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1. Everyday examples of uses of Acids, Alkalis & Hazard Signs

in the home, school/college laboratory, industry, your body!

Doc Brown's Chemistry  GCSE/IGCSE/O level Science–Chemistry Revision Notes

pH scale of acidity and alkalinity, acids, bases–alkalis, salts and neutralisation

1. Examples of everyday acids 7alkali chemistry in the home or industry, hazard warning symbols (safety signs!), examples of the pH of common materials in aqueous solution e.g. acids, alkalis, salts you may encounter from domestic products to the school or college laboratory

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.

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

1. Introducing a few examples of everyday acid–alkali chemistry and chemicals

In this introductory page of 'everyday' acid, alkali and salt chemistry, I have assumed that in your earlier school studies you have gained some idea of what the terms pH, acid, alkali, salt and neutralisation mean.

You should know that acids and bases/alkalis react together in a neutralisation reaction to form salts which occur in many domestic products for the home and garden.

If you are not sure any term used in section 1. revise the basics from section 2., which eventually goes on a bit further theoretically in section 2c.


a low pH is very acid, pH 7 is neutral, a very high pH means very alkaline

~ 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


EXAMPLES of the 'everyday life' of acids, alkalis and neutralisation to form salts


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.

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Theoretically the pH scale extends to <1 and >14 and there are solutions that are so acid or so alkaline!

You can pre–test the soil with pH paper and match the colour the paper turns with the pH number it indicates. These chemicals can be used on a larger scale in farming and treating acidic rivers and lakes.

(c) doc bThe alkali ammonia NH3 is a component in some oven cleaners and will react with fatty acids.

Citric acid is found in citrus fruits and is used as a food and drink flavouring, as is tartaric acid.

Table salt, used in preserving food and sprinkling over your fish and chips as a flavouring etc. is the chemical sodium chloride NaCl.

Hydrochloric acid HCl and phosphoric acid H3PO4 are components of limescale removers.

Salts are used to produce the colours in fireworks e.g. sodium chloride a yellow flame, calcium chloride makes a red flame and copper chloride can produce green and blue effects.

Antacid indigestion tablets are mild alkalis that react by neutralising excess stomach acid which is the 'strong' hydrochloric acid which your delicate stomach lining and upper gut can only take so much of. The antacids must be weak bases i.e. mild alkalis or harmless insoluble bases like calcium carbonate or magnesium hydroxide which readily react with hydrochloric acid. However, strong alkalis are not to be recommended as a suitable medication for 'heartburn' afflictions, since they can be just as irritating as strong acids! See hazard warning signs further down the page and also "Investigation of Indigestion Tablets".

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!

(c) doc bAcidic bee stings (pH 5.0–5.5) can be soothed, i.e. neutralised by calamine lotion, which is a mild alkali and antiseptic and anti–itching agent based (c) doc bon zinc oxide. You can also use baking soda ('bicarb of soda' or sodium hydrogen carbonate), another mild alkali.

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.

In the chemical INDUSTRY

Alkalis like lime (calcium oxide, CaO) and limestone (calcium carbonate, CaCO3) are used to reduce the acidity in soil, the neutralisation reaction produces the optimum pH for crops to grow.

Sodium hydroxide NaOH, one of the most commonly used alkalis, is used to neutralise aspirin making 'soluble aspirin'. Aspirin is an organic acid and not very soluble in water, but, its sodium salt is much more soluble and is absorbed faster by the body for more effective treatment.

(c) doc bAmmonia NH3 gas is a weak alkali and neutralised by sulfuric acid or nitric acid to form ammonium sulfate or ammonium nitrate salts. These are important agri–chemical fertilisers supplying nitrogen to the soil for better plant growth. Of course some people prefer organic growing using good old muck and compost, but it doesn't involve neutralisation, but it does involve my wife, who is a member of the Soil Association! NPK fertilisers for agriculture contain potassium, ammonium and phosphate salts.

Neutralising harmful sulfur dioxide gas (acidic, irritating and toxic SO2) in power station smoke from burning fossil fuels, by absorbing it in alkaline calcium hydroxide solution (limewater) to absorb it. Eventually harmless calcium sulfate solution is formed.

(c) doc bAcids can be used to clean corroded metal surfaces because of their reactivity to metals and metal oxides to form soluble salts which can be washed away to leave a cleaner metal surface. Concentrated acid solutions are used to remove limescale from the ceramic (unreactive) sides of toilets. Limescale is the build–up of a limestone like deposit in areas of hard water.

Alkalis are important chemicals in many industrial processes e.g.

Heating natural oils and fats with strong alkalis like sodium hydroxide produces soaps.

Alkalis are used either directly, or to make other chemicals that bind natural dyes to cloth and other fabrics.

The alkali sodium carbonate is used in making glass.

In the past alkalis have been obtained from burnt wood, burnt seaweed and stale urine, but they are now may made on a huge bulk scale from industrial processes e.g. sodium chloride is manufactured from the electrolysis of brine (sodium chloride solution) and is then used to make many other products. Sodium carbonate is made from calcium carbonate (limestone) and common salt (sodium chloride) by the Solvay Process.

So all of this is still pretty important chemistry even for the 21st century, with strong links to agriculture, the environment and leading a stressful life!

Of course there are 'downsides' to some of this 'acidic' chemistry: Acid rain increases the rate of corrosion of stonework (particularly limestone) and metal structures. Acid rain makes water too acid for some aquatic organisms to live and this in turn affects food chains e.g. salmon do not like water with a pH below 4.5! Living on Venus could be hard going, its atmosphere is mainly sulfuric acid, mind you, you should be ok in a plastic suit because plastics don't usually react with acids, which is why, as well as being cheaper, plastics are replacing water pipes, drain pipes and gutters etc.


The hazard signs for irritant, harmful and corrosive are those most appropriate when dealing with acids and alkalis

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)
hazard 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 will cause irritation of the skin and reddening and blistering.
hazard 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.
hazard Corrosive: All concentrated acidic and alkaline solutions will attack many materials and destroy living tissue too!


Highly flammable: Most organic solvents like hexane, propanone (acetone), petrol and other hydrocarbon fuels are easily ignited and catch fire. 
hazard 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!
hazard 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.
hazard Explosive: TNT, hydrogen, fireworks, peroxides
hazard Radioactive: Radioisotopes giving off dangerous ionising radiation

 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).



pH <2


pH 2–6


~pH 7


(weak soluble base)

pH 8–12


(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 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 chloride
* rain water naturally has a pH of pH ~5.5 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. 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

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