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E3.1 · Explain water polarity, hydrogen bonding, and solvent properties

Learn to explain water polarity, hydrogen bonding, and solvent properties through clear examples and targeted practice.

Ontario Grade 11 Chemistry

Solutions and Solubility

Ontario Grade 11 Chemistry — E3.1

When table salt is stirred into water, the solid can disappear as its particles spread through the liquid. Oil, by contrast, does not mix evenly with water. These observations are connected to how water molecules are built and how they attract other particles. This lesson links the visible behaviour of water to its polarity, hydrogen bonding, and role as a solvent.

What you will learn

Explain how the shape and bond polarities of a water molecule make the molecule polar.
Describe hydrogen bonding between water molecules and explain how it differs from a covalent bond.
Use water’s polarity to explain how it can dissolve many ionic and polar substances.
Explain why water does not dissolve every substance.

1. From bonds to a polar water molecule

A water molecule contains two hydrogen atoms bonded to one oxygen atom. A covalent bond is a bond in which atoms share electrons. The shared electrons in each water bond are not shared equally. Oxygen attracts them more strongly than hydrogen does.

This unequal sharing gives the oxygen end a small negative charge and the hydrogen ends small positive charges. These are partial charges: they are not full ionic charges. We show them with the symbols

\delta-

and

\delta+

The water molecule is bent rather than straight. Because of this shape, the two bond polarities do not cancel. One side of the molecule is slightly negative, and the other side is slightly positive. A molecule with this uneven charge distribution is called polar.

\mathrm{H_2O}

Oxygen attracts shared electrons more strongly than hydrogen does.
Water’s bent shape means its bond polarities do not cancel.
A polar molecule has slightly positive and slightly negative regions.

2. Hydrogen bonding between water molecules

Water molecules attract one another. In particular, the slightly positive hydrogen end of one water molecule is attracted to the slightly negative oxygen end of a nearby water molecule. This attraction is called a hydrogen bond.

A hydrogen bond is an attraction between molecules, not the covalent bond that holds the atoms inside one water molecule together. The covalent bonds are within a molecule; hydrogen bonds connect neighbouring molecules through attraction.

A simple particle sketch would show several bent water molecules. Each oxygen would be marked slightly negative and each hydrogen slightly positive. A dotted line could show an attraction from a hydrogen on one molecule toward an oxygen on a nearby molecule. The dotted line represents a hydrogen bond, not an extra atom-to-atom covalent bond.

\mathrm{H^{\delta+}\cdots O^{\delta-}}

Hydrogen bonding is an attraction between neighbouring molecules.
The attraction involves a slightly positive hydrogen and a nearby slightly negative oxygen.
Do not confuse hydrogen bonds between molecules with covalent bonds within a molecule.

3. Water as a solvent

A solvent is the substance that does the dissolving. A solute is the substance that dissolves. Together, the solvent and dissolved solute form a solution. Water is often called a solvent because it can dissolve many substances.

Water’s polarity helps explain this property. When an ionic substance such as sodium chloride is placed in water, the water molecules are attracted to the charged particles. The oxygen side, which is slightly negative, is attracted to positive ions. The hydrogen sides, which are slightly positive, are attracted to negative ions.

Water molecules can surround separated ions and help keep them spread through the solution. In the dissolved-state notation, the label (aq) means that a particle is in water solution. For sodium chloride, the symbolic representation shows the solid forming separate aqueous ions. The atoms and overall charge are conserved.

Water can also dissolve many polar molecular substances. Their slightly charged regions can attract water molecules. This does not mean every polar substance dissolves equally well; the identity of the solute also matters.

Nonpolar substances, such as oil, do not have separated positive and negative regions like water does. Water therefore does not attract their particles in the same way. Oil and water do not form one even mixture under ordinary conditions. So, water is a useful solvent, but it is not a universal solvent.

\mathrm{NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)}

A solvent dissolves a solute to form a solution.
Water’s partial charges attract positive and negative ions in different ways.
Many ionic and polar substances dissolve in water, but not all substances do.
The notation (aq) identifies a particle present in water solution.

4. Connect the particle model to what you observe

The particle model helps explain why a substance that seems to vanish may still be present. When salt dissolves, its particles are distributed through the water rather than disappearing. The mixture looks uniform because the dissolved particles are too small to see individually.

The model also explains why stirring oil and water does not make them into one lasting, even mixture. Water molecules attract one another and interact with polar or charged particles in ways that do not work as well with nonpolar oil particles.

To explain a solvent property, connect three ideas: the solute’s particles, water’s partial charges, and the attractions between them. Avoid saying only that water dissolves something because it is water. The molecular explanation is that water’s polarity allows particular attractions to form.

Dissolving spreads solute particles through a solvent; it does not make them cease to exist.
Water’s polarity helps determine which particles it can interact with.
The particle model explains visible mixing and dissolving behaviour.

Worked example

Explaining why salt dissolves in water

Use water’s polarity to explain what happens when sodium chloride is added to water. Include the particle-level attractions and a symbolic representation.

Identify the particles
Sodium chloride is an ionic substance. Its particles include positive sodium ions and negative chloride ions. Water molecules have a slightly negative oxygen end and slightly positive hydrogen ends.
$\mathrm{Na^+,\ Cl^-,\ H^{\delta+},\ O^{\delta-}}$
Match opposite charges
The slightly negative oxygen end of water is attracted to positive sodium ions. The slightly positive hydrogen ends are attracted to negative chloride ions. These attractions help water molecules surround the ions.
Represent the dissolved particles
The dissolved particles are represented as separate aqueous ions. The equation shows one sodium ion and one chloride ion, so the atoms and total charge are conserved.
$\mathrm{NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)}$

Answer: Water’s partial charges attract the oppositely charged ions. Water molecules surround the ions, allowing them to spread through the solution as aqueous sodium and chloride ions.

Check: The left side contains one sodium atom and one chlorine atom with zero net charge. The right side contains one of each atom, and the charges add to zero.

Common mistakes and how to avoid them

Calling water’s partial charges full ionic charges.

Correction: Water’s oxygen and hydrogen atoms have partial charges from unequal sharing in covalent bonds. They are not separate ions.

Saying a hydrogen bond is the bond holding the atoms inside one water molecule together.

Correction: The atoms within a water molecule are held by covalent bonds. Hydrogen bonds are attractions between neighbouring molecules.

Claiming that water dissolves every substance.

Correction: Water dissolves many ionic and polar substances, but it does not dissolve all substances. Nonpolar substances such as oil do not mix evenly with water.

Assuming dissolved particles have disappeared.

Correction: Dissolved particles remain in the solution, spread among the water molecules.

Lesson summary

Water is polar because its bonds share electrons unequally and its bent shape leaves the molecule with uneven charge regions.
Hydrogen bonding is an attraction between the slightly positive hydrogen end of one water molecule and a slightly negative oxygen end of a nearby molecule.
Water’s partial charges attract ions and many polar particles, helping water act as a solvent.
Water does not dissolve every substance; nonpolar oil, for example, does not mix evenly with water.

Check your understanding

Question 1

Why is a water molecule polar?

Its atoms share electrons equally and the molecule is straight.
Its bonds share electrons unequally, and its bent shape leaves uneven charge regions.
It is made of separate positive and negative ions.
Its hydrogen bonds give each atom a full ionic charge.

Show answer and explanation

Its bonds share electrons unequally, and its bent shape leaves uneven charge regions.

Oxygen attracts shared electrons more strongly than hydrogen, and water’s bent shape means the bond polarities do not cancel.

Question 2

Which statement best describes a hydrogen bond in water?

It is the covalent bond between oxygen and hydrogen within one water molecule.
It is an attraction between a slightly positive hydrogen on one molecule and a slightly negative oxygen on another.
It is a full charge transfer that makes water into ions.
It is an attraction between two oxygen atoms inside one molecule.

Show answer and explanation

It is an attraction between a slightly positive hydrogen on one molecule and a slightly negative oxygen on another.

A hydrogen bond is an attraction between neighbouring molecules, involving the partial charges on hydrogen and oxygen.

Question 3

Why can water help dissolve sodium chloride?

Water’s partial charges attract the positive and negative ions.
Water changes the ions into neutral atoms.
Water has no partial charges, so it cannot attract the ions.
Hydrogen bonds turn sodium chloride into water molecules.

Show answer and explanation

Water’s partial charges attract the positive and negative ions.

The slightly negative oxygen end is attracted to positive ions, while the slightly positive hydrogen ends are attracted to negative ions.

Key terms

Covalent bond: A bond in which atoms share electrons.
Partial charge: A small positive or negative charge caused by unequal sharing of electrons.
Polar molecule: A molecule with an uneven distribution of charge.
Hydrogen bond: An attraction between a slightly positive hydrogen on one molecule and a slightly negative region, such as oxygen, on a nearby molecule.
Solvent: The substance that does the dissolving.
Solute: The substance that dissolves in a solvent.
Solution: A uniform mixture formed when a solute dissolves in a solvent.
Aqueous: Present in water solution; shown by the state label (aq).

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation E3.1. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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