Liquids and Solids

Properties of Liquids

Liquids are a type of matter with definite volume but no definite shape. Particles of a liquid experience cohesive forces, which give rise to surface tension, and adhesive forces, which give rise to capillary action.

A liquid is a state of matter that has a definite volume but not a definite shape, consisting of particles that are close together and flow freely. However, intermolecular forces, such as hydrogen bonds and London dispersion forces, pull the particles together. These forces keep the substance from expanding continually outward, as a gas does.

Intermolecular forces are weak interactions between the atoms of molecules. They are caused by the distribution of electric charge on atoms and, thus, within molecules. When one side of a molecule or atom has a positive charge and the other side has a negative charge, it is known as a dipole. The positive side of one molecule will attract the negative side of another molecule, pulling the two molecules together. In a liquid, this causes one molecule to stick to the next one, which sticks to another one, and so on. Thus the molecules of a liquid stay together, although they freely slide past one another.

Where the surface of a liquid meets the surface of another substance, intermolecular forces lead to some interesting effects. For example, when liquid water meets air, the intermolecular forces in the liquid are stronger than the intermolecular forces in the gas. This means that the molecules of the water are more attracted to one another than they are to the molecules of the air. This attraction between the same type of molecules is called a cohesive force. Cohesive force creates surface tension—the tendency of a liquid surface to acquire the smallest possible surface area and resist outside forces, such as cohesive and intermolecular forces. A hydrogen bond is an intermolecular bond between a hydrogen with a slightly positive charge and a lone pair of electrons. This bond keeps molecules such as water held together. The hydrogen bonds in water are the root cause of surface tension. Surface tension is responsible for the "bubbling up" effect of water droplets—they form rounded edges because the surface tension encourages them to take the shape with the least surface area. It is also the mechanism by which many insects walk on water. Surface tension resists the outside force of pressure from the insect's foot, keeping the foot resting on the surface of the water.

Surface Tension in a Water Droplet

Surface tension is the tendency of a liquid surface to acquire the least possible surface area and resist outside forces. This is caused by the attraction between the slightly positive charge (δ+ \delta+) of the hydrogen atoms and the slightly negative charge (δ\delta- ) of the oxygen atoms of the water molecule. These intermolecular forces pull the water molecules together.
In contrast, when liquid water meets glass, intermolecular forces between the molecules of the liquid and the molecules of the solid are greater than intermolecular forces between molecules within the liquid. Attraction between unlike molecules, such as forces between a liquid and its container, is called adhesive force. If the surface is a thin tube, called a capillary, a phenomenon known as capillary action results. Capillary action is the ability of a liquid to move up a tube against other forces, such as gravity and intermolecular interactions, including surface tension and cohesion. Capillary action is an important mechanism in plant transport of nutrients. For example, plants use capillary action to bring up water from the roots to the rest of the plant. The water is attracted to the walls of the inside of the plant and adheres because of its intramolecular forces.

Capillary Action

Capillary action is the ability of a liquid to move up a tube against other forces, such as gravity.
Capillary action is actually caused by both adhesive and cohesive forces working together. Adhesive forces cause water molecules to stick to the sides of the capillary because the tube has atoms with a slightly negative charge that are attracting the hydrogen atoms in water, pulling them up. Cohesive forces between a hydrogen atom in one water molecule and the oxygen atom in a different water molecule give the water molecules a tendency to stick together and cause water currently not touching the sides of the capillary to rise in the middle of the tube. The height that a liquid will rise in a tube can be calculated according to Jurin's law.
h=2γcosθρgrh=\frac{2\gamma\cos\theta}{\rho gr}
The h in Jurin's law is the height in meters the liquid will rise up the capillary tube. The surface tension, represented by γ\gamma , is measured in newtons per meter (N/m). The contact angle, θ\theta , is the number of degrees between the liquid and the wall of the tube. The ρ\rho in Jurin’s law is the density of the liquid in kilograms per cubic meter (kg/m3). Acceleration due of gravity, g, is 9.8 m/s2, and r is the radius of the tube in meters. For example, suppose a glass tube with an inner radius of 0.25 millimeter (mm) is placed into a beaker of water. Assume the temperature of the system is 25.0°C. Since the capillary is immersed in the water, assume the angle of contact is 0.0°. Water has a surface tension of 0.072 N/m and a density of 997 kg/m3. Jurin’s law can be used to calculate how high up the capillary the water will rise.
h=2γcosθρgr=(2)(0.072N/m)(cos0.0°)(997kg/m3)(9.8m/s2)(0.00025m)=0.059m=5.9cm\begin{aligned}h&=\frac{2\gamma\cos\theta}{\rho{gr}}\\&=\frac{(2)(0.072\;\rm N/\rm m)(\cos0.0\degree)}{\left({997\;\rm{kg}/\rm m^3}\right)\!(9.8\;\rm m/\rm s^2)(0.00025\;\rm m)}\\&=0.059\;\rm m \\&=5.9\;\rm{cm}\end{aligned}
Another important property of liquids is their viscosity, the measure of the liquid's resistance to deformation by internal friction. In other words, a liquid's viscosity is its tendency to flow. Low-viscosity liquids, such as water and alcohol, flow freely and easily, while high-viscosity liquids, such as honey and liquid glue, flow much more slowly.