1a. Summary

Examples of everyday acids and alkali 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.

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 e.g. use of lime and antacids, chemistry of bee/wasp stings, and the functions of 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

An indicator is a dye molecule that changes colour depending on the pH of the solution.

Universal indicator is a mix of indicators to produce a wide range of colours to match a pH.

The diagram above illustrate the colours you can see with a typical universal indicator.

However its not as accurate as a specially calibrated instrument called a pH meter - see picture at the end of the page.

~ 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

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

You can use universal indicator paper to test the pH of a soil-water mixture to decide if alkaline lime needs to applied to soil that is too acidic for the healthy growth of plants.

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

All three chemicals react with acids to neutralise them - a neutral salt is formed.

For more see the Chemistry of Limestone

The pH scale is fully explained on the next page pH scale, indicator colours, ionic theory of acids, alkalis (bases) & neutralisation

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.

The alkali ammonia NH₃ is a component in some oven cleaners and will react with fatty acids to dissolve them away.

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 neutral chemical sodium chloride NaCl.

Concentrated hydrochloric acid HCl and phosphoric acid H₃PO₄ are components of limescale removers (take care - very corrosive on metal surfaces and skin too! - look out for hazard warning symbols on the container.

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 in the stomach.

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!

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. See ....

Manufacture and uses of ammonia-nitric acid-fertilisers (and preparation of ammonium salts)

and Use of NPK fertilisers and environmental problems

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. The salt form of aspirin is absorbed more readily than the relatively insoluble acid (salicylic acid)

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 small intestine is alkaline to help break down food.

Digestion and enzymes - section on human digestion, metabolism and synthesis

The strong alkali sodium hydroxide NaOH is used bleaches and other cleaning products, very corrosive, especially on the skin - look out for hazard warning symbols on the container.

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

TOP OF PAGE

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)
	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.
Households often contain a surprising number of acidic and alkaline chemicals—many of which are useful but potentially hazardous if mishandled or mixed. The hazard warning symbol on the label of a bottle of toilet cleaner. Here's a breakdown of common ones and the risks they pose: Acidic Household Chemicals These typically have a low pH and can cause burns, corrosion, or toxic reactions. Chemical Common Use Hazards Hydrochloric acid Toilet bowl cleaners Corrosive to skin and eyes; toxic fumes Sulfuric acid Drain cleaners Severe burns; reacts violently with water Acetic acid Vinegar Mild irritant; dangerous when mixed with bleach Citric acid Descaling agents, cleaners Mild irritant; can damage surfaces in high doses Oxalic acid Rust removers Toxic if ingested; skin and eye irritant Phosphoric acid Rust removers, cleaners Corrosive; can damage enamel and skin Vinegar solution (acetic acid, ethanoic acid) is weakly acidic substance used in food preparation and cooking Alkaline Household Chemicals These have a high pH and can be just as dangerous as acids. Chemical Common Use Hazards Sodium hydroxide Oven and drain cleaners Severe chemical burns; reacts with acids Ammonia Glass and surface cleaners Toxic fumes; dangerous when mixed with bleach Calcium hydroxide Lime-based products Skin and eye irritant; corrosive Sodium carbonate Laundry detergents Mild irritant; harmful in large quantities Potassium hydroxide Soap making, drain cleaners Highly corrosive; causes deep tissue damage Some neutral substances in the home Common salt, chemical name sodium chloride, is used in cooking and de-icing frozen paths and roads. Sugar is used as Dangerous Combinations to Avoid Bleach + Ammonia → Toxic chloramine gas Bleach + Vinegar → Chlorine gas Hydrogen peroxide + Vinegar → Peracetic acid (corrosive) Bleach + Alcohol → Chloroform (can cause unconsciousness or death) Safety Tips Always read labels and follow instructions. Store acids and alkalis separately. Use gloves and eye protection when handling strong cleaners. Never mix products unless explicitly instructed.
1f. The pH scale of acidity and alkalinity 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. 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). 1g. pH examples of common acidic, alkaline and neutral substances in aqueous solution 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 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     1h. How to measure pH accurately with a pH meter   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 standard buffer solutions of accurately known pH. These instruments are not cheap, this probe alone cost £350 in 2023 ($471 US dollars). It is a much more accurate method than universal indicator solution or paper.   It is an important indicator measurement for water pollution from industrial process waste and climate change e.g. the acidification of oceans and its effects of coral reefs from the increase in the weakly acidic gas carbon dioxide. The acidification of lakes and rivers e.g. from acid rain or industrial pollution, can disrupt ecosystems and kill certain 'sensitive' plants or animals and affect the food chains of other species.

1i. Some general learning objectives for Part 1 on Acids, bases, alkalis, pH, neutralisation an salts

Know that solutions with a pH <7 are acidic, pH = 7 solutions of are neutral and if a solution has a pH of >7 it is alkaline.

Know that the pH is a measure of the concentration of the hydrogen ion (H+) in a solution.

BUT, know the lower the pH, the more acidic the solution AND each pH unit is a factor of 10 (up or down) in the concentration of the hydrogen ion.

Appreciate the use of the pH scale colours and numbers as observed using universal indicator paper.

Be able to quote some useful chemicals in the home that are acidic, neutral and alkaline and what they are used for.

Be able to quote typical approximate pH values in aqueous solution for common acidic, neutral or alkaline solutions in the home or the school laboratory.

Be able to recognise and know the meaning of the eight hazard warning symbols tabled in section (1d).

Know that the pH of a solution can be accurately measured with an instrument called a pH meter, which is calibrated with buffer solutions of known pH.

Knowing the pH of water systems e.g. lakes and rivers, is one important indication of the health of them, e.g. acid rain can disrupt aquatic ecosystems and kill important animal and plant species.