The Impact of Salinity on the Surface Tension of Water
Introduction
Along with my study of Physics in the IB, I am also a keen student of Biology. Consequently,
I have studied the properties of water that make it so vital to life, one of these is its polarity;
due to the uneven distribution of electrons between its constituent hydrogen and oxygen
atoms, water forms intermolecular ‘hydrogen bonds’ that give it several emergent properties,
including capillarity: the ability of a liquid to rise up a capillary tube due to surface tension. It
is this simple characteristic that allows plants of incredible heights to be nourished by water
within the inputting of external energy. After discovering this, I was interested to learn more
about the interplay between the force of gravity and surface tension that govern this process.
When considering the variety of water conditions plants grow in globally, I was intrigued to
investigate the relationship between water salinity and surface tension, since surface tension
is a key component of capillary action. This led me to formulate the research question:
How does the salinity of water affect its surface tension?
Theory and Hypothesis
Salinity is defined as the mass of salt dissolved in one kilogram of water, expressed in parts
per thousand (ppt) [1]. Freshwater tends to have a salinity of less than 1ppt, whereas seawater
has roughly 35 ppt [2]. At room temperature, water is saturated with salt at 357ppt [3].
Surface tension is the propensity of certain liquids to
try and minimise their surface area. In the case of
water in air, this is caused by the attractive force
from hydrogen bonds between water molecules being
greater than the attraction between the water
molecules and air [4].
In Figure 1, non-surface water molecules are
attracted by hydrogen bonds in all directions, causing
Figure 1 – Forces at a water-air
a resultant force of zero on them. boundary [12]
1
, The molecules on the surface, however, are attracted in all directions but that of the air above,
leading to a resultant force pulling them in and causing the surface to shrink to a smaller area.
Surface tension, 𝛾, is measured in newtons per metre and is defined as the force required to
increase the surface area of a thin film of a liquid per unit length of the film [5].
The capillary action of water in a tube is governed by two forces: cohesion and adhesion.
Cohesive forces are the attractive forces between water molecules, whereas adhesive forces
are attractive forces between molecules of differing substances, such as glass and water [6].
The adhesion between the water molecules and tube walls causes an upward force at the edge
of the liquid. As water molecules rise up the tube walls, the surface tension caused by the
cohesive forces between molecules pulls the water’s surface up along with it. At a certain
point, the force of surface tension pulling the water column up is balanced with the
downward force of gravity on the water column [7].
This point of equilibrium is determined by Jurin’s Law [13]:
Where:
h is liquid height
ρ is the density of liquid
𝛾 is surface tension
g is acceleration due to gravity
θ is the contact angle between
r0 is the tube’s inner radius
the liquid’s surface and the tube
From this, it follows that a liquid’s surface tension and the height it rises in a capillary tube
are proportional. By using this formula and measuring the height of water for different
salinities, the relationship between surface tension and salinity of water can be determined.
Sodium chloride (NaCl), otherwise known as table salt, is an ionic compound comprised of a
sodium and a chlorine atom. It is held together by an ionic bond, formed by the donation of
an electron from the sodium atom to the chlorine atom. When sodium chloride is added to
water, the slightly positive sodium part is attracted to the slightly negative oxygen part of a
water molecule, and the slightly negative chlorine part is attracted to the slightly positive
2
Introduction
Along with my study of Physics in the IB, I am also a keen student of Biology. Consequently,
I have studied the properties of water that make it so vital to life, one of these is its polarity;
due to the uneven distribution of electrons between its constituent hydrogen and oxygen
atoms, water forms intermolecular ‘hydrogen bonds’ that give it several emergent properties,
including capillarity: the ability of a liquid to rise up a capillary tube due to surface tension. It
is this simple characteristic that allows plants of incredible heights to be nourished by water
within the inputting of external energy. After discovering this, I was interested to learn more
about the interplay between the force of gravity and surface tension that govern this process.
When considering the variety of water conditions plants grow in globally, I was intrigued to
investigate the relationship between water salinity and surface tension, since surface tension
is a key component of capillary action. This led me to formulate the research question:
How does the salinity of water affect its surface tension?
Theory and Hypothesis
Salinity is defined as the mass of salt dissolved in one kilogram of water, expressed in parts
per thousand (ppt) [1]. Freshwater tends to have a salinity of less than 1ppt, whereas seawater
has roughly 35 ppt [2]. At room temperature, water is saturated with salt at 357ppt [3].
Surface tension is the propensity of certain liquids to
try and minimise their surface area. In the case of
water in air, this is caused by the attractive force
from hydrogen bonds between water molecules being
greater than the attraction between the water
molecules and air [4].
In Figure 1, non-surface water molecules are
attracted by hydrogen bonds in all directions, causing
Figure 1 – Forces at a water-air
a resultant force of zero on them. boundary [12]
1
, The molecules on the surface, however, are attracted in all directions but that of the air above,
leading to a resultant force pulling them in and causing the surface to shrink to a smaller area.
Surface tension, 𝛾, is measured in newtons per metre and is defined as the force required to
increase the surface area of a thin film of a liquid per unit length of the film [5].
The capillary action of water in a tube is governed by two forces: cohesion and adhesion.
Cohesive forces are the attractive forces between water molecules, whereas adhesive forces
are attractive forces between molecules of differing substances, such as glass and water [6].
The adhesion between the water molecules and tube walls causes an upward force at the edge
of the liquid. As water molecules rise up the tube walls, the surface tension caused by the
cohesive forces between molecules pulls the water’s surface up along with it. At a certain
point, the force of surface tension pulling the water column up is balanced with the
downward force of gravity on the water column [7].
This point of equilibrium is determined by Jurin’s Law [13]:
Where:
h is liquid height
ρ is the density of liquid
𝛾 is surface tension
g is acceleration due to gravity
θ is the contact angle between
r0 is the tube’s inner radius
the liquid’s surface and the tube
From this, it follows that a liquid’s surface tension and the height it rises in a capillary tube
are proportional. By using this formula and measuring the height of water for different
salinities, the relationship between surface tension and salinity of water can be determined.
Sodium chloride (NaCl), otherwise known as table salt, is an ionic compound comprised of a
sodium and a chlorine atom. It is held together by an ionic bond, formed by the donation of
an electron from the sodium atom to the chlorine atom. When sodium chloride is added to
water, the slightly positive sodium part is attracted to the slightly negative oxygen part of a
water molecule, and the slightly negative chlorine part is attracted to the slightly positive
2
