Reactions of acids and saltsIB MYP Sciences: Revision notes
Section 1
Acid + metal
Many metals react with dilute acids to make a salt and hydrogen gas.
acid + metal → salt + hydrogen
Example: magnesium + sulfuric acid → magnesium sulfate + hydrogen
Mg + H₂SO₄ → MgSO₄ + H₂
You see fizzing (bubbles of gas) and the test tube may get warm. Magnesium, zinc and iron react. Very unreactive metals such as copper do not react with dilute acids. Very reactive metals such as sodium and potassium are far too dangerous to add to acid.
Metals do not make water or carbon dioxide with acids. The gas is hydrogen.
Section 2
Acid + metal oxide or hydroxide
Metal oxides and metal hydroxides are bases. They neutralise acids to give a salt and water.
acid + metal oxide → salt + water acid + metal hydroxide → salt + water
Examples:
- copper(II) oxide + sulfuric acid → copper(II) sulfate + water (CuO + H₂SO₄ → CuSO₄ + H₂O)
- sodium hydroxide + hydrochloric acid → sodium chloride + water (NaOH + HCl → NaCl + H₂O)
There is no gas, so no fizzing. You may see a colour change, for example black copper(II) oxide dissolving to a blue solution.
Section 3
Acid + carbonate
Carbonates react with acids to give a salt, water and carbon dioxide.
acid + carbonate → salt + water + carbon dioxide
Example: calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
The mixture fizzes because carbon dioxide is released. This is why acid damages limestone and marble, which are mostly calcium carbonate, and why descalers remove limescale.
Section 4
Naming salts
The name of a salt has two parts. The first part comes from the metal (or base) and the second part comes from the acid:
- hydrochloric acid → chlorides
- sulfuric acid → sulfates
- nitric acid → nitrates
Examples: zinc + hydrochloric acid → zinc chloride + hydrogen; copper(II) oxide + nitric acid → copper(II) nitrate + water; magnesium carbonate + sulfuric acid → magnesium sulfate + water + carbon dioxide.
Use the acid for the second word and the metal for the first word. Sulfuric acid never makes a sulfide.
Section 5
Tests for hydrogen and carbon dioxide
Hydrogen: hold a lit splint at the mouth of the test tube. Hydrogen burns with a squeaky pop.
Carbon dioxide: bubble the gas through limewater (calcium hydroxide solution). It turns milky or cloudy.
These tests let you decide whether a reaction involved a metal (hydrogen) or a carbonate (carbon dioxide).
Section 6
Making a salt and titration in outline
Making a soluble salt from an insoluble base (for example copper(II) sulfate):
- Warm the dilute acid and add the base a little at a time, stirring, until no more dissolves (the base is in excess)
- Filter to remove the unreacted solid
- Heat the filtrate to evaporate some water, then leave it to cool so crystals form
- Dry the crystals between filter paper
Titration is used when both reactants are soluble, for example an alkali and an acid. In outline:
- Measure a fixed volume of alkali into a flask using a pipette and add an indicator
- Add acid from a burette until the indicator changes colour (the end point)
- Record the volume and repeat until results agree
- Repeat without the indicator using that volume, then crystallise the salt
Do not heat the solution until it is completely dry, because this gives a powder instead of good crystals.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Reactions of acids and salts
- A student in Cairo adds a piece of magnesium ribbon to dilute sulfuric acid in a test tube. The mixture fizzes and the test tube becomes warm. When the student holds a lit splint at the mouth of the tube, it goes out with a squeaky pop.Write a word equation for the reaction between magnesium and dilute sulfuric acid.2 marks
- A student in Seoul adds small chips of marble (calcium carbonate) to dilute hydrochloric acid in a conical flask on a balance. The mixture fizzes and the reading on the balance slowly falls. The gas given off is collected and tested.Explain why the reading on the balance slowly falls during the reaction.2 marks
- Kofi, a student in Accra, makes copper(II) sulfate crystals. He warms 25 cm³ of dilute sulfuric acid, adds copper(II) oxide a spatula at a time while stirring, and keeps adding until some black solid remains undissolved. The solution turns blue. He filters the mixture, then heats the filtrate in an evaporating basin until all the water has gone. He obtains a small amount of pale blue powder instead of the large blue crystals he expected.Write a word equation for the reaction, and explain why Kofi adds copper(II) oxide until some remains undissolved.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).