Chapter 5 of 8

All 8 chapters
  1. 1Chemical formulae, readunderstood
  2. 2Balancing equations, readunderstood
  3. 3The mole, readunderstood
  4. 4Reacting masses and yield, readunderstood
  5. 5Acids, bases and salts, readunderstood
  6. 6The reactivity series, readunderstood
  7. 7Electrolysis, readunderstood
  8. 8Energy changes and rates, readunderstood

S4.7.5 Chapter 5

Acids, bases and salts

Why does vinegar fizz when I add baking soda?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Write equations for acids reacting with metals, bases and carbonates
  • Explain the pH scale in terms of hydrogen ions

Lemon juice, vinegar and the juice in your stomach are acids. Soap and baking soda solution are alkalis. This chapter shows what they have in common, and what they make when they react.

An acid is a substance that makes hydrogen ions, H⁺, when it dissolves in water. Hydrochloric acid, HCl, splits into H⁺ and Cl⁻ ions. Sulfuric acid is H₂SO₄ and nitric acid is HNO₃.

A base is a substance that reacts with an acid and cancels it. Metal oxides and metal hydroxides are bases. An alkali is a base that dissolves in water. It makes hydroxide ions, OH⁻. Sodium hydroxide, NaOH, makes Na⁺ and OH⁻ ions.

When an acid reacts, it makes a salt. A salt is a compound. It is made from the metal (or ammonium) part of the other substance and the negative ion of the acid. Hydrochloric acid makes chlorides. Sulfuric acid makes sulfates. Nitric acid makes nitrates.

A table with the columns Acid and Salt. Hydrochloric acid makes a chloride. Sulfuric acid makes a sulfate. Nitric acid makes a nitrate.AcidSalthydrochloricchloridesulfuricsulfatenitricnitrate
The name of the acid gives the end of the name of the salt. Zinc and hydrochloric acid make zinc chloride.

There are three kinds of reaction to know. An acid with a metal makes a salt and hydrogen. An acid with a base makes a salt and water. An acid with a carbonate makes a salt, water and carbon dioxide.

A table with the columns Acid with and Makes. A metal makes salt + H₂. A base makes salt + H₂O. A carbonate makes salt + H₂O + CO₂.Acid withMakesmetalsalt + H₂basesalt + H₂Ocarbonatesalt + H₂O + CO₂
The three reactions. H₂ is hydrogen, H₂O is water and CO₂ is carbon dioxide.

Worked example

Write a balanced equation for zinc reacting with hydrochloric acid.

  1. Word equation: zinc + hydrochloric acid → zinc chloride + hydrogen.
  2. Formulae: Zn + HCl → ZnCl₂ + H₂. Zinc chloride is ZnCl₂, because Zn²⁺ needs two Cl⁻.
  3. Count Cl: 1 on the left and 2 on the right. Write 2 in front of HCl.
  4. Now H is 2 on each side. The equation is balanced.

Answer: Zn + 2HCl → ZnCl₂ + H₂

Three discs in a row. A hatched disc is Zn²⁺. Two plain discs are Cl⁻. Under them the sum reads +2 −1 −1 = 0, and the formula ZnCl₂.Zn²⁺Cl⁻Cl⁻+2 −1 −1 = 0ZnCl₂
The salt zinc chloride. One Zn²⁺ ion goes with two Cl⁻ ions.

Worked example

Write a balanced equation for magnesium hydroxide reacting with hydrochloric acid.

  1. Word equation: magnesium hydroxide + hydrochloric acid → magnesium chloride + water.
  2. Formulae: Mg(OH)₂ + HCl → MgCl₂ + H₂O.
  3. Count Cl: 1 on the left and 2 on the right. Write 2 in front of HCl.
  4. Count H: 2 in Mg(OH)₂ and 2 in 2HCl, so 4 on the left. Write 2 in front of H₂O to make 4 on the right.
  5. Check O: 2 on each side. The equation is balanced.

Answer: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

Worked example

Write a balanced equation, with state symbols, for calcium carbonate reacting with hydrochloric acid.

  1. Word equation: calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide.
  2. Formulae: CaCO₃ + HCl → CaCl₂ + H₂O + CO₂.
  3. Count Cl: 1 on the left and 2 on the right. Write 2 in front of HCl.
  4. Now H is 2 on the left, from the 2HCl, and 2 on the right, in H₂O.
  5. C is 1 on each side. O is 3 in CaCO₃ on the left and 1 + 2 = 3 on the right.
  6. Add state symbols. The carbonate is a solid. The acid and the salt are dissolved. The water is a liquid, and the carbon dioxide is a gas.

Answer: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Now think about what makes a solution acid or alkaline. It is the number of H⁺ ions. The pH scale measures it. A solution of pH 7 is neutral, at room temperature. Below 7 is acid, and above 7 is alkaline.

A lower pH means more H⁺ ions in each dm³. Each step of 1 on the scale means 10 times as many H⁺ ions, or 10 times fewer. A solution of pH 3 has 10 times more H⁺ than one of pH 4.

The pH scale stood upright, from 0 at the top to 14 at the bottom, with acid, neutral and alkali regions. The marks read stomach acid, 1 to 2; lemon juice, about 2; vinegar, about 3; pure water, 7; baking soda solution, about 8; and soap, about 10.neutralacidalkali01234567891011121314pHstomach acid,1 to 2lemon juice,about 2vinegar, about 3pure water, 7baking sodasolution,about 8soap, about 10
The pH scale with some things from a home. The values are about, because they change with the brand and the strength.
A table with the columns pH and Times more H⁺, compared with pure water at pH 7. pH 6 has 10 times as many H⁺. pH 5 has 100 times as many. pH 4 has 1 000 times as many. pH 3 has 10 000 times as many.pHTimes more H⁺610 times5100 times41 000 times310 000 times
Compared with pure water, at pH 7. Each step down is 10 times more H⁺. Vinegar, at about pH 3, has about ten thousand times as many H⁺ ions as pure water.

Draw it first

An acid and an alkali meet

  1. Three steps, drawn in the same place. Step 1: a hatched disc H⁺ with the charge +1. Step 2: a plain disc OH⁻ is added and the sum reads +1 −1 = 0. Step 3: the formula H₂O appears under the sum.H⁺+1

    1 An acid supplies H⁺ ions. Each has a charge of +1.

  2. Three steps, drawn in the same place. Step 1: a hatched disc H⁺ with the charge +1. Step 2: a plain disc OH⁻ is added and the sum reads +1 −1 = 0. Step 3: the formula H₂O appears under the sum.H⁺OH⁻+1 −1 = 0

    2 An alkali supplies OH⁻ ions, each with a charge of −1. One of each makes a total charge of 0.

  3. Three steps, drawn in the same place. Step 1: a hatched disc H⁺ with the charge +1. Step 2: a plain disc OH⁻ is added and the sum reads +1 −1 = 0. Step 3: the formula H₂O appears under the sum.H⁺OH⁻+1 −1 = 0H₂O

    3 They join to make a water molecule, H₂O. This is neutralisation: H⁺ + OH⁻ → H₂O.

See it three ways

As a picture

A pH scale with the acid, neutral and alkali regions, and one mark, vinegar, about 3.neutralacidalkali01234567891011121314pHvinegar, about 3

In words

Lower pH, more H⁺. Every step down the scale is ten times more H⁺ ions in the same volume.

In symbols

pH 3 has 10 × more H⁺ than pH 4, and 100 × more than pH 5.

Try it

Red cabbage tells acid from alkali

You need: a few leaves of red cabbage, hot water, poured by an adult, six clear cups, or four if you wash and dry two, vinegar, water, baking soda and a spoon.

Be safe: An adult pours the hot water and lets the cabbage liquid cool. Vinegar and baking soda are the strongest acid and alkali to use. Taste nothing, keep the liquids away from your eyes, and wash your hands after.

  1. Tear the cabbage into small pieces in a cup. An adult pours hot water over them. Leave it for 15 minutes, then pour off the purple liquid into a second cup.
  2. Stir a spoon of baking soda into half a cup of cold water, in a third cup, to make baking soda solution.
  3. Put an equal amount of purple liquid into three clean cups.
  4. Add 2 spoons of vinegar to the first cup. Add 2 spoons of plain water to the second. Add 2 spoons of baking soda solution to the third.
  5. Look at the three colours.

What you will see: The vinegar turns the purple liquid pink or red. The water leaves it purple. The baking soda solution turns it blue, or blue green. The cabbage juice is an indicator. Its colour changes with the number of H⁺ ions.

If you cannot try it, this is what happens.
A table with the columns Added and Colour. Vinegar: pink or red. Water: purple. Baking soda solution: blue green.AddedColourvinegarpink or redwaterpurplebaking sodablue green

A wrong idea many people have

The pH scale goes in equal steps, so pH 3 is only a little more acidic than pH 4.

Why it feels right: The numbers 3 and 4 are close together, like 3 cm and 4 cm on a ruler.

The test that breaks it: Take a strong acid, such as hydrochloric acid, at pH 3. Add water until it has 10 times the volume. Its pH rises by 1, to 4, because the H⁺ ions are spread through ten times the space. A weak acid such as vinegar changes less.

What is true: Each step of 1 on the pH scale is a change of 10 times in the H⁺ ions. A solution of pH 3 has 10 times more H⁺ than one of pH 4.

Key words

acid
a substance that makes H⁺ ions in water.
base
a substance that reacts with an acid and cancels it.
alkali
a base that dissolves in water and makes OH⁻ ions.
salt
a compound made when an acid reacts, such as zinc chloride.
pH
a scale that measures the H⁺ ions in a solution.
neutralisation
the reaction of H⁺ and OH⁻ to make water.

Back to your guess

The answer is A: The acid in the vinegar reacts with the baking soda and makes a new gas.

Vinegar is an acid. It reacts with baking soda, sodium hydrogencarbonate, which reacts like a carbonate, and makes a new gas, carbon dioxide. The fizz is that gas leaving the liquid.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Zinc is added to hydrochloric acid and bubbles form. What gas is in the bubbles?

    Tap your answer.

    1. Yes, that is right. An acid with a metal makes a salt and hydrogen. Here zinc chloride and hydrogen gas form.

    2. Not this one. There is no oxygen to release. An acid with a metal makes a salt and hydrogen. Try another.

    3. Not this one. Carbon dioxide comes from a carbonate. Zinc is a metal, so it makes hydrogen. Go to S3.6.7. Try another.

    See the answer

    Hydrogen

    An acid with a metal makes a salt and hydrogen. Here zinc chloride and hydrogen gas form.

    • If you chose B: There is no oxygen to release. An acid with a metal makes a salt and hydrogen.
    • If you chose C: Carbon dioxide comes from a carbonate. Zinc is a metal, so it makes hydrogen. Go to S3.6.7.

  2. Which is the balanced equation for magnesium reacting with hydrochloric acid?

    Tap your answer.

    1. Not this one. The Cl is not equal: 1 on the left and 2 on the right. A number in front of HCl is missing. Try another.

    2. Yes, that is right. MgCl₂ has 2 Cl, so we need 2HCl. Then H is 2 on each side.

    3. Not this one. MgCl is not a formula that works: Mg²⁺ needs two Cl⁻, so the salt is MgCl₂. Go to S4.7.1. Try another.

    See the answer

    Mg + 2HCl → MgCl₂ + H₂

    MgCl₂ has 2 Cl, so we need 2HCl. Then H is 2 on each side.

    • If you chose A: The Cl is not equal: 1 on the left and 2 on the right. A number in front of HCl is missing.
    • If you chose C: MgCl is not a formula that works: Mg²⁺ needs two Cl⁻, so the salt is MgCl₂. Go to S4.7.1.

  3. An acid reacts with a carbonate. What does it make?

    Tap your answer.

    1. Not this one. That is what a metal makes. A carbonate makes carbon dioxide too. Try another.

    2. Not this one. That is what a base makes. A carbonate also gives off carbon dioxide gas, which is the fizz. Try another.

    3. Yes, that is right. The carbonate ion, CO₃²⁻, is changed by the acid into water and carbon dioxide. The rest makes the salt.

    See the answer

    A salt, water and carbon dioxide

    The carbonate ion, CO₃²⁻, is changed by the acid into water and carbon dioxide. The rest makes the salt.

    • If you chose A: That is what a metal makes. A carbonate makes carbon dioxide too.
    • If you chose B: That is what a base makes. A carbonate also gives off carbon dioxide gas, which is the fizz.

  4. How many times more H⁺ ions does a solution of pH 3 have than a solution of pH 5?

    Tap your answer.

    1. Yes, that is right. pH 3 and pH 5 are 2 steps apart. Each step is 10 times, so 10 × 10 = 100.

    2. Not this one. That is the difference between the two pH numbers, 5 − 3. Each step is 10 times, so the answer is bigger. Try another.

    3. Not this one. That would be 3 steps. 3 and 5 are 2 steps apart. Try another.

    See the answer

    100 times

    pH 3 and pH 5 are 2 steps apart. Each step is 10 times, so 10 × 10 = 100.

    • If you chose B: That is the difference between the two pH numbers, 5 − 3. Each step is 10 times, so the answer is bigger.
    • If you chose C: That would be 3 steps. 3 and 5 are 2 steps apart.

  5. A sample is at pH 9 on the scale. What is true of it?

    The pH scale stood upright, from 0 at the top to 14 at the bottom. One mark, labelled sample X, is at pH 9.01234567891011121314pHsample X
    Tap your answer.

    1. Not this one. Acids are below 7. Sample X is above 7, so it has few H⁺ ions. Try another.

    2. Not this one. Only pH 7 is neutral, with as many H⁺ ions as OH⁻ ions. Try another.

    3. Yes, that is right. A pH above 7 is alkaline. Higher pH means fewer H⁺ ions and more OH⁻ ions.

    See the answer

    It is alkaline, with fewer H⁺ ions than pure water.

    A pH above 7 is alkaline. Higher pH means fewer H⁺ ions and more OH⁻ ions.

    • If you chose A: Acids are below 7. Sample X is above 7, so it has few H⁺ ions.
    • If you chose B: Only pH 7 is neutral, with as many H⁺ ions as OH⁻ ions.

  6. What happens when an acid and an alkali neutralise each other?

    Tap your answer.

    1. Not this one. They cancel each other. The H⁺ ions and the OH⁻ ions are used up. Go to S3.6.8. Try another.

    2. Yes, that is right. The equation is H⁺ + OH⁻ → H₂O. The other ions stay in the solution as the salt.

    3. Not this one. Hydrogen comes from a metal and an acid. Acid and alkali make a salt and water. Try another.

    See the answer

    H⁺ and OH⁻ ions join to make water.

    The equation is H⁺ + OH⁻ → H₂O. The other ions stay in the solution as the salt.

    • If you chose A: They cancel each other. The H⁺ ions and the OH⁻ ions are used up. Go to S3.6.8.
    • If you chose C: Hydrogen comes from a metal and an acid. Acid and alkali make a salt and water.

  7. From an earlier chapter

    How many moles of hydrochloric acid, HCl, neutralise 4 g of sodium hydroxide, NaOH? Use NaOH + HCl → NaCl + H₂O. Mr of NaOH is 40.

    Tap your answer.

    1. Not this one. That is 40 ÷ 4, upside down. Moles are mass divided by Mr. Go to S4.7.3. Try another.

    2. Not this one. The 4 is the mass in grams, not moles. Divide by Mr first. Go to S4.7.3. Try another.

    3. Yes, that is right. Moles of NaOH = 4 ÷ 40 = 0.1 mol. The ratio of NaOH to HCl is 1 to 1, so 0.1 mol of HCl is needed.

    See the answer

    0.1 mol

    Moles of NaOH = 4 ÷ 40 = 0.1 mol. The ratio of NaOH to HCl is 1 to 1, so 0.1 mol of HCl is needed.

    • If you chose A: That is 40 ÷ 4, upside down. Moles are mass divided by Mr. Go to S4.7.3.
    • If you chose B: The 4 is the mass in grams, not moles. Divide by Mr first. Go to S4.7.3.

  8. Balance this equation: Mg + H₂SO₄ → MgSO₄ + H₂. What are the smallest whole numbers in front of each formula, in order?

    See the answer

    1, 1, 1 and 1

    Mg is 1 on each side. SO₄ is 1 on each side. H₂ is 2 on each side. No change is needed.

  9. How many times more H⁺ ions does a solution of pH 2 have than a solution of pH 3?

    See the answer

    10 times

    The two are 1 step apart. Each step is 10 times.

Keep it

Copy this into your exercise book.

Acid + metal makes salt and hydrogen. Acid + base makes salt and water. Acid + carbonate makes salt, water and carbon dioxide. Each pH step is ten times the H⁺.

Draw: The pH scale from 0 to 14 with vinegar, water and soap marked, and H⁺ + OH⁻ → H₂O underneath.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (907 KB). It opens on any phone.