Question Video: Remembering Whether Water Molecules Are Classed as Polar or Nonpolar Solvents | Nagwa Question Video: Remembering Whether Water Molecules Are Classed as Polar or Nonpolar Solvents | Nagwa

Question Video: Remembering Whether Water Molecules Are Classed as Polar or Nonpolar Solvents Chemistry • First Year of Secondary School

Is a water, H₂O, molecule a polar or nonpolar solvent?


Video Transcript

Is a water, H2O, molecule a polar or nonpolar solvent?

Before discussing whether water is polar or nonpolar, we need to recall the definition of electronegativity. Electronegativity is a measure of the ability of an atom to attract bonding pairs of electrons from covalent bonds. It is given the Greek symbol 𝜒, and the Pauling scale is often used to quantify it. Fluorine is the most electronegative atom. On the Pauling scale, it has a value of approximately four. Nitrogen, oxygen, and chlorine are also very electronegative. Generally speaking, electronegativity increases as group number increases and period number decreases. But we ignore the noble gases as they have a full valence shell, thus don’t attract electrons.

Now that we know what electronegativity is, let’s look at how it determines whether a bond is polar or nonpolar. In a covalent bond between an atom with high electronegativity and an atom with low electronegativity, for example, fluorine and hydrogen, the bonding electrons that make up the covalent bond would be pulled towards the fluorine atom, as in a covalent bond between two atoms that have significantly different electronegativities, the bonding electrons are pulled towards the more electronegative atom. As electrons are negatively charged and they’re pulled closer to the fluorine atom, the fluorine atom now has a partial negative charge. This means that the hydrogen atom has a partial positive charge. This difference in charges creates what we call a dipole.

A dipole is created from unequal sharing of electron density between atoms. This is the symbol we use to represent a dipole, where the arrow points to the partially negative end. We say that this bond is polar. In a covalent bond between two atoms which are the same, for example, a diatomic fluorine molecule, the electronegativities are the same. This means that the bonding electrons are attracted to both atomic nuclei equally. This bond is considered to be nonpolar. Some covalent bonds between different atoms are also considered to be nonpolar.

For bonds between atoms such as hydrogen and carbon, which have similar electronegativities, the bonding electrons are only slightly more attracted to the carbon than they are to the hydrogen. Thus, a carbon-hydrogen bond is also considered to be nonpolar. Generally speaking, for a bond to be polar, the electronegativity difference between the two atoms needs to be between 0.5 and 1.7.

Now that we know what makes a bond polar or nonpolar, let’s look at the bonds involved in the molecule given in the question. The question asks us about water, or H2O. Water molecules only contain one type of bond, an OH bond. Oxygen has a value of 3.44 on the Pauling electronegativity scale, and hydrogen has a value of 2.2. The difference between these values is 1.24. As this is a large difference, we can determine that the OH bond is a polar bond, although it’s more precise to have the actual electronegativity values. We could determine that this bond is polar by looking at the periodic table. As oxygen and hydrogen are far apart on the periodic table, it’s likely that their electronegativity values will be very different. Thus, the OH bond is polar.

It’s very important to note that although we’ve determined that the OH bond is polar, it doesn’t necessarily mean that a water molecule will be polar. To help explain this concept, let’s first look at a carbon-oxygen bond. A carbon-oxygen bond is polar. Oxygen is more electronegative than carbon, so the arrow that represents the dipole points towards the oxygen atom. A molecule of carbon dioxide contains two carbon-oxygen double bonds. So we also need to draw an arrow to represent the dipole between the carbon atom and the other oxygen atom. The bond dipoles are in opposite directions, so they essentially cancel each other out. So, although the bonds in CO2 have a dipole, carbon dioxide has no net dipole moment. Thus, it is nonpolar.

Like carbon dioxide, water has two polar bonds, but unlike carbon dioxide, which has a linear geometry, water has a bent geometry. The bond dipoles in water don’t cancel each other out, so there is a net molecular dipole moment. Therefore, water is a polar molecule. Water molecules are highly polar, and they have strong electrostatic interactions with other charged and polar molecules. Thus, many other charged and polar species will dissolve in water. We can therefore refer to water as a polar solvent.

So, the answer to the question “Is a water molecule a polar or nonpolar solvent?” is a polar solvent.

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