Water may look simple, but its molecular structure gives it a remarkable combination of physical and chemical properties. Surface tension, heat capacity, density, polarity, conductivity, pH, hardness, and its ability to dissolve other substances all help explain why water behaves the way it does in our bodies, ecosystems, plumbing systems, oceans, lakes, and atmosphere.
Water covers about 71% of Earth's surface, while most of Earth's total water is stored in the oceans. Only a small percentage is freshwater, and an even smaller amount is easily accessible in lakes, rivers, and shallow groundwater.
Quick answer: Water is H₂O—two hydrogen atoms bonded to one oxygen atom. Because the molecule is polar and can form hydrogen bonds, water has unusually strong cohesion, high surface tension, high heat capacity, strong solvent properties, and unusual density behavior. These characteristics help make water essential to Earth's climate and living systems.
In This Guide
What Is Water?
Water is a chemical compound made from hydrogen and oxygen.
Its chemical formula is:
H₂O
Each molecule contains:
- Two hydrogen atoms
- One oxygen atom
Pure water has no strong taste or odor and appears colorless in small quantities. In sufficiently deep or large volumes, however, pure water has a faint blue color because water molecules absorb some wavelengths of visible light more strongly than others.
On Earth, water exists naturally as:
- Liquid water
- Ice and snow
- Water vapor
These forms continually move through the water cycle.
Why Is the Water Molecule So Unusual?
The oxygen atom attracts shared electrons more strongly than the hydrogen atoms do.
This creates an uneven distribution of electrical charge:
- The oxygen side of the molecule has a slight negative charge.
- The hydrogen sides have slight positive charges.
This makes water a polar molecule.
Neighboring water molecules are attracted to each other through relatively weak interactions called hydrogen bonds.
Much of water's unusual behavior—including cohesion, surface tension, heat capacity, and solvent properties—comes from its polarity and hydrogen bonding.
Water as the “Universal Solvent”
Water is often called the universal solvent because it dissolves more substances than any other common liquid.
However, the term should not be taken literally.
Water does not dissolve everything.
Its polarity makes it particularly effective at dissolving many:
- Salts
- Minerals
- Acids
- Bases
- Polar organic compounds
For example, when sodium chloride dissolves in water, the positively charged sodium ions and negatively charged chloride ions become surrounded by water molecules.
This solvent ability is essential in nature because water can transport:
- Nutrients
- Minerals
- Dissolved gases
- Waste products
- Many chemical substances
Water's ability to dissolve substances is both useful and important for water quality. The same property that allows water to transport nutrients can also allow dissolved minerals and contaminants to move through soil, groundwater, rivers, plumbing, and biological systems.
1. Cohesion and Adhesion
Cohesion
Cohesion is the attraction between water molecules.
Hydrogen bonding causes neighboring water molecules to attract one another.
This helps explain:
- Droplet formation
- Surface tension
- Continuous columns of water moving through plant tissue
Adhesion
Adhesion is the attraction between water molecules and another material.
For example, water can adhere to:
- Glass
- Soil particles
- Plant tissues
- Many other polar surfaces
2. Surface Tension
Water molecules at the surface experience strong cohesive forces from neighboring molecules.
This produces relatively high surface tension.
Surface tension helps explain why:
- Water forms rounded droplets.
- Some small insects can remain on the water surface.
- A carefully placed lightweight object may temporarily rest on water despite being denser than water.
3. Capillary Action
Capillary action describes the movement of water through narrow spaces without an external pump.
It results from the combined effects of:
- Cohesion
- Adhesion
- Surface tension
You can observe capillary action when water rises:
- Inside a narrow glass tube
- Through a paper towel
- Through soil pores
- Within plant tissues
Capillary action is one of several processes that contribute to water movement through soils and plants.
4. Color of Water
A small glass of clean water generally appears colorless.
But a sufficiently deep body of pure water can appear blue.
Natural water may also appear:
- Brown from dissolved organic material
- Green from algae
- Orange or reddish from iron
- Cloudy from suspended particles
Water color alone cannot tell you whether a water supply meets drinking-water standards. Many important dissolved substances are invisible.
5. Compressibility
Like most liquids, water is only slightly compressible under ordinary conditions.
This property makes liquids useful for transmitting pressure in hydraulic systems.
Water does compress under sufficiently high pressure, so describing it as completely “incompressible” is an approximation rather than an absolute rule.
6. Density of Water
Density describes mass per unit volume.
A commonly used approximation for liquid water is:
1 gram per milliliter, or 1 g/cm³
The exact value changes with temperature.
Water Is Densest Near 4°C
One of water's most unusual properties is that liquid water reaches approximately its maximum density near 4°C (39°F).
As water cools below that temperature toward freezing, it begins to expand rather than continuing to become denser.
Why Does Ice Float?
When water freezes, hydrogen bonding arranges the molecules into a more open crystal structure.
This makes ice less dense than liquid water.
As a result:
Ice floats.
This unusual density behavior has major ecological importance. When lakes freeze, ice generally forms at the surface rather than from the bottom upward, helping liquid water remain underneath.
How Much Does a Gallon of Water Weigh?
A U.S. gallon of water weighs approximately 8.34 pounds near ordinary room temperatures.
The exact value varies slightly with temperature because density changes.
I would remove the original full engineering table of density in slugs/ft³. It adds complexity without helping most readers understand the property.
7. High Specific Heat Capacity
Water has a relatively high specific heat capacity.
This means a substantial amount of energy is required to change its temperature.
This property helps water:
- Moderate coastal climates
- Stabilize lake and ocean temperatures
- Transport heat through the body
- Serve as a coolant in many industrial applications
Heat of Vaporization
Water also requires substantial energy to change from liquid to vapor.
When water evaporates, it removes heat from the surface it leaves.
This explains much of the cooling effect of:
- Sweating
- Evaporation from plant leaves
- Evaporative cooling systems
8. Water Temperature
Temperature reflects the average kinetic energy of molecules in a substance.
Water temperature affects many physical, chemical, and biological processes.
For example, it can influence:
- Dissolved oxygen
- Reaction rates
- Density
- Viscosity
- Aquatic organisms
- Algal growth
Fish and other aquatic organisms have temperature ranges in which they function most effectively. Large temperature changes can therefore alter aquatic communities.
9. Electrical Conductivity
Electrical conductivity measures how easily water conducts electric current.
Pure water conducts electricity poorly.
Natural water becomes more conductive when dissolved substances create charged ions.
Examples include:
- Sodium
- Chloride
- Calcium
- Magnesium
- Sulfate
- Bicarbonate
In general:
More dissolved ions → higher conductivity
Conductivity is commonly reported in:
- Microsiemens per centimeter (µS/cm)
- Millisiemens per centimeter (mS/cm)
Conductivity does not identify individual contaminants. Two waters with similar conductivity can contain very different dissolved substances.
10. pH
pH describes how acidic or basic an aqueous solution is.
The familiar pH scale runs approximately from:
- Below 7: acidic
- 7: neutral under typical reference conditions
- Above 7: basic or alkaline
The scale is logarithmic.
A change of one pH unit represents approximately a tenfold change in hydrogen-ion activity.
Why pH Matters
Water pH can influence:
- Corrosion
- Mineral solubility
- Water-treatment chemistry
- Aquatic ecosystems
- Metal release from plumbing
See our related guide on metallic-tasting water and plumbing corrosion.
11. Water Hardness
Water hardness primarily reflects the concentrations of dissolved calcium and magnesium.
Hard water can contribute to:
- Scale on faucets
- Mineral buildup in water heaters
- Soap scum
- Reduced soap lather
- Deposits on glassware
Hardness is commonly reported as milligrams per liter of calcium carbonate equivalent.
Hardness and pH are different measurements. Hard water is not automatically alkaline, and soft water is not automatically acidic.
12. Turbidity
Turbidity describes the cloudiness of water caused by suspended particles.
Particles can include:
- Clay
- Silt
- Organic matter
- Algae
- Other suspended material
Turbidity is commonly measured in NTU—nephelometric turbidity units.
High turbidity can affect:
- Water appearance
- Aquatic habitats
- Drinking-water treatment
- Disinfection effectiveness
13. Dissolved Oxygen
Dissolved oxygen is molecular oxygen present in water.
It enters water through:
- Exchange with the atmosphere
- Photosynthesis
- Mixing and turbulence
Dissolved oxygen is essential for many aquatic organisms.
Its concentration is influenced by:
- Water temperature
- Salinity
- Atmospheric pressure
- Biological activity
- Organic pollution
Colder water can generally hold more dissolved oxygen than warmer water.
14. Salinity
Salinity describes the concentration of dissolved salts in water.
Ocean water has much higher salinity than freshwater.
Common dissolved ions in seawater include:
- Chloride
- Sodium
- Sulfate
- Magnesium
- Calcium
- Potassium
Salinity affects:
- Water density
- Freezing point
- Conductivity
- Osmotic conditions
- Aquatic ecosystems
How Are Water Properties Measured?
| Property | Common Measurement | What It Tells You |
|---|---|---|
| Temperature | °C or °F | Thermal condition of the water |
| pH | pH units | Acidity or alkalinity |
| Conductivity | µS/cm | Ability to conduct electricity due largely to dissolved ions |
| Hardness | mg/L as CaCO₃ | Primarily calcium and magnesium concentration |
| Turbidity | NTU | Cloudiness from suspended particles |
| Dissolved oxygen | mg/L | Oxygen available in the water |
| Salinity | ppt, PSU or conductivity-derived values | Amount of dissolved salts |
| Density | g/cm³ or kg/m³ | Mass per unit volume |
Water in Oceans, Lakes, Rivers, Rain, and Groundwater
Water chemistry changes as water moves through the environment.
Seawater
The oceans contain about 96.5% of Earth's total water.
Seawater contains large concentrations of dissolved salts, making it denser and much more conductive than freshwater.
Rain and Snow
Rain and snow are not chemically identical to distilled water.
As precipitation forms and falls through the atmosphere, it can interact with:
- Dust
- Sea salt
- Atmospheric gases
- Air pollutants
Rivers and Lakes
River and lake chemistry can be influenced by:
- Groundwater
- Rainfall
- Runoff
- Local geology
- Biological processes
- Human activity
There is no general rule that chemical concentrations simply increase downstream. River chemistry can increase, decrease, or change composition depending on tributaries, groundwater, geology, treatment discharges, rainfall, and other inputs.
Groundwater
Groundwater can remain in contact with rock and soil for long periods.
This can increase concentrations of dissolved minerals such as:
- Calcium
- Magnesium
- Iron
- Manganese
- Other naturally occurring substances
See our Well Water Testing & Treatment Guide.
Why Water's Properties Matter to Life and the Environment
Climate Regulation
Water's high heat capacity allows oceans and lakes to absorb and release large amounts of heat relatively slowly.
This helps moderate temperature changes in Earth's climate system.
Plant Growth
Plants depend on water for:
- Photosynthesis
- Nutrient transport
- Cell structure
- Temperature regulation
- Metabolic processes
Aquatic Ecosystems
Water temperature, pH, salinity, turbidity, and dissolved oxygen help determine which organisms can live in a particular aquatic environment.
Geology and Soil
Because water dissolves and transports substances, it contributes to:
- Weathering
- Erosion
- Mineral transport
- Groundwater chemistry
- Soil formation
The Water Cycle
The continuous movement of water among the atmosphere, land, oceans, ice, rivers, lakes, groundwater, plants, and living organisms links all of these properties.
See our Water Cycle Explained guide.
Water Availability and Freshwater Resources
Although Earth contains an enormous amount of water, most of it is saline ocean water.
Only about 2.5% is freshwater, and much of that freshwater is stored in:
- Glaciers
- Ice caps
- Groundwater
Rivers and lakes contain only a very small fraction of Earth's total water, even though they supply a significant portion of the freshwater people use.
This is why water-resource management depends not simply on how much water exists on Earth, but on:
- Where freshwater is located
- When it is available
- Water quality
- Infrastructure
- Population and demand
- Climate and drought
See our Water Conservation Guide and U.S. Water Resources Guide.
Frequently Asked Questions About Water Properties
What is the chemical formula for water?
Water is H₂O, meaning each molecule contains two hydrogen atoms and one oxygen atom.
Why is water polar?
Oxygen attracts shared electrons more strongly than hydrogen, creating an uneven distribution of charge across the water molecule.
Why is water called the universal solvent?
Water dissolves more substances than any other common liquid, particularly ionic and polar compounds. It does not literally dissolve everything.
What is cohesion in water?
Cohesion is the attraction between water molecules, largely resulting from hydrogen bonding.
What is adhesion?
Adhesion describes the attraction between water and another material or surface.
What causes water's surface tension?
Strong cohesive forces among water molecules create relatively high surface tension at the liquid surface.
What is capillary action?
Capillary action is the movement of water through narrow spaces, driven by interactions involving adhesion, cohesion, and surface tension.
What is the density of water?
A commonly used approximation is about 1 gram per cubic centimeter, although the exact density varies with temperature.
At what temperature is water most dense?
Liquid freshwater reaches its maximum density near 4°C (39°F).
Why does ice float?
Freezing creates a more open molecular structure, making ordinary ice less dense than liquid water.
How much does a gallon of water weigh?
A U.S. gallon of water weighs approximately 8.34 pounds near ordinary room temperature.
What is water conductivity?
Conductivity measures how readily water carries electrical current. Dissolved ions such as sodium, chloride, calcium, and magnesium increase conductivity.
Does pure water conduct electricity?
Pure water conducts electricity very poorly. Natural water becomes more conductive because it contains dissolved ions.
What is the pH of pure water?
Pure water is neutral around pH 7 under common reference conditions, although the exact neutral pH varies somewhat with temperature.
What causes hard water?
Hardness is primarily caused by dissolved calcium and magnesium.
Is hard water the same as high-pH water?
No. Hardness measures mainly calcium and magnesium, while pH measures acidity or alkalinity.
What is turbidity?
Turbidity is the cloudiness of water produced by suspended particles.
What is dissolved oxygen?
Dissolved oxygen is molecular oxygen present in water and available to aquatic organisms.
What is salinity?
Salinity describes the amount of dissolved salts in water.
Why can cold water hold more dissolved oxygen?
The solubility of oxygen generally decreases as water temperature rises, so colder water can usually hold more dissolved oxygen.
Is natural rainwater pure H₂O?
No. Rain interacts with atmospheric gases, particles, sea salt, dust, and pollutants as it forms and falls.
Final Thoughts
Water's importance comes not simply from how much of it exists, but from its unusual chemistry.
Its molecular structure gives water a combination of properties that include:
- Polarity
- Hydrogen bonding
- Strong solvent ability
- Cohesion and adhesion
- High surface tension
- High specific heat capacity
- Unusual density behavior
At the same time, measurements such as:
- Temperature
- pH
- Conductivity
- Hardness
- Turbidity
- Dissolved oxygen
- Salinity
help scientists and water professionals understand what is happening in drinking water, rivers, groundwater, lakes, and oceans.
Water looks simple, but its unusual molecular behavior is one of the foundations of Earth's climate, ecosystems, chemistry, and life.