data: a major structural rearrangement of the solvent would affect volumetric properties in complicated ways.

The current evidence, though, points strongly in the other direction. Molecular dynamics simulations using a nearest-neighbor approach to analyze water structure have shown that urea does not break the local tetrahedral structure of water, even at high concentrations.⁹ Vibrational spectroscopy experiments back this up: the absorption spectra of water are remarkably insensitive to urea concentration, consistent with the idea that urea only very weakly perturbs water structure.¹⁰ Urea does slow down the rotational dynamics of nearby water molecules slightly, but it integrates into the hydrogen bond network rather than destroying it.

This is good news for anyone interpreting density data. Because urea fits into water’s structure without drastically rearranging it, the density of urea solutions behaves in a regular, predictable way. There are no anomalous density jumps at particular concentrations that would signal a sudden structural reorganization. The smooth, nearly linear relationship between urea concentration and solution density holds precisely because urea is a gentle guest in the water lattice, not a disruptive one.

Measuring Urea Solution Density Accurately

If you need to measure the density of a urea solution yourself, the standard laboratory tool is a vibrating-tube densimeter. These instruments determine density by measuring the oscillation frequency of a small U-shaped tube filled with the sample. Modern versions can resolve density differences of a few parts per million, which is more than enough for any practical urea application.

For field work or quality control on a production line, hydrometers and handheld digital density meters are common. AdBlue quality checks, for example, often rely on a refractometer or a simple hydrometer calibrated for the expected density range. Temperature control or correction is important in all cases. Since the density of a urea solution shifts with temperature by roughly 0.0004 to 0.0005 g/cm³ per degree Celsius, a measurement taken at 30 °C will read lower than one taken at 20 °C for the same solution. Most instruments either control temperature automatically or apply a correction factor.

When comparing your measurements against published reference data, pay attention to whether the values are reported at atmospheric pressure or at elevated pressures, and whether the concentration is expressed as weight percent, molality, or molar concentration. A “10% urea solution” by weight is a different thing from a “10 mol% urea solution,” and mixing up concentration units is one of the most common sources of confusion when looking up density data.

Urea Density at Extreme Conditions

Most day-to-day applications involve urea at atmospheric pressure and temperatures between about 0 °C and 60 °C. But certain industrial processes push into more extreme territory. In high-pressure fertilizer reactors, urea solutions may experience pressures of tens or even hundreds of megapascals. The density data collected up to 100 MPa confirm that urea solutions compress in a well-behaved way, growing denser as pressure increases without any phase transitions or anomalous behavior in the liquid state.²

The solid, on the other hand, does undergo a structural change under extreme compression. As noted earlier, Form I transforms to Form IV near 3.10 GPa, a pressure far beyond anything encountered in normal manufacturing but relevant to fundamental materials science and planetary chemistry. Researchers studying how simple organic molecules behave deep inside icy planetary bodies or under shock conditions need to know which crystal polymorph is stable and what its density is at those pressures.¹ For those purposes, the ambient-condition density of 1.32 g/cm³ is just the starting point on a compressibility curve that stretches into much denser, structurally distinct territory.

1
PubMed Central. Can We Predict the Pressure Induced Phase Transition of Urea? Application of Quantum Molecular Dynamics
2
The Journal of Chemical Thermodynamics. Density and volumetric properties of the aqueous solutions of urea at temperatures from T = (278 to 333) K and pressures up to 100 MPa
3
The Journal of Chemical Thermodynamics. Densimetric and ultrasonic characterization of urea and its derivatives in water
4
PubMed. Structure and thermodynamics of aqueous urea solutions from ambient to kilobar pressures: From thermodynamic modeling, experiments, and first principles simulations to an accurate force field description
5
Chemical Papers. Comparative analysis of physical and thermal properties of prilled and granulated urea
6
PubMed. Relative density of urine: methods and clinical significance
7
Journal of Chemical & Engineering Data. Densities and Viscosities of (Choline Chloride + Urea) Deep Eutectic Solvent and Its Aqueous Mixtures in the Temperature Range 293.15 K to 363.15 K
8
Fluid Phase Equilibria. Thermodynamic properties of aqueous salt containing urea solutions
9
PubMed. Molecular dynamics simulation of aqueous urea solution: is urea a structure breaker?
10
PubMed Central. Structure and dynamics of urea/water mixtures investigated by vibrational spectroscopy and molecular dynamics simulation