Why water behaves the way it does
HESI A2 topic: Water
- Water is polar: the oxygen end is slightly negative, hydrogens slightly positive.
- Hydrogen bonds between water molecules cause cohesion, surface tension, and adhesion.
- High specific heat means water warms and cools slowly.
- Ice floats: hydrogen bonds hold its molecules farther apart, so it is less dense.
- Each pH unit is a tenfold change; blood stays near pH 7.4.
Key facts
- Oxygen is more electronegative than hydrogen, so water's polar covalent bonds leave oxygen slightly negative and the hydrogens slightly positive.
- A hydrogen bond is a weak attraction between a slightly positive hydrogen of one molecule and a slightly negative atom of another; it is weaker than a covalent bond.
- Cohesion is water sticking to water (it causes surface tension); adhesion is water sticking to other surfaces, such as glass, which lets it climb thin tubes.
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- Water's specific heat is 1 calorie per gram per degree Celsius, about five times that of sand, so water changes temperature slowly.
- Evaporating water absorbs a lot of heat, which is why sweating cools the body.
- Ice is less dense than liquid water because hydrogen bonds hold its molecules farther apart, so ice floats.
- pH 7 is neutral, below 7 is acidic, above 7 is basic, and each step of one unit is a tenfold change in hydrogen ion concentration; blood is about pH 7.4.
Quick check1 question · 2 more at the end
A water strider insect can stand on the surface of a pond without sinking. Which property of water mainly explains this?
Correct answer: B.
Cohesion, the attraction of water molecules to each other through hydrogen bonds, creates surface tension at the surface of water. Surface tension lets light objects such as a needle or a water strider rest on the surface. Adhesion is water sticking to other surfaces, and specific heat is about temperature change, not support.
A Specific heat describes how much heat water absorbs before its temperature changes; it does not hold objects up.
B Correct: cohesion between water molecules creates surface tension, which supports a water strider.
C Adhesion is water attaching to other surfaces, such as climbing a glass tube; it confuses water-to-surface with water-to-water attraction.
D The low density of ice explains why ice floats, not why an insect can stand on liquid water.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Water is polar: its oxygen end is slightly negative and its hydrogen ends slightly positive. Neighboring molecules attract each other through hydrogen bonds. Those bonds explain cohesion and surface tension, adhesion, water's high specific heat, evaporative cooling, ice floating because it is less dense, and water's power to dissolve polar and ionic substances.
1 Why is water a polar molecule?Water is polar because its oxygen pulls on shared electrons harder than its hydrogens do. 1 min
Water is polar because its oxygen pulls on shared electrons harder than its hydrogens do. Oxygen is more electronegative, so the electrons in each oxygen–hydrogen bond spend more time near the oxygen. These are polar covalent bonds.
The result is a molecule with a slightly negative oxygen end (δ−) and slightly positive hydrogen ends (δ+). Almost every special property of water comes from this uneven charge.
2 What are hydrogen bonds?A hydrogen bond is a weak attraction between the slightly positive hydrogen of one molecule and the slightly negative atom of another. 1 min
A hydrogen bond is a weak attraction between the slightly positive hydrogen of one molecule and the slightly negative atom of another. In water, the δ+ hydrogen of one molecule is drawn to the δ− oxygen of a neighbor.
Each hydrogen bond is weak and easily broken, much weaker than a covalent bond. In liquid water they constantly form, break, and re-form.
There are so many of them, though, that together they give water its unusual behavior. Hydrogen bonds also help stabilize the shapes of proteins and DNA.
3 How do cohesion and adhesion work?Water molecules stick to each other and to other charged surfaces, and that stickiness shows up in everyday life. 1 min
Water molecules stick to each other and to other charged surfaces, and that stickiness shows up in everyday life.
| Property | What sticks to what | Example |
|---|---|---|
| Cohesion | Water to water | Water forms a dome above the rim of a full glass |
| Surface tension | Cohesion at the surface | A needle or a water strider rests on the surface |
| Adhesion | Water to another surface | Water climbs up a thin glass tube (capillary action) |
In plants, cohesion and adhesion together help pull water up from the roots to the leaves.
4 Why does water resist temperature change?Water changes temperature slowly because its hydrogen bonds soak up heat before the molecules can speed up. 1 min
Water changes temperature slowly because its hydrogen bonds soak up heat before the molecules can speed up. Specific heat is the heat one gram must absorb or lose to change by 1 °C. For water it is 1 calorie, about five times more than sand, which is why land cools faster than the sea at night.
Turning liquid water into vapor also takes a lot of energy, because hydrogen bonds must be broken (a high heat of vaporization). As water evaporates, it absorbs that heat, so the surface left behind cools. This evaporative cooling is why sweat, which is mostly water, cools your skin.
5 Why does ice float?Ice floats because it is less dense than liquid water. 1 min
Ice floats because it is less dense than liquid water. When water freezes, hydrogen bonds lock the molecules into a pattern that holds them farther apart than in the liquid.
This matters for life. Floating ice forms an insulating layer on top of a pond, so the water underneath stays liquid and organisms survive the winter.
6 Why is water such a good solvent?Water dissolves many substances because its charged ends can surround other charged or polar particles. 1 min
Water dissolves many substances because its charged ends can surround other charged or polar particles. Substances that dissolve easily in water are hydrophilic. When table salt (NaCl) dissolves, it separates into Na⁺ and Cl⁻ ions, and water molecules cluster around each ion in a sphere of hydration.
Nonpolar substances such as oils and fats are hydrophobic. They have no charges for water to grab, so they stay separate.
7 What does pH measure?pH tells you how acidic or basic a water solution is, based on its hydrogen ion (H⁺) concentration. 1 min
pH tells you how acidic or basic a water solution is, based on its hydrogen ion (H⁺) concentration. Pure water is neutral at pH 7. Below 7 is acidic and above 7 is basic (alkaline).
The scale is logarithmic: one pH unit is a tenfold change in H⁺, and two units is a hundredfold change. The stomach sits around pH 1 to 2, while blood stays near 7.4 thanks to buffers such as bicarbonate. For more on acids and bases, see our acids and bases lesson.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Hydrogen bonds form between water molecules; the bonds holding H and O together inside one molecule are covalent, not hydrogen bonds.
Watch out: Cohesion is water-to-water attraction and adhesion is water-to-other-surface attraction, not the other way around.
Watch out: A high specific heat means water warms and cools slowly, not quickly.
Watch out: Ice floats because hydrogen bonds hold its molecules farther apart, making it less dense than liquid water, not because frozen water contains trapped air.
Watch out: Oils do not dissolve in water because they are nonpolar (hydrophobic), not because water is a poor solvent.
How to remember it
Memory hook: "Cohesion = co-workers (water with water); Adhesion = adds something else (water with a surface)." Then trace every property back to one cause: polar molecule → hydrogen bonds.
Self-check:
- Explain in one sentence why the oxygen end of water is slightly negative.
- Say why a lake freezes from the top down instead of the bottom up.
- A solution goes from pH 6 to pH 4. By what factor did its H⁺ concentration change?