On earth, there are three dominant faces of matter; solids, liquids and gases. The predominant distinction between these types lies in the force between and the energy within their molecules.

In gases, the molecules are extremely energetic and are essentially independent of each other and easily wonder past each other. Both of these faces take on the shapes of their containers, with gases taking on the containers volume. They are also grouped together as fluids i.e. they are materials that flow readily under the action of an applied force. In solids, the molecules vibrate about their fixed position and are so strongly attracted that the material appears to be rigid.

Properties of A Fluid

  1. Density
  2. Compressibility.
  1. Density:-It is defined as the ratio of mass per unit volume. It is measured in kg/m3.

                          Density= mass/volume

Since the volume of a fluid expands and contracts, the density of fluids vary with temperature. The most common fluid is water has maximum density of 1000kg/m3 at 4oc. Air, a mixture composed principally of the gases; Nitrogen (78%), Oxygen (21%) has a density of 1.29kg/m3 at 0oc and 1.20kg/m3 at 20oc.

How a liquid density compares to that of water at 4oc is called its specific gravity. If a liquid has a specific gravity of 0.9 then its density is 900kg/m3.

  • Compressibility:- How easily a material can be compressed or reduced in volume by a change in pressure. It is generally represented by the letter R and is measured in terms of Pascal.


Air has a compressibility value of 1.0 x 10-5/Pa while H20 has a compressibility value of 4.5 x10-10/ Pa. Because of this extremely small value, liquids are often referred to as being incompressible and we do not need to worry about volume changes in bar calculations.


Surface Tension

 This is a property of the surface of a liquid that allows it to resist an external force. It is revealed for example, in floating of same objects on the surface of water, and in the ability of some insects (e.g. water striders) to run on the water surface. This property is caused by cohesion of like molecules, as its responsible for many of the behaviours of liquids.

Surface Tension has the dimension of force per unit length i.e.

                Surface Tension =Dimension of Force/ dimension of Length

                                           = MLT-2/L = MT-2

or energy per unit area. The 2 are equipment but when referring to energy per unit area, people used the term surface energy which is a more general term in the sense that it applies also to solids and not just liquids.


The cohesive force among the liquid molecules are responsible for thus phenomenon of surface tension. In the bulk of the liquid, earth molecules are pulled equally in every direction by neighbouring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have other molecules on them but have molecules acting on the other side of them and therefore pulled toward. This creates some internal pressure and force liquid surfaces to contract to the minimal area.

Surface tension is responsible for the shape of droplets of liquid although easily deformed droplets of water tend to be pulled into a spherical sphere by the cohesive force of the surface layer. In the absence of other forces including gravity, drops of virtually all liquids would be perfectly spherical.



Several effects of surface tension can be seen with ordinary water.

  1. Beading of rain water on the surface of a waxy surface such as an automobile. Water adheres weakly to wax and strongly to itself, so water clusters into deep. Surface tension gives them their mere spherical shape, because a sphere as the smallest possible surface area to volume ratio.


                            Figure: Beading of rain water on waxy surface

  • Formation of drops occurs when a mass of liquid is stretched. The diagram, below shows water adhering to the faucet (tap) gaining mass until it is stretched to a point where the surface tension can no longer bind it to the faucet. It then separates and surface tension forms the drop into a sphere. If a stream of water were running from the faucet, the stream will break up into drops during its fall. Gravity stretches the stream then surface tension pinches it into sphere.

                         Figure: Water adhering to the faucet

  • Floatation of objects denser than water occurs when the object is non-wetable and its weight is small enough to be borne by the force acting from the surface tension. For example, water striders use surface tension to walk on the surface of a pond. The surface of water behaves like an elastic film. The insect feet cause indentation in the water surface increasing in surface area.

                                 Figure: Water Strider walking on water

  • Separation of oil and water is caused by tension in the surface between dis-similar liquids. This type of surface tension is called “Interface Tension” but its physics are the same.
  • Tears of wine is the formation of drops and rivulets on the side of a glass containing an alcoholic beverage. Its cause is a complex interaction between the different surface tension of water and ethanol. It is induced by the combination of surface tension modification of water by ethanol together with ethanol evaporation faster than water.


This is the tendency for a liquid to rise up to a narrow tube when one end of the tube is immersed in the liquid.

                              Figure: Capillarity rise in water and fall in mercury.

Dipping one end of a capillary tube into water and dipping one end of another capillary tube into mercury. It will be observed that the water level rises in the capillary tube while the mercury level falls in the tube.


The adhesive force between water molecules and glass molecules is more than the cohesive force between the water molecules. The meniscus thus curves upwards and the water level rises in the tube. The narrower the capillary tube, the more the rise on the other hand, the cohesive force between mercury molecules and glass molecules is more than the adhesive force between the mercury and glass molecules. The meniscus thus curves inwards and the mercury level falls in the tube. The narrow the capillary tube the more the depression. Any other liquid which wets glass (palm oil, alcohol) behaves like water in the capillary tube experiment while any other liquid which doesn’t wet glass (e.g. molten wax) behaves like mercury in the capillary tube experiment.