Chemical Reactions, Acids and Bases

Chemistry questions on this subtest ask you to recognize a reaction type, balance a simple equation, or place something on the pH scale. The chemistry beyond that is out of scope.

Conservation of mass

Atoms are neither created nor destroyed in a chemical reaction. They are rearranged into different molecules.

That is why the mass of the products equals the mass of the reactants, and it is the reason equations must balance: the same count of each element has to appear on both sides.

A question saying a reaction produced less mass than it started with is describing something that did not happen - or, more usually, a gas escaping unmeasured.

Balancing equations

Change the coefficients - the big numbers in front. Never change the subscripts.

The subscript in H2O says water is two hydrogens and one oxygen. Change it and you are no longer describing water, so the equation stops being about the reaction you were given.

H2 + O2 gives H2O

Count: left has 2 hydrogen and 2 oxygen. Right has 2 hydrogen and 1 oxygen. Oxygen is short on the right.

Put a 2 in front of the water: H2 + O2 gives 2 H2O. Now the right has 4 hydrogen and 2 oxygen, and hydrogen is short on the left.

Put a 2 in front of the hydrogen: 2 H2 + O2 gives 2 H2O.

Check: left 4 hydrogen, 2 oxygen. Right 4 hydrogen, 2 oxygen. Balanced.

The method is to count, fix one element, recount, and repeat. Balancing is iterative, and doing it in your head without recounting is how errors happen.

Reaction types

Recognize by counting the pieces. Many into one is synthesis; one into many is decomposition; an element swapping into a compound is single replacement; two compounds trading is double.

Combustion always involves oxygen and, for a hydrocarbon fuel, always produces carbon dioxide and water. If oxygen is a reactant and CO2 and water are products, it is combustion whatever else is going on.

Endothermic and exothermic

Exothermic releases energy, usually as heat, and the surroundings get warmer. Burning, and most reactions you would notice.

Endothermic absorbs energy, and the surroundings get cooler. An instant cold pack is the standard example.

Exo is exit; endo is into. That prefix reading is reliable.

Acids and bases

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The scale runs 0 to 14.

Acids release hydrogen ions in solution. Bases release hydroxide ions. That is the definition behind the scale.

Each whole number is a factor of ten. pH 3 is ten times more acidic than pH 4 and a hundred times more acidic than pH 5. A question asking how much stronger one is than another wants a power of ten, and answering "one unit" is the error.

Properties: acids taste sour and react with metals; bases feel slippery and taste bitter. Neither should be tested that way, and the test asks about the properties rather than recommending them.

Litmus paper is the indicator to know: acids turn blue litmus red; bases turn red litmus blue. Phenolphthalein is the other one asked: colorless in acid, pink in a base.

The common ones by formula, because some questions give only the formula:

A buffer is a solution that resists changes in pH when acid or base is added. Your blood is buffered, which is why its pH barely moves.

Neutralization

Acid + base gives a salt + water.

Hydrochloric acid plus sodium hydroxide gives sodium chloride plus water. The hydrogen ion from the acid and the hydroxide ion from the base combine into water, and what is left is the salt.

"Salt" in chemistry means more than table salt. It is any ionic compound formed this way, which is a question.

Compounds and mixtures

A compound is chemically bonded in a fixed ratio and can only be separated by a chemical reaction. A mixture is physically combined in any ratio and can be separated physically, by filtering, evaporating or settling.

Water is a compound. Salt water, air and steel are mixtures.

A solution is a mixture where one substance dissolves evenly in another: the solute dissolves, the solvent does the dissolving. In salt water the salt is the solute and the water is the solvent.

Water is called the universal solvent because it dissolves more substances than any other common liquid.

Kinds of mixture

How to separate them: filtration removes solid particles from a liquid; distillation separates liquids by boiling point, or a dissolved solid from its solvent, by boiling off and recondensing.

A saturated solution holds all the solute it can at that temperature; add more and it sits undissolved on the bottom. To make something dissolve faster: stir it, heat it, or crush it to expose more surface. Dissolved substances that carry electricity in water, like salt, are electrolytes.

Physical or chemical change?

A physical change alters form but not identity: melting, cutting, dissolving. A chemical change makes a new substance, and the signs are a gas given off, a color change, heat or light, or a precipitate - a solid forming out of two clear liquids. Burning shows flammability, a chemical property; density and boiling point are physical properties.

Reaction rates and carbon compounds

Four things speed up a reaction: higher temperature, higher concentration, more surface area (a powder reacts faster than a lump), and a catalyst - a substance that speeds the reaction without being used up. The enzymes in your body are catalysts. The total mass of reactants is the distractor: it does not change the rate.

Organic compounds are built on carbon. Every organic compound contains it. The simplest are the hydrocarbons - hydrogen and carbon only - such as methane, CH4, the main part of natural gas. Burning a hydrocarbon completely produces carbon dioxide and water.

What you can skip

Across all 2,104 General Science questions in our bank:

• Moles and molar mass calculations do not appear as calculations. "Mole" shows up inside other words far more than as the unit; Avogadro's number comes up twice.
• Oxidation and reduction as electron bookkeeping. Reduction never appears and oxidation only three times, as rusting and burning. Know that rust is iron combining with oxygen.
• Chromatography never appears; filtration and distillation carry the separation questions.
• Balancing hard equations. The balancing questions use small whole numbers, as in the section above. There is no need to drill long equations.

Where people lose points

Changing subscripts to balance an equation.

Saying mass is lost in a reaction.

Reading the pH scale backward. Low is acidic.

Treating each pH step as one unit rather than a factor of ten.

Confusing strong with concentrated.

Swapping solute and solvent.

Work one in under a minute

A solution has a pH of 2. Another has a pH of 5. How much more acidic is the first?

Three steps down the scale, and each step is a factor of ten.

Ten times ten times ten is one thousand times more acidic.

The answer "three times" is the one the question is built to catch.

Where this leads

The valence electrons from the atoms lesson are what these reactions rearrange, and the energy released or absorbed is the subject of the heat lesson.