(I once asked a class, "What is the most important chemical on Earth?" They guessed oxygen, carbon, even gold. Then I held up a glass of water and said, "You are 60% of this. Every living thing is mostly this. And it is the only substance on Earth that exists naturally as a solid, a liquid, and a gas — all at the same time." That class never looked at water the same way again. I hope you won't either.) Every morning, you wake up, brush your teeth, drink a glass of water, and get on with your day. You rarely stop to think that the water you just drank is one of the strangest and most important substances in the universe. It dissolves more substances than any other liquid. It expands when it freezes — which is why ice floats, and why fish survive in frozen lakes. It has an unusually high boiling point for such a small molecule. And without it, no chemical reaction in your body could happen. In this chapter, we're going to look at water like a chemist — its structure, its properties, its types, and how we make it safe to drink. Then we'll meet water's lighter cousin: hydrogen — the most abundant element in the universe and a potential fuel of the future. Let's dive in.
Water is a compound, not an element. Its chemical formula is H₂O — two hydrogen atoms covalently bonded to one oxygen atom.
But the shape of the molecule matters. The two hydrogen atoms are not in a straight line with the oxygen — they're arranged at an angle of about 104.5°. This bent shape, combined with the fact that oxygen is more electronegative than hydrogen, makes the oxygen end slightly negative (δ⁻) and the hydrogen ends slightly positive (δ⁺).
This creates a polar molecule — a molecule with a positive end and a negative end. This polarity is the secret behind almost every unusual property of water.
(Think of a magnet — north on one end, south on the other. A water molecule is like a tiny magnet: negative near the oxygen, positive near the hydrogens. When water molecules meet, they attract each other, negative-to-positive. That attraction is called hydrogen bonding, and it explains everything that follows.)
Because water molecules are polar, they attract each other. The slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of another. This attraction is called a hydrogen bond.
Hydrogen bonds are much weaker than covalent bonds, but there are so many of them in water that they have a huge effect on its properties.
| Property | Explanation Using Hydrogen Bonding |
|---|---|
| High boiling point (100°C) | Extra energy is needed to break the hydrogen bonds between molecules before they can escape as a gas. |
| High surface tension | Water molecules at the surface pull together, creating a "skin" that can support small insects. |
| Ice floats on water | In ice, hydrogen bonds hold molecules in an open, hexagonal lattice — so ice is less dense than liquid water. |
| High specific heat capacity | Water absorbs a lot of heat before its temperature rises — this is why oceans moderate coastal climates. |
| Universal solvent | Polar water molecules surround and separate ions and other polar molecules. |
| Property | Value / Description |
|---|---|
| Appearance | Colourless, odourless, tasteless liquid |
| Boiling point | 100°C at standard atmospheric pressure |
| Freezing point | 0°C at standard atmospheric pressure |
| Density | 1 g/cm³ at 4°C (maximum density) |
| Solubility | Dissolves many ionic and polar substances; poor solvent for non-polar substances |
| Conductivity | Pure water is a very poor conductor of electricity; the presence of dissolved ions makes it a good conductor |
Water is often called the universal solvent because it dissolves more substances than any other liquid. But it's not really universal — it doesn't dissolve everything. Oil, for example, doesn't dissolve in water. What water dissolves best are ionic compounds (like salt) and polar molecules (like sugar).
Why does water dissolve salt? When salt (NaCl) is placed in water, the slightly negative oxygen ends of water molecules surround the positive sodium ions (Na⁺), and the slightly positive hydrogen ends surround the negative chloride ions (Cl⁻). These attractions pull the ions away from the crystal, and the salt dissolves. The process is called hydration.
(Here's a useful way to think about it: water is like a crowd of tiny magnets that surround and pull apart the ions in a crystal. If the ions are strongly attracted to water, the crystal dissolves. If the substance is non-polar — like oil — the water molecules have no grip, and the substance stays separate.)
Not all water is the same. The water from your tap might be soft or hard depending on where you live and where the water has come from.
| Type | Definition | Behaviour with Soap |
|---|---|---|
| Soft water | Water that contains few or no dissolved calcium or magnesium ions. | Lathers easily with soap — produces a good amount of foam with little soap. |
| Hard water | Water that contains dissolved calcium ions (Ca²⁺) and/or magnesium ions (Mg²⁺). | Does not lather easily — forms a scum (precipitate) with soap, wasting soap. |
The hardness of water comes from the rocks it has passed through. Water flowing over limestone (calcium carbonate) or dolomite (calcium magnesium carbonate) picks up calcium and magnesium ions.
There are two types of hardness — and the difference matters because they're removed in different ways.
| Type | Cause | How to Remove |
|---|---|---|
| Temporary hardness | Caused by dissolved calcium hydrogencarbonate, Ca(HCO₃)₂, or magnesium hydrogencarbonate, Mg(HCO₃)₂. | Boiling — the hydrogencarbonates decompose to form insoluble carbonates, which precipitate out. |
| Permanent hardness | Caused by dissolved calcium sulphate (CaSO₄) or magnesium sulphate (MgSO₄), or other calcium/magnesium salts that are not hydrogencarbonates. | Cannot be removed by boiling. Requires chemical treatment — washing soda (sodium carbonate) or ion exchange. |
Important: Boiling removes temporary hardness but leaves permanent hardness untouched. That's why hard water that stays hard after boiling is called "permanent".
| Advantages | Disadvantages |
|---|---|
| Contains calcium, which is good for bones and teeth. | Wastes soap — forms scum, so more soap is needed. |
| Some studies suggest hard water may reduce the risk of heart disease. | Forms scale (limescale) in kettles, pipes, and boilers — reduces efficiency and can block pipes. |
| Tastes better to many people than soft water. | Scale deposits in industrial boilers can cause overheating and even explosions if not managed. |
| Provides a source of dietary calcium and magnesium. | Not ideal for washing — leaves deposits on dishes, clothes, and skin. |
If you live in a hard-water area, you'll want to soften the water for washing, laundry, and industrial use. Here are the main methods:
| Method | How It Works | Removes |
|---|---|---|
| Boiling | Decomposes hydrogencarbonates into insoluble carbonates, which settle out. | Temporary hardness only |
| Adding washing soda (Na₂CO₃) | Carbonate ions react with Ca²⁺ and Mg²⁺ to form insoluble precipitates. | Both temporary and permanent hardness |
| Adding slaked lime (Ca(OH)₂) | Reacts with hydrogencarbonates to form insoluble calcium carbonate. | Temporary hardness only |
| Ion exchange | Hard water is passed through a resin containing sodium ions. The resin swaps sodium ions for calcium and magnesium ions, softening the water. | Both types of hardness |
| Distillation | Water is boiled and condensed, leaving all dissolved salts behind. | All hardness (but expensive) |
Before water reaches your tap, it goes through a treatment process to make it safe to drink. Here are the main stages:
(Here's a sobering fact: nearly 2 billion people worldwide still lack access to safely managed drinking water. Every stage of water treatment you just read about is something many people can only dream of. Never take your tap for granted.)
Hydrogen is the lightest and most abundant element in the universe. It's a colourless, odourless, tasteless gas — and it's highly flammable. On Earth, it rarely exists on its own; it's almost always combined with other elements, most commonly with oxygen (in water) and carbon (in hydrocarbons).
In the laboratory, hydrogen is usually prepared by reacting a reactive metal with a dilute acid. The most common method uses zinc and dilute hydrochloric acid:
Zn + 2HCl → ZnCl₂ + H₂
The hydrogen gas is collected by downward delivery (also called upward displacement of air) because hydrogen is less dense than air. Alternatively, it can be collected over water since it is only slightly soluble.
Test for hydrogen: Hold a lit splint at the mouth of the test tube. If hydrogen is present, you'll hear a distinctive "pop" sound. This is the classic test — learn it.
Safety warning: Hydrogen is highly flammable and forms explosive mixtures with air. Never collect hydrogen near a naked flame. Always ensure the apparatus is airtight before lighting the gas.
| Property | Description |
|---|---|
| Appearance | Colourless, odourless, tasteless gas |
| Density | Least dense of all gases — about 1/14th the density of air |
| Solubility | Slightly soluble in water |
| Reactivity | Highly flammable; burns in oxygen to form water |
| Reaction with oxygen | 2H₂ + O₂ → 2H₂O (releases a lot of energy — used in fuel cells and rockets) |
| Reducing agent | Can remove oxygen from metal oxides (e.g., CuO + H₂ → Cu + H₂O) |
| Use | Explanation |
|---|---|
| Ammonia production (Haber process) | Hydrogen reacts with nitrogen to make ammonia, which is used in fertilisers. |
| Fuel cells | Hydrogen reacts with oxygen to produce electricity, with water as the only by-product — a clean energy source. |
| Rocket fuel | Liquid hydrogen is used as a rocket propellant because it releases enormous energy when burned. |
| Hydrogenation of oils | Hydrogen is added to unsaturated vegetable oils to make margarine and other solid fats. |
| Welding | Oxy-hydrogen flames are used for welding and cutting metals. |
| Weather balloons | Because it is so light, hydrogen was historically used in balloons and airships (though helium is safer). |
(Hydrogen is often called the fuel of the future. When it burns, the only product is water — no carbon dioxide, no pollution. The challenge is producing it cleanly and storing it safely. If we solve those problems, hydrogen could change the world. Maybe you'll be the one to solve them.)
Question: Explain, in terms of hydrogen bonding, why water has a much higher boiling point than hydrogen sulphide (H₂S), even though they have similar molecular sizes.
Solution: Oxygen is more electronegative than sulphur, so the O–H bond in water is more polar than the S–H bond in hydrogen sulphide. This means water molecules form stronger hydrogen bonds with each other than H₂S molecules do. Extra energy is needed to break these hydrogen bonds before water can boil, so water has a much higher boiling point than hydrogen sulphide.
Question: Describe a simple test to distinguish between hard water and soft water in the laboratory.
Solution: Add a few drops of soap solution to a sample of each water in separate test tubes and shake. Soft water lathers easily and produces a good amount of foam. Hard water produces little lather and forms a white scum (precipitate). The more scum and the less lather, the harder the water.
Question: A sample of water contains dissolved calcium sulphate. Explain why boiling does not soften it, and state one method that would.
Solution: Calcium sulphate is not a hydrogencarbonate, so boiling does not decompose it. Boiling only removes temporary hardness caused by hydrogencarbonates. To remove permanent hardness caused by calcium sulphate, you would add washing soda (sodium carbonate), which reacts with the calcium ions to form insoluble calcium carbonate. Alternatively, ion exchange or distillation would also work.
1. Draw or describe the structure of a water molecule, including its shape and polarity.
A water molecule consists of one oxygen atom covalently bonded to two hydrogen atoms. The molecule is bent, with an angle of about 104.5° between the hydrogen atoms. Because oxygen is more electronegative than hydrogen, the oxygen end is slightly negative and the hydrogen ends are slightly positive, making water a polar molecule.
2. Why does ice float on water?
In ice, hydrogen bonds hold water molecules in an open, hexagonal lattice structure. This arrangement is less dense than the arrangement of molecules in liquid water, so ice floats.
3. What is the difference between temporary and permanent hardness?
Temporary hardness is caused by dissolved calcium or magnesium hydrogencarbonates and can be removed by boiling. Permanent hardness is caused by other calcium or magnesium salts (such as sulphates) and cannot be removed by boiling.
4. Describe the test for hydrogen gas.
Hold a lit splint at the mouth of the test tube. If hydrogen is present, a "pop" sound is heard.
5. Name two methods used to soften hard water.
Any two of: boiling (for temporary hardness), adding washing soda, adding slaked lime, ion exchange, distillation.
6. State two uses of hydrogen gas.
Any two of: production of ammonia, fuel cells, rocket fuel, hydrogenation of oils, welding, weather balloons.
7. Why is chlorination an important step in water treatment?
Chlorination kills bacteria and other microorganisms that can cause disease, making the water safe to drink.
Question 1 [10 marks]
(a) Describe the structure of a water molecule and explain why it is polar. [3]
(b) Explain, using hydrogen bonding, why water has a high boiling point. [3]
(c) Explain why water is described as a "universal solvent" and give one substance it does not dissolve. [2]
(d) State one reason why water's high specific heat capacity is important in the natural world. [2]
(a) A water molecule has one oxygen atom covalently bonded to two hydrogen atoms in a bent shape (about 104.5°). Oxygen is more electronegative than hydrogen, so the oxygen end is slightly negative and the hydrogen ends are slightly positive. This makes the molecule polar. (3 marks: 1 for structure, 1 for electronegativity, 1 for polarity.)
(b) Water molecules form hydrogen bonds with each other — the slightly positive hydrogen of one molecule attracts the slightly negative oxygen of another. These bonds must be broken before water molecules can escape as a gas. This requires extra energy, so the boiling point is higher than expected. (3 marks: 1 for hydrogen bonds, 1 for "must be broken", 1 for "extra energy".)
(c) Water is a good solvent for ionic and polar substances because its polar molecules surround and separate ions. It does not dissolve non-polar substances such as oil. (2 marks: 1 for explanation, 1 for example.)
(d) Water's high specific heat capacity means it absorbs a lot of heat before its temperature rises significantly. This moderates the climate of coastal areas and helps keep aquatic ecosystems stable. (2 marks: 1 for statement, 1 for consequence.)
Question 2 [10 marks]
(a) Distinguish between soft water and hard water. [2]
(b) Describe a simple test to distinguish between the two. [2]
(c) Describe one method used to soften permanently hard water, and explain how it works. [4]
(d) State one advantage and one disadvantage of hard water. [2]
(a) Soft water contains few or no dissolved calcium or magnesium ions. Hard water contains dissolved calcium and/or magnesium ions. (2 marks: 1 for each.)
(b) Add soap solution to a sample of each water and shake. Soft water lathers easily; hard water forms a scum and produces little lather. (2 marks: 1 for method, 1 for observation.)
(c) One method: add washing soda (sodium carbonate). The carbonate ions react with dissolved calcium and magnesium ions to form insoluble precipitates (e.g., calcium carbonate), which settle out and can be filtered off. (4 marks: 2 for method, 2 for explanation.)
(d) Advantage: hard water provides calcium, which is good for bones and teeth. Disadvantage: hard water wastes soap and forms scale in pipes and kettles. (2 marks: 1 for each.)
Question 3 [10 marks]
(a) Describe how hydrogen is prepared in the laboratory from zinc and hydrochloric acid, including the equation. [3]
(b) Describe the test for hydrogen gas. [2]
(c) State two uses of hydrogen. [2]
(d) Explain why hydrogen is considered a clean fuel, and state one challenge associated with its use. [3]
(a) Zinc granules are placed in a flask with dilute hydrochloric acid. The reaction is: Zn + 2HCl → ZnCl₂ + H₂. Hydrogen gas is collected by downward delivery (or over water). (3 marks: 1 for method, 1 for equation, 1 for collection method.)
(b) Hold a lit splint at the mouth of the test tube. If hydrogen is present, a "pop" sound is heard. (2 marks: 1 for method, 1 for sound.)
(c) Any two of: production of ammonia, fuel cells, rocket fuel, hydrogenation of oils, welding. (2 marks: 1 for each.)
(d) Hydrogen is a clean fuel because when it burns, the only product is water — no carbon dioxide or other pollutants. One challenge is that it is highly flammable and forms explosive mixtures with air, so it must be stored and handled very carefully. Producing it cleanly and economically is another challenge. (3 marks: 1 for "clean fuel" reason, 2 for one valid challenge.)
Challenge 1: A student says, "Water is a universal solvent because it dissolves everything." Is the student correct? Explain, using examples of substances water does and does not dissolve.
The student is not correct. Water is a very good solvent for ionic compounds and polar molecules — for example, it dissolves salt (an ionic compound) and sugar (a polar molecule) easily. However, water does not dissolve non-polar substances such as oil, grease, and wax. These substances have no charged ends for water molecules to attract, so they remain separate. Water is a good solvent, but not a universal one.
Challenge 2: Explain why temporary hardness can be removed by boiling, but permanent hardness cannot.
Temporary hardness is caused by dissolved calcium and magnesium hydrogencarbonates. When boiled, these hydrogencarbonates decompose into insoluble carbonates (e.g., calcium carbonate), which precipitate out and can be filtered off — so the water is softened. Permanent hardness is caused by other calcium and magnesium salts, such as sulphates and chlorides, which do not decompose on boiling. They remain dissolved in the water, so boiling has no effect on them. Removing permanent hardness requires chemical treatment (e.g., washing soda or ion exchange).
Challenge 3: Water expands when it freezes, which is very unusual. Explain how hydrogen bonding causes this, and why it is important for life in cold climates.
In liquid water, molecules are close together and constantly moving, with hydrogen bonds continually forming and breaking. When water freezes, the molecules settle into a regular lattice held together by hydrogen bonds. This lattice has open spaces between the molecules, making ice less dense than liquid water. Because ice is less dense, it floats on top of liquid water rather than sinking. This is crucial for life in cold climates: the floating ice layer insulates the water below, allowing aquatic organisms to survive in unfrozen water beneath. If ice sank, lakes and rivers would freeze solid from the bottom up, and most aquatic life would die.
"Water is the quiet hero of chemistry — it dissolves, it protects, and it makes life possible. Respect it, and never take it for granted."
