Science notes, chapter 6
Acids and alkalis
Textbook Science Form 2 (DLP), pages 124–139
What this chapter covers
- Give the operational definition of an acid and an alkali from their properties (pH, taste, corrosiveness, litmus paper, reaction with metals) and explain the role of water (6.1.1)
- Classify acidic and alkaline substances and choose an indicator (litmus paper, universal indicator, methyl orange, phenolphthalein, pH meter) to test them (6.1.2)
- Use the pH scale to compare the strength of acids and alkalis (6.1.3)
- Describe the uses of acids and alkalis in daily life, agriculture and industry (6.1.4)
- Explain neutralisation, write its word equation and describe titration (6.2.1)
- Give examples of the applications of neutralisation in daily life, agriculture and industry (6.2.2)
6.1Properties of acids and alkalis
Acid: a substance that has a pH value less than 7, tastes sour, turns blue litmus paper red and reacts with metals to produce hydrogen gas.
Alkali: a substance that has a pH value more than 7, tastes bitter and turns red litmus paper blue. It does not react with metals.
Alkali: a substance that has a pH value more than 7, tastes bitter and turns red litmus paper blue. It does not react with metals.
The word acid comes from the Latin acidus (sour). The word alkali comes from the Arabic alqali (ashes of plants). Both words describe what people noticed first: taste and origin.
| Property | Acid | Alkali |
|---|---|---|
| pH value | Less than 7 | More than 7 |
| Taste | Sour | Bitter |
| Corrosiveness | Corrosive | Corrosive |
| Effect on litmus paper | Turns blue litmus paper red | Turns red litmus paper blue |
| Reaction with metals | Produces hydrogen gas | No reaction |
| Examples | Hydrochloric acid, vinegar, lemon juice | Sodium hydroxide solution, soap water, baking soda |
Properties: Science Form 2 (DLP) textbook, Table 6.2, p. 128.
How the properties are tested (Activity 6.1)
- pH value: put the substance on pH paper and compare the colour with a pH chart.
- Reaction with metals: a cleaned magnesium ribbon in dilute hydrochloric acid gives bubbles of gas. A lighted splinter at the mouth of the tube gives a "pop" sound. This is hydrogen. There is no gas with sodium hydroxide solution.
- Corrosiveness: concentrated acid and concentrated alkali both damage filter paper. This test is a teacher's demonstration in a fume chamber, with goggles on.
Clean the magnesium ribbon with sandpaper first. This removes the oxide layer on the surface, so the acid can reach the metal itself.
Never taste a substance in the laboratory unless the teacher says it is safe. Taste is a property of food acids such as lime juice, not a test for chemicals. Both strong acids and strong alkalis are corrosive.
The role of water
Acids and alkalis show their properties only in the presence of water. Glacial ethanoic acid (no water) does not change the colour of blue litmus paper, but ethanoic acid in water turns it red. Solid sodium hydroxide does not change red litmus paper, but sodium hydroxide in water turns it blue.
Textbook Figure 6.4, p. 128.
Dry citric acid crystals are placed on a piece of dry blue litmus paper. The paper stays blue. Explain why.
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An acid shows its acidic properties only in the presence of water. With no water, the crystals do not turn blue litmus paper red.
Acidic and alkaline substances, and indicators
Substances that contain an acid are acidic substances. Substances that contain an alkali are alkaline substances. Apples, coffee, vinegar and lemon juice are acidic. Baking soda, soap and toothpaste are alkaline.
An indicator is a colouring (or a mixture of colourings) that changes colour depending on whether the substance tested is acidic, neutral or alkaline.
| Indicator | Acid | Neutral | Alkali |
|---|---|---|---|
| Blue litmus paper | Red | Blue | Blue |
| Red litmus paper | Red | Red | Blue |
| Universal indicator | Red | Green | Blue |
| Methyl orange | Red | Yellow | Yellow |
| Phenolphthalein | Colourless | Colourless | Pink |
Textbook Table 6.3, p. 129.
Only acids change blue litmus, only alkalis change red litmus. A neutral substance changes neither. Universal indicator gives a colour for every pH, so it tells you how acidic or alkaline, not just which. This is its advantage over litmus paper.
Methyl orange is yellow in both neutral and alkaline solutions, and phenolphthalein is colourless in both acid and neutral solutions. One of these indicators alone cannot separate all three. Use litmus paper, universal indicator or a pH meter as well.
Strength of acids and alkalis: the pH scale
The pH scale runs from 0 to 14. pH 7 is neutral (pure water and salt solution). Below 7, the lower the pH, the stronger the acid. Above 7, the higher the pH, the stronger the alkali. The pH of a solution is measured with pH paper (compared with a pH chart), universal indicator or a pH meter.
| Substance | Typical pH | Type |
|---|---|---|
| Hydrochloric acid (0.1 M) | 1 | Strong acid |
| Ethanoic acid (0.1 M) | 3 | Weak acid |
| Salt solution, pure water | 7 | Neutral |
| Ammonia solution (0.1 M) | 11 | Weak alkali |
| Sodium hydroxide solution (0.1 M) | 13 | Strong alkali |
The values above are the kind of readings Activity 6.3 gives when all solutions have the same concentration. Always keep the concentration the same, or the comparison is not fair.
Worked example
Pineapple juice pH 4, fresh milk pH 6, vinegar pH 2. Arrange in increasing acid strength.Stronger acid = lower pH, so the weakest acid has the highest pH. Order from weakest to strongest: fresh milk (pH 6), pineapple juice (pH 4), vinegar (pH 2).
Read the pH, not the label. Weak acids are still acids (pH below 7) and weak alkalis are still alkalis (pH above 7). Acid rain in an industrial area has a pH lower than 7 because gases such as sulphur dioxide dissolve in the rain water.
Solution X has pH 12 and solution Y has pH 9. Which is the stronger alkali, and what colour is X with universal indicator?
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X, because it has the higher pH. Universal indicator is blue (towards purple) in an alkali.
Uses of acids and alkalis in daily life
| Acid | Where it is found / used | Alkali | Where it is found / used |
|---|---|---|---|
| Tartaric acid | Cola drinks | Potassium hydroxide | Soap |
| Carbonic acid | Fizzy drinks | Ammonia | Fertilisers (made by reacting with acids) |
| Sulphuric acid | Car batteries | Magnesium hydroxide | Antacid pills |
| Ethanoic acid (vinegar) | Cooking, pickles | Sodium hydroxide | Detergent |
Textbook Photograph 6.3, p. 132.
Fertilisers in agriculture are produced from the reaction between acidic and alkaline substances. Industry and medicine also use acids and alkalis. Ants such as fire ants have formic acid in their sting; green apples contain malic acid.
Bottles of acid and alkali carry a corrosive hazard symbol because both strong acids and strong alkalis damage skin, metals and other materials. Gases such as ammonia show their properties only when they dissolve in water, so a moist litmus paper is used to test them.
Science blog: human skin has an acid mantle made of sebum and sweat. It makes the skin slightly acidic and helps it fight pathogens.
6.2Neutralisation
Neutralisation is the reaction between an acid and an alkali to produce salt and water. The acid loses its acidity and the alkali loses its alkalinity. The method used to carry it out is titration.
Word equation
Acid + alkali → salt + waterHydrochloric acid + sodium hydroxide → sodium chloride + water
Sulphuric acid + potassium hydroxide → potassium sulphate + water
Nitric acid + sodium hydroxide → sodium nitrate + water
Textbook p. 133.
Name the salt in two parts: the metal part from the alkali (sodium, potassium) then the name from the acid (hydrochloric → chloride, sulphuric → sulphate, nitric → nitrate).
Titration (Activity 6.5)
- Fill the burette with 0.5 M hydrochloric acid (use a filter funnel) and note the initial reading.
- Use a pipette to put 25 mL of 0.5 M sodium hydroxide solution into a conical flask. Add three drops of phenolphthalein (the solution turns pink).
- Add acid drop by drop from the burette, shaking the flask gently.
- Stop when the colour changes from pink to colourless. Record the final reading.
Worked example
Initial burette reading 2.40 mL, final reading 27.80 mL.Volume of acid used = 27.80 − 2.40 = 25.40 mL.
Volume used = final − initial reading. Do not add them. Burettes are read downwards, so the final reading is the bigger number.
The pipette is used for the alkali because it delivers a fixed volume exactly; the burette gives the acid slowly and measures the volume accurately.
Applications of neutralisation
| Situation | Problem | Neutralisation |
|---|---|---|
| Dental care | Bacteria in the mouth produce acid that damages teeth (dental caries) | Toothpaste is alkaline and neutralises the acid |
| Hair care | Shampoo makes hair slightly alkaline | Slightly acidic conditioner neutralises shampoo residue and makes hair smooth |
| Skin care | Alkaline face cleanser dries the skin | Acidic toner neutralises it |
| Agriculture | Acidic soil makes plants grow poorly | Slaked lime (alkaline) is added |
| Laundry | Fabric is alkaline after washing with detergent | Acidic fabric softener lowers the pH of the fabric |
| Industry | Acidic factory waste harms rivers | Treated with alkali before it is discharged |
Textbook Figure 6.8 and Photographs 6.4 and 6.5, pp. 134–135.
Reasoning from a case (p. 138)
Amran's jellyfish sting hurt more after soap and toothpaste were applied. Both are alkaline, so the sting must be alkaline too, and adding more alkali makes it worse. To reduce the pain, apply a weak acid such as vinegar, which neutralises the sting.Write the word equation for sulphuric acid reacting with sodium hydroxide solution, and give one use of neutralisation in farming.
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Sulphuric acid + sodium hydroxide → sodium sulphate + water.
Slaked lime (an alkali) is added to acidic soil so plants can grow well.
Slaked lime (an alkali) is added to acidic soil so plants can grow well.