Learning Biology Through Inquiry: The Biological Properties of Water

The first topic in the new IB Biology syllabus is A1.1 Water. Water featured as a subtopic in the previous syllabus but is now present in its own expanded topic that explores water at a deeper level with more explicit links to other parts of the syllabus, especially the origin of life.

Students will enter IB Biology with varying levels of understanding of water and its properties, depending on their previous studies. An understanding of some of the biological properties of water depends on some knowledge of chemistry and physics. 

This blog shows how inquiry-based approaches can be used to teach this topic, using a series of activities. Some common misconceptions are also outlined, as well links to ToK.

A1.1.2 Hydrogen bonds as a consequence of the polar covalent bonds within water molecules

Activity 1

An introductory question as a starting activity could be for students to: Write down and summarise what they already know about the behaviour and properties of water. (These all of course depend on the presence of relatively strong hydrogen bonds).

This could be followed up with a ”circus” of practical activities, such as:

• Making a paper clip float on still water (the paper clip can be made to sink by changing the surface tension of the water by addition of a few drops of a washing up liquid/detergent).
• Putting talcum powder on a clean flat surface of water and observing it spread, then adding a drop of soap and watching the surface tension forces.
• Adding ice cubes to water (relative densities) and dissolving salt (sodium chloride) and sugar (sucrose) into water to show many ionic and many simple molecular substances are soluble.
• Using pipettes or straws to draw up water to demonstrate cohesion (via a reduction in pressure and the cohesive and adhesive nature of water molecules).

Students could be asked to deduce conclusions about the properties observed.

Activity 2

This is a simple activity where students put ice on liquid water.

• Students will be familiar with ice being of lower density than water (Figure 1), but usually fail to acknowledge the almost unique nature of this phenomenon which is contrary to simple molecular kinetic theory. Another almost unique property is that pressure can melt the solid (ice) rather than freezing the liquid (water).

Figure 1 The structures of water and ice (https://commons.wikimedia.org/wiki/File:Liquid-water-and-ice.png)

• A simple explanation is that hydrogen bonding in ice leads to a very ”open” structure which not only holds the water molecules together, but also apart.  This ”open” but regular structure of ice collapses on heating.

A student discussion could be: Imagine that ice did not float on water. Predict how you think that would affect the biological world?

The unique properties of water and its ability to form crystalline ice with a regular and ordered lattice is believed to depend on its almost tetrahedral distribution  (Figure 2) of electron pairs (two bonding pairs) and two lone (non-bonded) pairs which allow water to form hydrogen bonds with four other water molecules.

Figure 2 The tetrahedral distribution of electrons pairs around the central oxygen atom of a water molecule (https://commons.wikimedia.org/wiki/File:Tetrahedral_Structure_of_Water.png)

An extension of the practical activity is for students to investigate the relative densities by adding ice to ethanol and then ice to water. Students will enjoy varying the proportions of water and ethanol until the ice ‘floats’ in the middle of the liquid in its container. Unfortunately, a freezer (around -20 °C) is not cold enough to freeze ethanol (freezing point of -114 °C).

Activity 3

A Molymod kit could be used to model the structure and the V-shape of water molecules (Figure 3). This could be used to introduce or remind students about chemical concepts such as atom, electron, covalent bond (sharing of electron pairs), lone pair (non-bonded pair) and molecule.

Figure 3 Molymod kit showing water molecules

• Students could then be asked to draw and label a water molecule including the polar bonds showing the dipole (separation of charge) using the delta symbol. It should be stressed that two d⁺’s on the hydrogens are neutralized by the single d^- on the oxygen.  It should be stressed that water molecules are overall electrically neutral.
• If the delta symbol is drawn between the two hydrogen atoms, then any confusion about the magnitude of the partial charges should be avoided (i.e., when the individual bond dipoles are summed they add to the overall molecular dipole).

Activity 4

Deflection of a polar liquid (following a risk assessment), e.g., propanone, ethanol, and water can be achieved by running these liquids through separate burettes and bringing a freshly rubbed dry polythene rod close to the stream as it falls into a beaker below. The experiment works best when there is low humidity and if the polythene rods have been thoroughly dried with a hairdryer prior to use. There must be no naked flames in the laboratory.

• Students can evaluate the relative values of deflection and arrange the liquids in order of polarity [(least polar) propanone, ethanol and water (most polar)]. The greatest deflection will be from water since its molecules have the greatest molecular polarity.
• Students may ask what makes oxygen able to form hydrogen bonds and which other atoms can form hydrogen bonds and why?  A simple answer is that only nitrogen, oxygen and fluorine can form strong hydrogen bonds due to their relatively small values of atomic radius and relatively high values of electronegativity.

Activity 5

One possible student activity is for students to research and compare and contrast the physical properties of water (H₂O) and hydrogen sulfide (H₂S), which have similar molecular structures.  The crucial difference is that hydrogen sulfide does not engage in hydrogen bonding, but its molecules are associated by a similar but weaker interaction known as a dipole-dipole force.

The importance of hydrogen bonding in nucleic acids and protein folding should also be mentioned to “sign post” future topics and concepts, such as base pairing in nucleotides, alpha helices and the beta pleated sheets in proteins.

Activity 6

Students will be very familiar with the solvent properties of water which can best be explained by using a commercial water kit (Figure 4) involving molecular models to explain the solubility of sodium and chloride ions (via hydration of ions) and the low solubility of ethane (due to its non-polar nature and its inability to hydrogen bond with water molecules).

Figure 4 Water model kit

This hydration process (Figure 5) occurs because the partially negatively charged oxygen atoms of water molecules are attracted to the positive sodium ions; conversely, the partially positively charged hydrogen atoms of water molecules are attracted to the negative chloride ions. 

The forces of attraction between the water molecules and the ions are stronger than the forces of attraction between the ions. The hydration shell of water molecules also effectively screens the interionic electrostatic attraction and prevents precipitation of the salt.

Figure 5 The modeling of hydrated ions by water molecules (the chloride ion is green and the sodium ion is dark blue)

Activity 7

The role of water as a reactant and product in hydrolysis and condensation reactions could be introduced, using Molymod kits or at a simpler level using interconnecting bricks.

Activity 8

The thermal properties of water can be demonstrated to students using simple experiments: 

• Specific heat capacity can be demonstrated by placing ice cubes and hot water into large beakers of room temperature water (with the same volume) and measuring the temperature changes.   Specific heat is the amount of energy required to change 1 gram of a substance by 1 degree Celsius. 
• Latent heat can be demonstrated by placing a damp paper towel on the back of their hands and ask students to observe the decrease in skin temperature. The cooling effect is due to the molecules with the highest kinetic energy overcoming the attractive forces (hydrogen bonds) with their neighbours and entering the gas phase. The latent heat is the amount of heat energy needed to convert 1 g of water (at 100 °C) to steam (at 100 °C).

A1.1.3 Cohesion of water molecules due to hydrogen bonding and consequences for organisms

Activity 9

The cohesion of water can be demonstrated by students slowly dropping water onto a coin (or leaf with thick cuticle) and observing the water droplets forming a drop by cohesion. Eventually the drop breaks and spills off the coin under the force of gravity.

Activity 10

Cohesion can also be demonstrated by students carefully filling a glass to the top and then very carefully adding more water until there is an arc of water slightly over the glass. There will come a point when the cohesive forces are overcome and the water spills over.

A1.1.4 Adhesion of water to materials that are polar or charged for organisms

Students should be made clear about the distinction between cohesion and adhesion. Cohesive and adhesive forces are important for the transport of water from the roots to the leaves in plants via the xylem. Water can adhere to other substances via hydrogen bonding, e.g., with cellulose, but adhesion will also involve weaker interactions, known as London (dispersion) forces.

Activity 11

The adhesive properties of water resulting in capillary action can be demonstrated by placing fine capillary tubes into water stained with a dye.

Additional Higher Level

A1.1.7 Relationship between the search for extraterrestrial life and the presence of water

Activity 12

The IB Biology syllabus implies that life can only evolve in the presence of liquid water. On that basis students could be asked to suggest which places in the solar system have or might have liquid water. Alternatively, each small group could be given one of the following to investigate: Mars, Ceres, Europa, Ganymede, Callisto, Enceladus, Titan, Triton or Pluto.

A table could be generated along the following lines: 

• Body (and photo)
• Type of body and average distance from the Sun
• Why it might have life

Misconceptions

Students often have a number of documented misconceptions about hydrogen bonding, which teachers should be aware of, and the correct scientific concept:

• Misconception:  Hydrogen bonding occurs within water molecules. Scientific concept: hydrogen bonding occurs between water molecules.
• Misconception: Covalent bonds (-OH) are broken during melting and boiling. Scientific concept: hydrogen bonds are broken during melting and boiling and formed during condensing and freezing.
• Misconception: The hydrogen bond is any bond that involves hydrogen. Scientific concept: strong hydrogen bonds, an intermolecular force, are only formed by molecules that have hydrogen atoms bonded directly to oxygen, nitrogen, or fluorine.
• Misconception: Hydrogen bonds are induced and formed by neighbouring water molecules. Scientific concept: hydrogen bonds in water molecules are due to the presence of a permanent dipole in the polar –O-H bond.

Theory of Knowledge (TOK)

A TOK case study for confirmation bias (or pathological science) is polywater. In 1961, the Soviet physicist Nikolai Fedyakin, who had been repeatedly forcing ultra-pure water through very thin quartz tubes, discovered small quantities of a type of water with significantly different properties to normal water, e.g., higher boiling and lower freezing point. Polywater was suggested to be a covalently bonded polymerised form of water. 

Students could be encouraged to research this topic and outline how the story ends.

Learn more about Skills-Based Learning in this blog by Andrew Davis.

Resources and Acknowledgements.

For more information, visit titlewave.com.

About the Author:

Chris Talbot has taught chemistry, biology and TOK at schools in Singapore for more than 20 years. He is the author of numerous science textbooks, including Biology for the IB Diploma, Third Edition.