Acid Attack
Four things an acid can attack, and the salt each one leaves behind. Then the pH scale, and the method for growing a pure dry sample of blue copper sulfate crystals.
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One pattern, four reactions 🧪
Acid questions look varied and are not. An acid attacks four kinds of substance, and **every one of them makes a salt**. What changes is what comes out alongside it. Learn the four patterns and you can predict the products of a reaction you have never seen, which is exactly what the exam asks you to do.
The four patterns 🗂️
Each of these is a general equation. Learn them as a set:
Match each reaction to what it produces
- Hydrochloric acid + zinc
- Sulfuric acid + copper oxide
- Nitric acid + calcium carbonate
- Hydrochloric acid + sodium hydroxide solution
- A salt and hydrogen gas
- A salt and water, from an insoluble base
- A salt, water and carbon dioxide
- A salt and water, from a base dissolved in solution
Which one fizzes? 🧂
A student adds an acid to a white solid and sees vigorous bubbling. A gas is produced that turns limewater milky. What was the solid?
- A metal carbonate
- A metal oxide
- A metal
- A metal hydroxide
Where the salt's name comes from 🏷️
A salt's name is built from two halves, and each half comes from one reactant: • The **metal** part comes from the metal, oxide, hydroxide or carbonate. • The **second** part comes from the acid: **hydrochloric acid** makes **chlorides**, **sulfuric acid** makes **sulfates**, **nitric acid** makes **nitrates**. So copper oxide plus sulfuric acid gives **copper sulfate**. Note the reactant there is copper **oxide**, not copper metal: copper is too unreactive to attack a dilute acid on its own.
Name the salt 📝
Zinc reacting with hydrochloric acid gives zinc _____. Copper oxide reacting with sulfuric acid gives copper _____. Calcium carbonate reacting with nitric acid gives calcium _____.
The pH scale 🌈
pH runs from 0 to 14 and measures how concentrated the **hydrogen ions (H+)** are in a solution. Pure water and neutral solutions sit at **pH 7**, where universal indicator is green.
Neutralisation ⚖️
Select the TWO statements that are correct about neutralisation.
- Hydrogen ions and hydroxide ions react together to make water
- A solution that has been exactly neutralised has a pH of 7
- Neutralising an acid with an alkali releases carbon dioxide
- The acid is destroyed and no new substance forms
Making copper sulfate crystals 🔀
An interactive activity.
Why keep adding it? ➕
In that method, why is the copper oxide added until no more will dissolve?
- So that all of the acid is used up and none is left in the solution
- So that the reaction happens faster
- So that the crystals grow larger
- So that the solution turns the right shade of blue
What the pH does as you add a base 📉
In **CP 3.6** you add calcium hydroxide, or calcium oxide, a little at a time to dilute hydrochloric acid and follow the pH. The pH starts low, **rises as the acid is neutralised**, passes through 7 at the point where the acid is exactly used up, then **levels off above 7** once there is excess base and no acid left to react with. The shape of that curve, not any single reading, is what the question is usually about.
Run the practical 🧭
An interactive activity.
The summary ✅
An acid plus a metal gives a salt and _____. An acid plus a carbonate gives a salt, water and _____. Sulfuric acid always makes a _____. In the crystal preparation, the base is added in _____, and the solution must never be evaporated to _____.
Write the method ✍️
An interactive activity.