Acids, alkalis and salts are different types of chemicals with a huge variety of uses. Alkalis The chemistry of acids and bases–alkalis is introduced by looking at common domestic examples in the home and not just in industry or the chemical laboratory. Lime, antacids, lime, bee/wasp stings, sodium bicarbonate, ammonia, sodium hydroxide, hydrochloric acid all get a mention!

 These revision notes on the use of acids, alkalis, pH meters, pH paper and examples of the pH of many solutions and hazard signs (hazard warning symbols) for acids and alkalis should prove useful for the new AQA chemistry, Edexcel chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses. Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE  chemistry, O level & ~US grades 8, 9, 10 school science courses for ~14-16 year old science students for national examinations in chemistry topics including acids bases alkalis salts preparations reactions

~ means approximately.

but the terms used on this page like acid, alkali and pH are explained in more theoretical detail in Part 2  ...

pH scale, indicators, ionic theory of acids, alkalis (bases) & neutralisation

In the HOME: Alkaline lime/quicklime (CaO, calcium oxide) or slaked lime (Ca(OH)₂, calcium hydroxide), are put on soil that is too acid for healthy plant growth, they raise the pH by neutralising the acidity in the soil.

Powdered limestone (CaCO₃, calcium carbonate) is slower and less effective.

All three chemicals react with acids to neutralise them.

Bicarbonate or (sodium hydrogencarbonate NaHCO₃, sodium bicarbonate, baking powder) can be used with sour milk (acidic) for raising action in baking. The acidic milk reacts with 'Bicarb' to form carbon dioxide gas giving the rising action. You can easily demonstrate this by adding any common laboratory acid to baking powder or any other carbonate!

Acidic bee stings (pH 5.0–5.5) can be soothed, i.e. neutralised by calamine lotion, which is a mildly alkaline antiseptic and anti–itching agent based on zinc oxide. You can also use baking soda ('bicarb of soda' or sodium hydrogen carbonate), another mild alkali. Hydrocortisone, an anti-inflammatory agent helps too.

Wasp stings are supposed to be alkaline, but apparently not so! they are almost neutral at pH 6.8–6.9 but are 'traditionally' treated with vinegar which is a weak acid (and then perhaps you need the calamine too!).

I've come across references on the web to say that wasp stings are not alkaline so 'English folklore' and mild–weak acid treatment has no real scientific basis.

It should be pointed out that sting venom is a complex mixture, including many protein–enzymes, which, with other 'foreign' substances, might well trigger a response from the bodies immune system.

So, in all honesty, I'm not quite sure what the truth is! However, what is known is that (i) bees and wasps have glands that can secrete either acids or alkalis with other substances and (ii) ants sting venom often contains methanoic acid ('formic acid') which can have a pH of 3 and is presumably 'soothed' by mild alkalis and just to confuse matters more, (iii) many people claim the 'folklore' remedies work! and maybe they do!

Ammonium salts, phosphate salts and magnesium/potassium sulfate salts are used in fertilisers for the garden.

Soluble aspirin is made by neutralising the acidic form of the medication with sodium hydroxide to make a soluble salt, or its made in situ with a bicarbonate 'fizzing' mixture.

Acids and alkalis are useful in your body! Your stomach produces hydrochloric acid to help in digestion of proteins.

Certain digestive enzymes only function properly in very acid conditions i.e. a low pH <2. Pancreatic fluids are alkaline to suit the conditions required by enzymes breaking down starches, fats and proteins.

The hydrochloric acid in your stomach kills a large % of potentially harmful bacteria, minimising the risk of food poisoning and irritation of the gut system.

However, as mentioned above, if you produce too much acid you get indigestion and need to take an antacid indigestion tablet to neutralise the excess. More body chemistry, preferably to be avoided!

The strong alkali sodium hydroxide NaOH is used bleaches and other cleaning products.

The equally strong alkali potassium hydroxide KOH is used in alkaline batteries.

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HAZARD WARNING SIGN	For all experiments, appropriate risk assessments should be done and hazcards studied etc. This table illustrates the use of hazard warning signs with common examples, and may NOT provide sufficient detail for specific laboratory experiments and detailed safe procedures, concentration factors (e.g. dilute or concentrated, 'doing labs', coursework write up from school/college investigations etc.
Symbol	Examples of what might be labelled/classified with this hazard warning sign (definitions above)
	Irritant: Most acidic and alkaline solutions unless very dilute, VERY small quantities of acidic gases like chlorine, sulfur dioxide, nitrogen dioxide, very dilute bleaches. These may not be that corrosive BUT they are irritating e.g. will cause irritation of the skin and reddening and blistering.
	Harmful: Some acids e.g. nitric acid; acidic gases like chlorine, sulfur dioxide, nitrogen dioxide; bleaches; heavy metal ions e.g. of lead, barium; some salts e.g. silver nitrate, copper sulfate. They are not quite as harmful as toxic chemicals but they can certainly make you ill.
	Corrosive: Any substance like concentrated acidic or alkaline solutions which will attack many materials and destroy living tissue too! Also includes substances like bromine.
	Highly flammable: Most organic solvents like hexane, propanone (acetone), petrol and other hydrocarbon fuels are easily ignited, easily catch fire.
	Toxic: Chlorine, sulfur dioxide, hydrogen cyanide can cause death if breathed in with sufficient quantity, absorbed through the skin or ingested by swallowing. Salts of hydrogen cyanide e.g. potassium cyanide are highly toxic – you only have a short time to take an antidote mixture!
	Oxidising: Chemicals that can act as oxidising agents e.g. chlorine gas/solution and oxygen gas/liquid, potassium manganate(VII), potassium chlorate (in some weed killers). Many oxidising agents donate oxygen to materials that burn and can be dangerously reactive. Many can cause combustion if mixed with an oxidisable combustible material. They may cause materials to burn more fiercely.
	Explosive: TNT, hydrogen, fireworks, peroxides
	Radioactive: Radioisotopes giving off dangerous ionising radiation
	Harmful to the environment. e.g. chemicals toxic to aquatic wildlife an in general harmful to organisms and the environment e.g. toxic metals like mercury, old pesticides like DDT.
pH examples of common acidic, alkaline and neutral substances in aqueous solution The pH scale and indicators are explained in more detail in section 2., The diagram illustrates what you might see when a universal indicator (mixture of several different colour changing indicators) is added to a variety of solutions with a wide variation of pH i.e. a wide range of acidity and alkalinity. Basically a low pH 0-1 is very acid, pH 7 is neutral (e.g. pure water), and a high pH 13-14 means very alkaline (~ means approximately in the table below). An acid solution is defined as one with a pH of less than 7, neutral solutions have a pH of 7 (or as near as make no difference if the solutions has acidic or alkaline properties) and alkaline solutions have a pH of over 7. The 'opposite' of an acid is a base, and a soluble base is called an alkali. The lower the pH the more acid the solution. The higher the pH the more alkaline the solution. It is important to understand two things about pH and pH scale: (i) pH is a measure of the hydrogen ion (H+) concentration (see section 2. ionic theory) Acidic solutions are formed when a substance dissolves in water and forms hydrogen ions. The lower the pH, the higher the hydrogen ion concentration, that is more acid. I know this seems confusing, but that's the way pH scale has been defined. (ii) Each pH unit change is equivalent to a 10x change in concentration of the hydrogen ion (1 x 10). For example changing the pH of a solution from pH 3 to pH 4 makes it 10x less acidic. Changing a solution's pH from 7 to 5 makes it 100x more acidic (10 x 10). STRONG ACID pH <2 WEAK ACID pH 2–6  ~NEUTRAL ~pH 7 WEAK ALKALI (weak soluble base) pH 8–12 STRONG ALKALI (strong soluble base) pH >12 H2SO4 sulfuric acid (car battery acid) pH 1 sulfuric acid vitamin C (ascorbic acid) pH ~3 very pure deionised H2O water pH7 toothpaste pH 8 NaOH sodium hydroxide pH 13–14 HCl hydrochloric acid (in the lab is same as your stomach!) pH 0–1 CH3COOH acetic/ethanoic acid (vinegar) pH ~3 NaCl sodium chloride (salt water) ~pH 7 Ca(OH)2 calcium hydroxide (limewater, slaked lime) pH 12 KOH potassium hydroxide pH 13–14 HNO3 nitric acid pH 1 fruit juices e.g. orange juice and lemon juice contain citric acid pH 2–3 MgSO4 magnesium sulfate (Epsom salts) pH ~6.5 – 7.0 Na2CO3 sodium carbonate (washing soda) pH 11 oven cleaner if it contains NaOH, pH can be >12, so take care! Limescale remover, pH <1, so take care! milk pH 6 C6H12O6 (glucose and other sugars) pH 7 NH3 ammonia pH 11, in some domestic cleaning fluids bleach solution might also be over pH 12 * wine/beer pH 3 – 6 C2H5OH ethanol ('alcohol') pH 7 soap powder pH 11 Caustic soda drain cleaner can be pH 13–14, based on sodium hydroxide * rain water naturally has a pH of pH ~5.6 due to the dissolved weakly acidic gas carbon dioxide from the atmosphere ('carbonic acid'), but it can fall to pH ~3.5 due to even more acidic sulfur dioxide gas from fossil fuel burning. See my local acid rain project some 'natural fluids' e.g. blood ~7.2 – 7.4 saliva ~6.4 – 6.9 cows milk ~6.6 human milk 6.6 – 7.6 Sea water has a pH of ~7.5 – 8.5 and has many different salts dissolved in it and bicarbonates causing the very slight alkalinity.  * * cider 2.9 – 3.3 Was sting, pH 6.8 – 6.9 NaHCO3 sodium hydrogen carbonate ('bicarb', baking soda, bread soda) pH 8 * * Bee sting, pH 5.0 – 5.5 * Mg(OH)2 magnesium hydroxide ('milk of magnesia') pH 10 – 10.5 * * * * Pancreatic juice for your digestive system can be as alkaline as pH 10   * * * Washing–up liquid ~pH 8–9       You can measure the pH of a solution very accurately using a pH meter and a glass membrane pH probe. The pH meter is calibrated against a standard buffer solution of accurately known pH It is a much more accurate method than universal indicator

GCSE/IGCSE Acids & Alkalis revision notes sub–index: Index of all pH, Acids, Alkalis, Salts Notes 1. Examples of everyday acids, alkalis, salts, pH of solution, hazard warning signs : 2. pH scale, indicators, ionic theory of acids–alkali neutralisation : 4. Reactions of acids with metals/oxides/hydroxides/carbonates, neutralisation reactions : 5. Reactions of bases–alkalis like ammonia & sodium hydroxide : 6. Four methods of making salts : 7. Changes in pH in a neutralisation, choice and use of indicators : 8. Important formulae of compounds, salt solubility and water of crystallisation : 10. More on Acid–Base Theory and Weak and Strong Acids

See also Advanced Level Chemistry Students Acid–Base Revision Notes – use index

Multiple choice revision quizzes and other worksheets

• GCSE/IGCSE foundation–easier multiple choice quiz on pH, Indicators, Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE higher–harder multiple choice quiz on pH, Indicators, Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE Structured question worksheet on Acid Reaction word equations and symbol equation questions

•  Word equation answers and symbol equation answers)

• GCSE/IGCSE word–fill worksheet on Acids, Bases, Neutralisation and Salts

• GCSE/IGCSE matching pair quiz on Acids, Bases, Salts and pH

• See also Advanced Level Chemistry Students Acid–Base Revision Notes – use index

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