Water, essential for life, is unique in its molecular structure, leading to intriguing properties such as cohesion, adhesion, and surface tension. Grasping these is fundamental for IB Biology students.
Cohesive Properties of Water
What is Cohesion?
- Cohesion is the attractive force between molecules of the same type.
- Within water, it's primarily due to the hydrogen bonding amongst water molecules.
The Intricacies of Hydrogen Bonding in Water
- A water molecule, H₂O, consists of two hydrogen atoms bonded to one oxygen atom.
- Oxygen's higher electronegativity gives it a stronger pull on shared electrons, creating a polar molecule. This polarity results in a partial negative charge on the oxygen and partial positive charges on the hydrogens.
- The partially positive hydrogen from one water molecule attracts the partially negative oxygen of another, resulting in a hydrogen bond.
- Each water molecule can form up to four hydrogen bonds with neighbouring molecules, leading to a structured, interconnected network.
IB Biology Tutor Tip: Understanding water's cohesion and adhesion is crucial for grasping how life-sustaining processes, like plant nutrient transport and animal thermoregulation, fundamentally depend on water's unique molecular properties.
Implications of Cohesion in Water
- Surface tension: A direct consequence of cohesive forces, surface tension makes water resistant to external forces. This is why certain small organisms, like water striders, can 'walk' on water.
- Droplet formation: When droplets form, it's cohesion pulling water molecules together, causing droplets to be spherical, as this shape minimises surface area.
Adhesive Properties of Water
Unpacking Adhesion
- Adhesion denotes the attractive forces between unlike molecules.
- Water's polarity lets it bond easily with other polar or charged materials.
The Mechanics of Water's Adhesion
- Water's polarity arises from the uneven distribution of electron density within the molecule. This allows it to form hydrogen bonds with other polar substances or ions.
- As a result, water tends to "stick" to surfaces that are charged or polar.
Image courtesy of USGS.gov
The Manifestations of Adhesion in Water
- Capillary action: A blend of cohesion and adhesion, capillary action sees water moving against gravity through narrow spaces. The adhesive forces between the water and the tube material draw the liquid upwards, while the cohesive forces ensure the column of water remains intact.
- Example: Plants exploit capillary action, drawing water from the soil through their roots and stems.
- Wetting: Materials get "wet" when in contact with water due to adhesion. If adhesive forces (between water and the surface) are stronger than the cohesive forces (amongst water molecules), water spreads out, leading to wetting.
Image courtesy of X.com
Surface Tension: A Deeper Dive
Dissecting Surface Tension
- Surface tension is the outcome of cohesive forces acting at a liquid’s surface. It manifests as a force that holds the liquid surface intact, reminiscent of an "elastic sheet."
Elements Impacting Surface Tension
- Temperature: Elevated temperatures boost the kinetic energy of molecules, potentially weakening cohesive forces and thereby reducing surface tension.
- Impurities: External contaminants can disrupt hydrogen bonding patterns. For instance, detergents drastically diminish water's surface tension by disrupting its molecular network.
IB Tutor Advice: For exams, illustrate your understanding of cohesion and adhesion by applying these concepts to real-world biological examples, such as water transport in plants or sweating in animals.
Practical Outcomes of Surface Tension
- Pouring dynamics: Ever noticed water dribbling down a bottle's side when pouring? Water's high surface tension is responsible for this.
- Behaviour on surfaces: Depending on the surface, water might form beads (attributed to cohesion) or spread out (owing to adhesion).
Image courtesy of Watthana Tirahimonch
Water's Cohesion and Adhesion in Biology
Transport in Plants
- Capillary action plays a role in the transpiration stream in plants. Water is drawn up from the roots to the leaves, a feat made possible by the cohesive and adhesive properties of water.
Image courtesy of GeeksforGeeks
Homeostasis in Animals
- Water’s adhesive properties play a part in thermoregulation. Sweating, for instance, sees water adhering to the skin, providing a cooling effect when it evaporates.
The Role in Ecosystems
- Cohesive and adhesive properties ensure water availability in various ecosystems, from soil moisture for terrestrial plants to maintaining water columns in aquatic habitats.
FAQ
Humans unconsciously and consciously harness water's cohesive and adhesive properties in various ways. For instance, in painting, adhesion ensures paint sticks to surfaces while cohesion prevents it from breaking apart. In medical technologies, capillary tubes exploit capillary action (a result of cohesion and adhesion) to draw up small quantities of blood for tests. Agriculture employs these properties when irrigating crops, relying on water's ability to move through soil (capillary action) and its retention in soil due to adhesive forces. Furthermore, many cleaning mechanisms, from simple mopping to advanced industrial cleaning, use water's adhesive properties to remove dirt and contaminants from surfaces.
Soaps and detergents have a unique molecular structure with a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. When added to water, these molecules disrupt the hydrogen bonding network amongst water molecules by inserting themselves at the water's surface. The hydrophilic heads remain in the water, while the hydrophobic tails project outwards, breaking the continuity of the water's surface. This reduces the cohesive forces amongst the surface water molecules, subsequently reducing surface tension. This reduced surface tension enhances the wetting ability of water, making it easier for water to spread on surfaces and penetrate dirt or grease.
Increasing the temperature of water boosts the kinetic energy of its molecules. With enhanced kinetic energy, water molecules move more vigorously, reducing the number and strength of hydrogen bonds. Surface tension is directly related to these hydrogen bonds at the water's surface. When these bonds are weakened or reduced in number, the cohesive forces between the surface water molecules decrease. As a result, surface tension diminishes with rising temperature. This is why hot water can be more effective in certain cleaning scenarios: its reduced surface tension allows it to spread and penetrate surfaces more readily, making it a better solvent.
Water's designation as the "universal solvent" stems from its ability to dissolve a myriad of substances. Its polar nature, where oxygen has a partial negative charge and hydrogens have partial positive charges, allows water to surround and dissolve ionic and other polar molecules effectively. When water encounters a soluble substance, the adhesive properties allow the water molecules to surround and bond with the particles of that substance. The cohesive forces then help keep these newly-formed aqueous solutions stable. The combination of cohesion and adhesion ensures that substances remain uniformly dissolved in water.
Water droplets behave differently on various surfaces due to a balance between cohesive and adhesive forces. On hydrophobic (water-repelling) surfaces, cohesive forces dominate, causing water to bead up and minimise contact with the surface. This results in spherical droplets because the shape reduces surface area. Conversely, on hydrophilic (water-attracting) surfaces, adhesive forces are stronger. Water molecules are more attracted to the surface than to each other, leading the droplets to spread out. A familiar example is the difference between water droplets on a waxed car (hydrophobic) versus on a paper towel (hydrophilic).
Practice Questions
Water's cohesive properties arise from the attraction between like molecules, primarily due to hydrogen bonding between water molecules. In contrast, adhesion denotes the attraction between different molecules. Water, being a polar molecule, has the capability to bond with other polar or charged surfaces. Capillary action in plants is a result of these two forces working together. Adhesive forces pull water upwards against the walls of the narrow tubes (xylem) in plants, while cohesive forces ensure that water molecules remain together, maintaining a continuous column of water from the roots to the leaves. This mechanism aids in the transportation of water throughout the plant.
Surface tension in water arises primarily due to its cohesive forces, where water molecules at the surface experience an inward force, leading to the minimisation of the surface area and acting as a sort of "elastic sheet". This is attributed to the hydrogen bonding between water molecules. For certain aquatic organisms, such as water striders, this property is of paramount importance. The high surface tension of water provides these organisms with a "platform" to move on, allowing them to "walk" on water without sinking, which is critical for their survival and day-to-day activities in their habitat.
