5.1Chapter 5: Acids and Alkalis

Acids and alkalis — Alhussan Chemistry - Stage 7 Revision Notes & Topical Questions

5 min read 10 parts Lesson 1 of 7 4 quizzes

📚 Alhussan Chemistry - Stage 7 Hub / Topical Revision

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Lesson summary

5.1 Acids and alkalis — Hazard Symbols, Properties and Litmus — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe the properties of acids and alkalis, state that acidity is a chemical property and use litmus and other indicators.Acids — lemons, limes and vom…

Laboratory chemicals
Universal indicator
Making an indicator
Precautions
Dilute solutions
Pure water

Part 1 of 8Acids and alkalis in daily life

Acids

Acids lemons, limes and vomit taste sour because they contain acids.
Lemon juice and vinegar are acids, and so are fizzy drinks and vitamin C.

Alkalis

Alkalis toothpaste, soap, baking soda and oven cleaner are alkaline.

Neutral

Neutral some substances are not acidic or alkaline, and pure water is neutral.

Chemical property

Chemical property the acidity or alkalinity of a substance is a chemical property.

Laboratory chemicals

Laboratory chemicals hydrochloric, sulfuric and nitric acid are acids,
- and sodium hydroxide, potassium hydroxide and calcium hydroxide (limewater) are alkalis.

Base

Base a substance that neutralises an acid, and an alkali is a base that dissolves in water.

Part 2 of 8Properties

Property	Acids	Alkalis
Taste (never test in the lab)	Sour	Bitter, soapy feel
pH	Less than 7	More than 7
Litmus	Turns red	Turns blue
Reactions	React with metals, carbonates and bases	React with acids
Ions in water	Hydrogen ions, H⁺	Hydroxide ions, OH⁻

Part 3 of 8Indicators

Indicator

Indicator a solution of a dye that changes colour in acidic and alkaline solutions.

Litmus

Litmus litmus solution is red in acid, blue in alkali and purple in neutral solution.
Paper soaked in litmus makes litmus paper: blue litmus paper turns red in acid and red litmus paper turns blue in alkali.
Litmus cannot tell you how strong the acid is.

Other indicators

Other indicators phenolphthalein is colourless in acid and pink in alkali, and methyl orange is red in acid and yellow in alkali.

Universal indicator

Universal indicator it gives a range of colours showing the pH value.

Part 4 of 8Plant indicators

Indicator	Colour in acidic solution	Colour in alkaline solution
Red cabbage	Red	Yellow, green or blue
Hibiscus flower	Dark pink or red	Dark green
Beetroot	Red or purple	Yellow

Making an indicator

Femi added hibiscus flowers to warm water in a cooking pot, poured the mixture through a colander and collected the solution in a cup.
Adamma heated hibiscus flowers in a beaker with a Bunsen burner, filtered the mixture and collected it in a conical flask.
Both made a working indicator, and hibiscus flowers make a good acid-alkali indicator.

Testing

Femi could not see the colours on a green plate, so her apparatus was not suitable there, and Adamma's test tube was a better choice.

Part 5 of 8Hazard symbols and safety

1Corrosive

Corrosive a hand and a surface being eaten away means it destroys skin and eyes.

2Irritant

Irritant an exclamation mark means it causes redness or irritation.

3Toxic

Toxic a skull and crossbones means it can kill if swallowed or inhaled.

4Flammable

Flammable it catches fire easily.

5Oxidising

Oxidising it helps other things burn.

6Precautions

Precautions wear eye protection, clear up spills with plenty of water and wash hands afterwards.

7Dilution

Dilution always add acid slowly to water with stirring, because the reaction releases heat and could spit.

8Dilute solutions

Dilute solutions they are usually irritant or less hazardous than concentrated ones.

Part 6 of 8Strong and weak

Strong and weak refer to how far an acid or alkali ionises in water, not to concentration.

Examples

Examples hydrochloric acid is a strong acid and ethanoic acid (vinegar) is a weak acid.

Part 7 of 8Acids and alkalis at the particle level

Acids

Acids when dissolved in water the molecules release hydrogen ions (H⁺), and more hydrogen ions means more acidic.

Alkalis

Alkalis they release hydroxide ions (OH⁻).

Pure water

Pure water it has equal tiny amounts of both, so it is neutral.

Neutralisation

Neutralisation hydrogen ions and hydroxide ions combine to make water.

Conduction

Conduction acids conduct electricity when dissolved because ions carry charge.

Part 8 of 8Everyday uses

Sulfuric acid

Sulfuric acid the most widely made industrial chemical, used for fertilisers, paints and batteries.

Sodium hydroxide

Sodium hydroxide used in making soap and paper.

Citric acid

Citric acid added to sweets and drinks as a preservative and for flavour.

Ammonia solution

Ammonia solution an alkali used in cleaning products and fertilisers.

Key terms

Term	Meaning
Acid	A substance with pH below 7 that releases hydrogen ions in water.
Alkali	A soluble base with pH above 7.
Indicator	A dye solution that changes colour in acidic and alkaline solutions.
Neutral	Neither acidic nor alkaline, with pH 7.
Hazard symbol	A symbol showing how a substance can cause harm.
Base	A substance that neutralises an acid.