Physical ScienceChemical ChangeYear 9 Science

Acids and bases: pH scale, alkalis and indicators

What this lesson covers

  • How acids and bases differ, and why both can be dangerous
  • Common acids and the main types of base
  • What makes a base an alkali
  • The pH scale from 0 to 14
  • How litmus and universal indicator show acidity
  • Matching pH to everyday uses
Key idea

Two groups with opposite properties

  • Acids and bases are two families of substances that behave in opposite ways.
  • When an acid and a base are mixed in the right amounts they cancel each other out, a reaction called neutralisation.
  • Acids often taste sour, like lemon juice and vinegar.
  • Bases often taste bitter, like baking soda, and solutions of them feel slippery or soapy on skin.
  • Strong acids and strong bases are both corrosive, meaning they damage skin, eyes and other materials. In the lab you never taste or touch a chemical to identify it.
Key idea

Common acids

  • Three acids turn up again and again in labs and industry.
  • Weaker acids are found in food: citric acid in citrus fruit, ethanoic acid in vinegar, and carbonic acid in fizzy drinks.
AcidFormulaWhere you meet it
Hydrochloric acidHClYour stomach makes it to help digest food; cleaning concrete and bricks
Sulfuric acidH₂SO₄Car batteries; making fertilisers
Nitric acidHNO₃Making fertilisers and explosives
Key idea

Types of base, and which ones are alkalis

  • Most bases belong to a few types: metal oxides, metal hydroxides, metal carbonates, metal hydrogencarbonates (bicarbonates) and ammonia.
  • A base that dissolves in water is called an alkali, and its solution has a pH above 7.
  • So every alkali is a base, but not every base is an alkali: copper oxide and calcium carbonate are bases that barely dissolve.
  • Insoluble bases still neutralise acids, which is why farmers spread ground limestone on acidic paddocks.
Type of baseExampleDissolves in water?Use
Metal oxideCopper(II) oxide, CuONo, so not an alkaliMaking copper salts in the lab
Metal hydroxideSodium hydroxide, NaOHYes, an alkaliDrain and oven cleaners
Metal carbonateCalcium carbonate, CaCO₃No, so not an alkaliAgricultural lime, antacid tablets
BicarbonateSodium hydrogencarbonate, NaHCO₃Yes, a weak alkaliBaking soda
AmmoniaNH₃Yes, an alkaliHousehold glass cleaners
Nested ovals: the alkalis sodium hydroxide, ammonia and sodium hydrogencarbonate sit inside the larger group of bases, while copper oxide and calcium carbonate are insoluble bases outside it
Key idea

The pH scale

  • pH is a number, usually from 0 to 14, that says how acidic or alkaline a solution is.
  • A solution under 7 is acidic, one at exactly 7 is neutral, and one over 7 is alkaline.
  • The lower the pH, the more acidic the solution; the higher it is above 7, the more alkaline.
  • Each step of 1 on the scale is a tenfold change, so pH 3 is ten times more acidic than pH 4.
pH (about)SubstanceType
1Stomach acidStrongly acidic
2Lemon juiceAcidic
3VinegarAcidic
5Black coffeeWeakly acidic
7Pure waterNeutral
8Sea water around NZWeakly alkaline
9Baking soda solutionAlkaline
11Ammonia cleanerAlkaline
13 to 14Drain cleanerStrongly alkaline
Worked example

How much more acidic?

  • Some geothermal lakes near Rotorua, such as Inferno Crater at Waimangu, are about pH 2. Black coffee is about pH 5. How many times more acidic is the lake?
A pH number line from 1 to 6 with Inferno Crater at pH 2 and black coffee at pH 5; three jumps of ×10 between them multiply to ×1000
  1. 5 − 2 = 3 steps on the pH scalefind the gap
  2. 10 × 10 × 10 = 1000each step is a factor of 10
  3. The lake is about 1000 times more acidic than black coffee
Key idea

Indicators: colour tells you pH

  • An indicator is a dye that changes colour depending on whether it is in an acid or an alkali.
  • Litmus comes as paper strips. Blue litmus turns red in an acid; red litmus turns blue in an alkali; in a neutral solution neither changes.
  • Litmus only says acid or alkali, not how strong.
  • Universal indicator is a mixture of dyes that shows a different colour at each part of the scale, so you match the colour against a chart to read an approximate pH.
  • The exact shades differ a little between brands, so always use the chart that comes with the bottle.
pHUniversal indicator colourMeaning
0 to 2RedStrong acid
3 to 4OrangeAcid
5 to 6YellowWeak acid
7GreenNeutral
8 to 9Blue-green to blueWeak alkali
10 to 11Blue to indigoAlkali
12 to 14VioletStrong alkali
Universal indicator colours across the pH scale from 0 to 14: red, orange and yellow for acids, green at neutral 7, then blue through violet for alkalis
Key idea

Choosing the right way to measure pH

  • Pick the method by the question you need to answer.
  • If you only need to know acid or alkali, litmus is quick and cheap.
  • If you need a rough pH number, use universal indicator, but it is hard to read in a solution that already has a strong colour, such as blackcurrant juice.
  • If you need an exact value, for example checking a swimming pool or soil for a crop, use a calibrated pH meter.
MethodTells youGood forWeakness
Litmus paperAcid, neutral or alkaliQuick sortingNo number
Universal indicatorpH to about 1 unitComparing strengthsHard to read in coloured liquids
pH meterpH to 0.1 or betterAccurate measurementCosts more, needs calibrating
Key idea

Linking pH to uses

  • Antacid tablets contain weak bases such as calcium carbonate, which neutralise extra stomach acid without harming the stomach lining.
  • Drain cleaners are strongly alkaline (pH 13 or more) so they can break down the fats and hair that block pipes, which is also why they burn skin.
  • Hydrangeas are a living indicator: in acidic soil their flowers turn blue, while in alkaline soil they turn pink.
Worked example

Identifying unknown solutions from evidence

  • Three unlabelled colourless solutions, A, B and C, are tested. Use the results to decide what each could be.
SolutionBlue litmusRed litmusUniversal indicator
ATurns redStays redRed
BStays blueStays redGreen
CStays blueTurns blueBlue
  1. A: blue litmus to red, indicator red, so pH about 1 or 2a strong acid
  2. B: no litmus change, indicator green, so pH 7neutral
  3. C: red litmus to blue, indicator blue, so pH about 8 or 9a weak alkali
  4. A could be dilute hydrochloric acid, B pure water or salt water, C baking soda solution

Common mistakes

  • pH 2 is only a little more acidic than pH 3, because the numbers are just 1 apart.
    Each step on the pH scale is a tenfold change, so pH 2 is ten times more acidic than pH 3.
  • The higher the pH number, the stronger the acid.
    Acids are below 7 and get stronger as the number goes down; a high pH means a strong alkali.
  • Alkali and base mean the same thing.
    An alkali is a base that dissolves in water, so every alkali is a base, but insoluble bases such as copper oxide are not alkalis.
  • Litmus paper tells you the pH of a solution.
    Litmus only shows acid or alkali; you need universal indicator for a rough pH number or a pH meter for an exact one.
Try it at home

Make a red cabbage indicator

You need: 2 red cabbage leaves, hot tap water, a jug, 4 clear glasses or cups, vinegar, lemon juice, baking soda, a teaspoon

  1. Tear the cabbage leaves into small pieces in the jug, cover them with hot tap water and leave it for 10 minutes until the water is dark purple.
  2. Let it cool, then pour the purple water into the four glasses, leaving the cabbage behind, so each is about a quarter full.
  3. Leave the first glass as it is. Add a teaspoon of vinegar to the second and a teaspoon of lemon juice to the third.
  4. Stir half a teaspoon of baking soda into the fourth glass.
  5. Look for the colours: the vinegar and lemon glasses turn pink or red because they are acids, the baking soda glass turns blue or green because it is alkaline, and the plain glass stays purple, close to neutral.

Safety: Hot tap water can scald and the purple dye stains clothes and benches, so pour slowly over the sink, and do not drink any of the mixtures.

Your turn

  • 1. A solution has pH 4. Is it acidic, neutral or alkaline, and what colour would universal indicator turn?
  • 2. Magnesium oxide hardly dissolves in water. Is it a base, an alkali, or both?
  • 3. You need to check whether a farmer's paddock soil is pH 5.8 or 6.2. Which method should you use, and why?
  • 4. Why is it a bad idea to identify a liquid in the lab by tasting it, even though acids taste sour?
  • 5. Oven cleaner has pH 13 and lemon juice has pH 2. Which is further from neutral, and what does that tell you about handling it?
Show answer

1. Acidic, because it is below 7; universal indicator turns orange. 2. A base only. It neutralises acids, but an alkali must dissolve in water. 3. A pH meter, because universal indicator cannot tell apart values only 0.4 apart. 4. The liquid could be corrosive or poisonous, and tasting is never a safe lab test. 5. Oven cleaner: it is 6 units above 7, while lemon juice is 5 units below. It is a strong alkali and corrosive, so wear gloves and eye protection and rinse any splash with plenty of water.

Try it

pH Scale: Basics

Full screen · PhET Interactive Simulations, University of Colorado Boulder, CC BY 4.0

Summary

Recap

  • Acids and bases have opposite properties and neutralise each other; strong ones of either kind are corrosive.
  • Hydrochloric, sulfuric and nitric acids are common acids; oxides, hydroxides, carbonates, bicarbonates and ammonia are bases.
  • An alkali is a base that dissolves in water.
  • pH under 7 means acidic, exactly 7 means neutral and over 7 means alkaline; each unit is a tenfold change.
  • Litmus shows acid or alkali, universal indicator gives an approximate pH by colour, and a pH meter gives an exact value.
  • A substance's pH explains its uses, from antacids to drain cleaners.

How is Year 9 maths going?

See the Year 9 maths guide