## The Structure and Properties of Water

Water (H2O) has many interesting and unique properties.

### Learning Objectives

Describe the structure and properties of water.

### Key Takeaways

#### Key Points

• Water is a liquid at standard temperature and pressure (25 degrees Celsius and 1 atm, for liquids).
• Water is is tasteless and odorless.
• Water is transparent in the visible part of the electromagnetic spectrum.
• Water can act as either an acid or a base.
• Water is a universal solvent, dissolving many substances found in nature.

#### Key Terms

• amphoteric: A molecule that can act as either an acid or a base depending on its chemical environment. For example, water (H2O) is amphoteric.
• dipole: Any molecule or radical that has delocalized positive and negative charges.
• equilibrium: The state of a reaction in which the rates of the forward and reverse reactions are equal.
• phase diagram: A graph showing the phase a sample of matter has under different conditions of temperature and pressure.

### The Properties of Water

Water is the most abundant compound on Earth’s surface. In nature, water exists in the liquid, solid, and gaseous states. It is in dynamic equilibrium between the liquid and gas states at 0 degrees Celsius and 1 atm of pressure. At room temperature (approximately 25 degrees Celsius), it is a tasteless, odorless, and colorless liquid. Many substances dissolve in water, and it is commonly referred to as the universal solvent.

Properties of Water
Molecular formula H2O
Molar mass 18.01528(33) g/mol
Appearance white solid or almost colorless, transparent, with a hint of blue, crystalline solid or liquid
Density 1000 kg/m3, liquid (4º C)(62.4 lb/cu. ft)
Melting point 0º C, 32º F, (273.15 K)
Acidity (pKa) 15.74 ~35–56
Basicity (pKb) 15.74
Refractive index (nD) 1.3330
Viscosity 0.001 Pas at 20º C
Structure of Water
Crystal structure Hexagonal
Molecular shape Bent
Dipole moment 1.85 D

### The Phases of Water

Similar to many other substances, water can take numerous forms. Its liquid phase, the most common phase of water on Earth, is the form that is generally meant by the word “water.”

### Solid Phase (Ice)

The solid phase of water is known as ice and commonly takes the structure of hard, amalgamated crystals, such as ice cubes, or of loosely accumulated granular crystals, such as snow. Unlike most other substances, water’s solid form (ice) is less dense than its liquid form, as a result of the nature of its hexagonal packing within its crystalline structure. This lattice contains more space than when the molecules are in the liquid state.

The hexagonal structure of ice: As a naturally occurring crystalline inorganic solid with an ordered structure, ice is considered to be a mineral. It possesses a regular crystalline structure based on the molecular structure of water, which consists of a single oxygen atom covalently bonded to two hydrogen atoms: H-O-H.

The fact the density of ice is less than that of liquid water’s has the important consequence that ice floats.

The density of ice and water as a function of temperature: The solid form of most substances is denser than the liquid phase; therefore, a block of a given solid will generally sink in its corresponding liquid. However, a block of ice floats in liquid water because ice is less dense than liquid water. The inset shows the curve in more detail in the range of 0-10 degrees Celsius. Liquid water is most dense at 4 degrees Celsius.

### Liquid Phase (Water)

Water is primarily a liquid under standard conditions (25 degrees Celsius and 1 atm of pressure). This characteristic could not be predicted by its relationship to other, gaseous hydrides of the oxygen family in the periodic table, such as hydrogen sulfide. The elements surrounding oxygen in the periodic table – nitrogen, fluorine, phosphorus, sulfur, and chlorine – all combine with hydrogen to produce gases under standard conditions. Water forms a liquid instead of a gas because oxygen is more electronegative than the surrounding elements, with the exception of fluorine. Oxygen attracts electrons much more strongly than does hydrogen, resulting in a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom. The presence of such a charge on each of these atoms gives a water molecule a net dipole moment.

The electrical attraction between water molecules caused by this dipole pulls individual molecules closer together, making it more difficult to separate the molecules, and therefore raising the boiling point. This type of attraction is known as hydrogen bonding. The molecules of water are constantly moving in relation to each other, and the hydrogen bonds are continually breaking and reforming at intervals briefer than 200 femtoseconds (200 x 10-15 seconds).

Arrangement of water molecules in the liquid phase: Water molecules align based on their polarity, forming hydrogen bonds (signified by “1”).

Many of the physical and chemical properties of water (including its capacity as a solvent) are partly to the acid-base reactions it can be part of. Water can be described as an amphoteric molecule, meaning that it can react as both a Brønsted-Lowry acid or base. This can be shown in the reaction between two water molecules that produces the hydronium ion (H3O+) and a hydroxide ion (OH):

$\text{H}_2\text{O}(l)+\text{H}_2\text{O}(l)\rightleftharpoons\text{H}_3\text{O}^+(aq)+\text{OH}^- (aq)$

### Gas Phase (Water Vapor)

The gaseous phase of water is known as water vapor (or steam) and is characterized by a transparent cloud. Water also exists in a rare fourth state called supercritical fluid, which occurs only in extremely uninhabitable conditions. When water achieves a specific critical temperature and a specific critical pressure (647 K and 22.064 MPa), the liquid and gas phases merge into one homogeneous fluid phase that shares properties of both gas and liquid.

### Phase Diagram of Water

Water freezes to form ice, ice thaws to form liquid water, and both water and ice can transform into the vapor state. Phase diagrams help describe how water changes states depending on the pressure and temperature.

Phase diagram of water: The three phases of water – liquid, solid, and vapor – are shown in temperature-pressure space.

Note the following key points on a phase diagram:

• The critical point (CP), above which only supercritical fluids exist.
• The triple point (TP), a well-defined coordinate where the curves intersect, at which the three states of matter (solid, liquid, gas) exist at equilibrium with each other.
• Well-defined boundaries between solid and liquid, solid and gas, and liquid and gas. During the phase transition between two phases (i.e, along these boundaries), the phases are in equilibrium with each other.

### The Polarity of Water

The polar nature of water is a particularly important feature that contributes to the uniqueness of this substance. The water molecule forms an angle with an oxygen atom at the vertex and hydrogen atoms at the tips. Because oxygen has a higher electronegativity than hydrogen, the side of the molecule with the oxygen atom has a partial negative charge. An object with such a charge difference is called a dipole (meaning “two poles”). The oxygen end is partially negative, and the hydrogen end is partially positive; because of this, the direction of the dipole moment points from the oxygen toward the center position between the two hydrogens. This charge difference causes water molecules to be attracted to each other (the relatively positive areas are attracted to the relatively negative areas), as well as to other polar molecules. This attraction contributes to hydrogen bonding and explains many of water’s properties (including its ability to act as a solvent to many substances).

Polarity of the water molecule: Owing to the electronegativity difference between hydrogen (H) and oxygen (O) atoms, and the bent shape of the H2O molecule, a net dipole moment exists. The figure indicates the partial charges that the atoms possess.

A water molecule can form a maximum of four hydrogen bonds by accepting two hydrogen atoms and donating two hydrogen atoms. Although hydrogen bonding is a relatively weak attraction compared to the covalent bonds within the water molecule itself (intramolecular bonds), it is responsible for a number of water’s physical properties. One such property is its relatively high melting and boiling points; more energy is required to break the hydrogen bonds between molecules in order to change to a higher energy phase.

## Surface Tension

Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force.

### Learning Objectives

Define surface tension.

### Key Takeaways

#### Key Points

• Surface tension is a fundamental property of the surface of liquid.
• Surface tension is responsible for the curvature of the surfaces of air and liquids.
• Surface tension is responsible for the ability of some solid objects to “float” on the surface of a liquid.
• Surface tension is responsible for the shape of the interface between two immiscible liquids.

#### Key Terms

• curvature: The degree to which a bent shape is curved.
• energy: A quantity that denotes the ability of a system to do work. It has dimensions of mass × distance²/time², such as 1 kg m2/sec2 = 1 Joule (J).
• tension: Force transmitted through a rope, string, cable, or similar object (used with prepositions on, in, or of to convey that the same magnitude of force applies to objects attached to both ends).

### Liquids are Fluids

Liquids and solids share a common attribute: a clear and discernible phase boundary that gives the sample a simple but definite shape. Liquids and solids share also something else: most of their molecular units are to some extent in relatively close contact. However, liquids, like gases, are fluids, meaning that their molecular units can move more or less independently of each other. Whereas the volume of a gas depends entirely on the pressure, the volume of a liquid is largely independent of the atmospheric pressure. Therefore, gases are compressible while liquids are very nearly not.

### Cohesive Forces Result in Surface Tension

Diagram of forces on molecules in a liquid: In the bulk of the liquid, the forces are the same in all directions, while at the surface, the net effect is “downward,” into the interior.

The molecules in a sample of a liquid that find themselves fully in the interior volume are surrounded by other molecules and interact with them based on the attractive intermolecular forces that are present for molecules of this type. However, the molecules at the interface with another medium (usually air) do not have other like molecules on all of their sides (namely, on top of them), so they cohere more strongly to the molecules on the surface and immediately below them. The result is a surface film which makes it more difficult for an object to pierce through the surface than for it to move once submerged in the sample of liquid. Therefore, the cohesive forces result in the phenomenon of surface tension.

### The Young-Laplace Equation

Surface tension is responsible for the shape of a liquid droplet. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. In the absence of other forces, including gravity, drops of virtually all liquids would be perfectly spherical. If no force acts normal (perpendicular) to a tensioned surface, the surface must remain flat. But if the pressure on one side of the surface differs from pressure on the other side, the pressure difference times the surface area results in a normal force. In order for the surface tension forces to cancel out this force due to pressure, the surface must be curved. When all the forces are balanced, the curvature of the surface is a good measure of the surface tension, which is described by the Young-Laplace equation:

$\Delta P = \gamma \left (\frac{1}{R_{1}}+\frac{1}{R_{2}} \right )$

where $\Delta P$ is the pressure differential across the interface, $\gamma$ is the measured surface tension, and $R_1, R_2$ are the principal radii of curvature, which indicate the degree of curvature.

This equation describes the shape and curvature of water bubbles and puddles, the “footprints” of water-walking insects, and the phenomenon of a needle floating on the surface of water. Even though the needle is denser than water, it floats because surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force. This property is caused by cohesion of similar molecules and is responsible for many of the behaviors of liquids.

Resolution of forces on a “floating needle”: For a needle floating on the surface of a liquid, the downward force of the needle’s weight is balanced by the upward forces of surface tension from the liquid. Note that the forces from the surface tension are symmetrical.

### Potential Energy of Molecules in a Liquid

In imagining the shape of a liquid droplet or the curvature of the surface of a liquid, one must keep in mind that the molecules at the surface are at a different level of potential energy than are those of the interior. That is to say, there is an energy difference between the interior and the surface: to move a molecule from the interior to the surface requires energy. Liquids (for instance, in the form of a droplet) are shaped in a way that minimizes the energy at the surface. Again, since the energy at the surface is due in large part to the intermolecular attractive forces between particles on the surface and those in the interior, the surface tension is an indicator of the extent of those forces. Different liquids and solutions have different surface tensions.

The surface tensions of a few common liquids and solutions are as follows, in dyne/cm (note the particularly high surface tension of water):

• water, H(OH): 72.7
• benzene, C6H6: 40.0
• glycerin, C3H2(OH)3: 63.0
• sucrose solution (85% in water): 76.4

### Units of Surface Tension

Surface tension is expressed in units of force per unit length or of energy per unit area (for instance, N/m or J/m2). The two are equivalent, but when referring to energy per unit area, people use the term “surface energy,” which is a more general term in the sense that it applies to solids as well as to liquids.

An example of surface tension: Water striders can “walk” on water as a result surface tension.

## Viscosity

Viscosity is a measure of a fluid’s resistance to flow.

### Learning Objectives

Describe viscocity.

### Key Takeaways

#### Key Points

• A fluid’s resistance to flow can be characterized by the viscosity.
• Viscosity depends on the intermolecular forces present within the fluid.
• Viscosity is highly dependent on temperature.

#### Key Terms

• viscosity: A quantity expressing a fluid’s resistance to flow. It can be interpreted as a measure of the internal friction in a fluid.
• Laminar Flow: Smooth flow of a fluid in a tube or pipe.

### Laminar Flow

When fluids flow smoothly through a tube or pipe, the motion can be thought of as consisting of layers, or lamina. This is called laminar flow (also known as streamlined flow), and the velocity of the fluid’s flow varies from close to zero near the pipe’s boundaries to its greatest in the center.

Laminar flow: Velocity of a fluid’s layers, or lamina, during smooth flow. The velocity is greatest at the center of the tube.

If we consider one of those layers as a moving plate within the fluid, then the velocity is highest right underneath the moving plate but gets progressively smaller as we move away from the plate in a direction perpendicular to the flow.

Moving plate in a fluid: The velocity of the liquid (in the x-direction) is highest near the moving plate and gets progressively smaller as we move away from the plate in the perpendicular, or y-, direction. Note the magnitude of the velocity vectors for layers increasingly away from the moving plate.

### Description of Viscosity

The force required to move the plate at a constant speed is directly proportional to the area of the plate and to the fluid’s velocity gradient as we move at a greater perpendicular distance from the plate (meaning how fast the velocity of the layers is changing as we move away from the plate). This implies the following quantitative relationship:

$F=\eta \cdot A \cdot \frac {dv_x} {dy}$

where F is the force required to move the plate (at constant speed), A is the area of the plate, $\frac {dv_x} {dy}$is the change in laminar velocity with respect to the perpendicular distance to the plate (y-direction). The term $\eta$ is called the viscosity, and it is a measure of a fluid’s resistance to ﬂow. The units of viscosity in the SI system are $N \cdot \text{sec /m}^2$ (or $Pa \cdot\text{ sec}$ ).

Viscosity is highly dependent on temperature, and therefore, when the value of the quantity is reported, the temperature at which the measurement was made must be included.

Viscosity dependence on temperature: The viscosity of water varies greatly over the temperature range of the liquid form.

### Experimental Measurements

Viscosity can be measured by observing the time required for a given volume of liquid to flow through the narrow part of a viscometer tube, a special instrument used for such measurements.

There are various types of viscometers and rheometers. A rheometer is used for fluids that cannot be defined by a single value of viscosity. They require more parameters to be set and measured than is the case for a viscometer. Close temperature control of the fluid is essential to acquire accurate measurements, particularly in materials like lubricants, whose viscosity can double with a change of only 5 °C.

When measuring viscosity, strain is applied at a certain rate, called shear rate. The resulting stress is measured as deformation, in the case of liquid, as the ease of flow. There are several ways in which the stress changes with the strain. In simple linear systems, it is directly proportional and sometimes constant. In other systems, it increases or decreases with shear rate.

Fluids that display a constant viscosity over a range of shear rates are called Newtonian, while those with a non-constant viscosity are non-Newtonian. In the latter category, fluids that decrease in viscosity as shearing rate increases are called shear thinning (examples include toothpaste and house paint), and those that increase in viscosity as shearing rate increases are called shear thickening.

Viscosity plot: This figure shows the relative relationships between shear stress and shear strain for a variety of materials.

### Intermolecular Forces Affect the Viscosity of a Substance

The viscosity of a substance is related to the strength of the intermolecular forces acting between its molecular units. In the case of water, these forces are primarily due to hydrogen bonding. Liquids such as syrups and honey are much more viscous because the sugars they contain are studded with hydroxyl groups (–OH). They can form multiple hydrogen bonds with water and with each other, producing a sticky disordered network which makes flow much more difficult (resulting in high viscosity).

In much of our daily experience, we act as viscometers, sensing the viscosity of liquids and assessing their fitness for a particular purpose. For example, some people add cream and sugar to coffee or tea not only for the taste but for what they term the mouth feel. In this case, we are forcing the fluid through a small gap between our tongue and palate, sensing the thickness and smoothness. Cream and sugar add particulate solids to the watery tea and coffee, which add to the viscosity of the solution.

In another setting, we judge the quality of a sauce by the way it flows and adheres to certain foods – salad dressing to lettuce, jam or jelly to toast, ketchup or mayonnaise to fried foods – the flow and cohesion are related to the viscosity. If you were to place honey or corn syrup in the refrigerator, it thickens considerably relative to its thickness at room temperature. We can see that viscosity is highly dependent on temperature.

In everyday terms, viscosity is described as thickness or internal friction within the substance. Therefore, we say water is thin, having a low viscosity, while honey is thick, having a high viscosity. When a fluid is less viscous, it flows more easily.

Interactive: Molecular View of a Liquid: Explore the structure of a liquid at the molecular level.

## Capillary Action

Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces.

### Learning Objectives

Distinguish capillary action from other forces

### Key Takeaways

#### Key Points

• The rise or fall of a fluid in a capillary tube is governed by the balance of cohesive and adhesive forces.
• Inermolecular forces are responsible for cohesion and adhesion.
• The narrower the bore of a glass tube, the greater the extent of raising or lowering of the liquid.

#### Key Terms

• capillary: Pertaining to a narrow tube.
• cohesion: Various intermolecular forces that hold solids and liquids together.
• adhesion: The ability of a substance to stick to an unlike substance.
• meniscus: The curved surface of liquids in tubes, whether concave or convex, caused by the surface tension of the liquid.

The molecules in any sample of matter experience intermolecular forces, which are attractive or repulsive forces between atoms or molecules within the sample. Such forces are responsible for many observable behaviors of substances, such as the phase they are in under certain conditions of temperature and pressure. When attractive forces occur between like molecules, they are referred to as cohesive forces, or resulting in cohesion, because they hold the molecules of sample close together. These cohesive forces are especially strong at the surface of a liquid, resulting in the phenomenon of surface tension. For example, the hydrogen bonds between water molecules are responsible for the cohesion observed in water droplets.

On the other hand, when intermolecular forces occur between different types of molecules (especially when they are part of different phases of matter), they are referred to as adhesive forces, or resulting in adhesion. The molecules in a sample of water in contact with a glass surface experience attractive forces toward the glass molecules. Water has a tendency to adhere to such surfaces because of those interactions.

### Capillary Action

Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces such as gravity. This effect can be seen in the drawing-up of liquids between the hairs of a paintbrush, in a thin tube, in porous materials such as paper, in some non-porous materials (such as liquified carbon fiber), or in a cell. It occurs when the intermolecular attractive forces between the liquid and the solid surrounding surfaces (adhesive forces) are stronger than the cohesive forces within the liquid. If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and container act together to lift the liquid. The height (h) of a liquid column is given by:

$h = \frac {2T} {\rho r g}$

where T is the surface tension, $\rho$ is the density of liquid, g is the acceleration due to gravity, and r is radius of the tube. Notice that the height to which the liquid is lifted is inversely proportional to the radius of the tube, which explains why the phenomenon is more pronounced for smaller tubes.

Consider a water-filled glass tube with a radius of 2 cm (0.02 m) in air at standard laboratory conditions, T = 0.0728 N/m at 20 °C, $\rho$ is 1000 kg/m3, and g = 9.81 m/s2. For these values, the height of the water column is 0.74 mm. By comparison, for a 4 m diameter glass tube in the lab conditions given above (radius 2 m, or 6.6 ft), the water would rise an unnoticeable 0.007 mm (0.00028 in). For a 0.4-mm (0.016-in) diameter tube (radius 0.2 mm, or 0.0079 in), the water would rise 70 mm (2.8 in).

A common apparatus used to demonstrate capillary action is the capillary tube. When the lower end of a vertical glass tube is placed in a liquid, a concave meniscus forms. Adhesion forces between the fluid and the solid inner wall pull the liquid column up until there is a sufficient mass of liquid for gravitational forces to counteract these forces.

Capillary action in glass tubes: The rise or fall of liquids in a capillary tube depends upon the interactions between the tube and the liquid.

The meniscus is the curve caused by surface tension in the upper surface of a liquid. It can be either convex or concave. A convex meniscus occurs when the molecules have a stronger attraction to each other (cohesion) than to the material of the container (adhesion), causing the surface of the liquid to cave downward. This may be seen between mercury and glass in barometers and thermometers. Conversely, a concave meniscus occurs when the molecules of the liquid are attracted to those of the container, causing the surface of the liquid to cave upwards. This can be seen in a glass of water.

The meniscus curve on a column of fluid in a capillary tube: The curvature of the surface at the top of a column of fluid in a narrow tube is caused by the relative strength of the forces responsible for the surface tension of the fluid (cohesive forces) and the adhesive forces to the walls of the container.

Capillary action acts on concave menisci to pull the liquid up, increasing the favorable contact area between liquid and container, and on convex menisci to pull the liquid down, reducing the amount of contact area.

When considering how liquids will behave on surfaces, if the liquid molecules are strongly attracted to the solid molecules then the liquid drop will completely spread out on the solid surface. This is often the case for water on bare metallic or ceramic surfaces.

The phenomenon of capillary action is important in the transport of water and nutrients in plants through the process of transpiration.