Chemical Reactions — Rearranging Atoms

1.Physical changes and chemical changes

Ice melting or paper being cut are physical changes, in which only the shape or state of a substance changes. By contrast, paper burning into ash and smoke, iron rusting, and meat changing color and flavor as it cooks are chemical changes, in which new substances different from the original ones are formed.

What a chemical change really is, is a rearrangement of atoms. Bonds between atoms break and the atoms bond with new partners, creating substances with different properties. A change in color, the release of a gas, the formation of a precipitate, and heat or light flowing in or out are clues that a chemical change has taken place.

2.Conservation of mass and chemical equations

The kinds and numbers of atoms do not change before and after a chemical reaction. So the total mass of the substances before the reaction equals the total mass of the substances after it. This is the law of conservation of mass.

When you burn wood, the ash looks much lighter than the original wood. The mass didn't disappear; carbon dioxide and water vapor escaped into the air. Conversely, when you burn steel wool, it combines with oxygen from the air and gains mass. If you run the reaction inside a sealed container, you can confirm that the mass is the same before and after.

A chemical equation writes the reactants on the left and the products on the right as chemical formulas, joined by an arrow. Then coefficients are placed in front of the formulas so that the number of atoms is the same on both sides. The small numbers inside a formula (the 2 in H₂O) change the substance itself, so you never alter them; you adjust only the coefficients in front.

3.Calculating amounts from an equation — how much carbon dioxide does burning fuel make?

A balanced equation tells you how many atoms react, so if you know the relative masses of the atoms, you can also calculate how many grams of each substance react and form. Here we use approximate atomic masses: hydrogen 1, carbon 12, oxygen 16. The mass of a molecule is the sum of the atomic masses of all its atoms.

This calculation turns up one surprising result: the carbon dioxide produced by burning a fuel is heavier than the original fuel. That is because each carbon atom in the fuel combines with two oxygen atoms from the air. This doesn't violate the law of conservation of mass; the mass of the oxygen taken from the air has been added.

4.Acids, bases, and pH

An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water; acids taste sour and often react with metals to give off hydrogen gas. Vinegar, lemon juice, and soda are acidic. A base is a substance that releases hydroxide ions (OH⁻) when dissolved in water; bases taste bitter and feel slippery. Soap, baking soda dissolved in water, and drain cleaner are basic. Outside a lab, never test by taste or touch.

How acidic or basic something is, is expressed as pH. At 25 °C, pH 7 is neutral, below 7 is acidic, and above 7 is basic. pH is a logarithmic scale on which each difference of 1 means a 10-fold difference in hydrogen ion concentration. So a solution at pH 3 has 10 × 10 = 100 times the hydrogen ion concentration of a solution at pH 5. Logarithmic scales are covered in Math Basics, Lesson 5.

When an acid and a base meet, the hydrogen ions and hydroxide ions combine into water and each weakens the other's properties; this is called a neutralization reaction. Using lemon juice to cut the fishy smell of fish (caused by basic substances) is an everyday example.

5.Oxidation, reduction, and combustion

A reaction in which a substance gains oxygen or loses electrons is called oxidation, and a reaction in which a substance loses oxygen or gains electrons is called reduction. Whenever one substance is oxidized, another must be reduced, so the two always happen together. Iron rusting when it meets oxygen and water in the air is a classic example of oxidation. Removing the oxygen from iron ore in a blast furnace to get iron is reduction. Phone batteries, too, store and release energy through oxidation-reduction reactions when they charge and discharge.

A cut apple turning brown is also related to oxidation: substances in the apple react with oxygen in the air with the help of an enzyme. Sprinkling on lemon juice or briefly soaking the slices in weak salt water slows the browning.

Combustion is an oxidation reaction in which a substance combines rapidly with oxygen and gives off heat and light. For combustion to happen, all three conditions are needed: fuel, oxygen, and a temperature at or above the ignition point. Putting out a fire means removing one of them: clearing away the firewood (removing the fuel), covering it with sand or a wet blanket (cutting off oxygen), or spraying water on it (lowering the temperature).

But if you pour water on burning oil, the water boils instantly and spatters, spreading burning oil everywhere and making things more dangerous. For an oil fire in a frying pan, the basic response is to cover it with a lid to cut off the oxygen and put out the fire.