Antacids: calming extra stomach acid
Your stomach makes hydrochloric acid (HCl) to digest food and kill germs. Its pH is about 1–2. Too much acid causes acidity: a burning feeling and, over time, sores (ulcers).
An antacid is a weak base that neutralises some of the acid: acid + base → salt + water.
- Magnesium hydroxide (milk of magnesia): Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
- Aluminium hydroxide: Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O
- Sodium hydrogencarbonate (baking soda): NaHCO₃ + HCl → NaCl + H₂O + CO₂ (gas, so it can cause burping)
A strong base like sodium hydroxide is never used; it would burn the stomach. Antacids only treat the symptom. Some modern medicines (like omeprazole) reduce the amount of acid made; a doctor decides which to use.
Medicines and how they are grouped
A drug is any chemical that changes how the body works; a medicine is a drug used to prevent, treat or cure illness in the right dose. Too much of any medicine is harmful.
- Analgesics (painkillers): paracetamol, ibuprofen, aspirin. Many also lower fever (antipyretics). Strong ones like morphine (narcotics) are used only in hospitals because they can cause addiction.
- Antibiotics: kill or stop bacteria, for example penicillin, amoxicillin. They do not work on viruses like the common cold. Always finish the course, or tough bacteria survive (antibiotic resistance).
- Antiseptics: kill germs on living tissue: dilute iodine, Dettol (chloroxylenol), boric acid, hydrogen peroxide.
- Disinfectants: kill germs on non-living things: bleach, phenol 1%. The same chemical can be an antiseptic when very dilute (phenol 0.2%) and a disinfectant when stronger.
- Antihistamines: block histamine, the body chemical that causes allergy symptoms: cetirizine.
- Tranquilisers: calm anxiety and stress and help sleep; only with a doctor's prescription.
Chemicals in food
- Preservatives slow the growth of microbes: salt, sugar, vinegar and oil (traditional, used in pickles and jams), and sodium benzoate, potassium sorbate, sulfites in bottled foods.
- Antioxidants stop fats and oils going rancid (oxidised): vitamin C, vitamin E, BHA, BHT.
- Artificial sweeteners taste very sweet with almost no energy, useful for people with diabetes: saccharin, aspartame (not for cooking, it breaks down in heat), sucralose (stable in heat).
- Colours, flavours and emulsifiers make food look, taste and mix well.
Every food additive has an approved safe daily limit set by food-safety authorities; packets list them, often as E-numbers or INS numbers.
Soaps and detergents: how cleaning works
A soap is the sodium or potassium salt of a long-chain fatty acid. It is made by heating oil or fat with sodium hydroxide (saponification). Each soap molecule has a head that loves water and a long tail that loves oil.
In water, the tails dig into a drop of grease and the heads stay outside in the water. The grease is trapped in a tiny ball called a micelle, which is rinsed away.
In hard water (with Ca²⁺ and Mg²⁺ ions), soap forms a sticky scum and gives little lather. Synthetic detergents work in hard water too. Choose biodegradable ones; branched-chain detergents break down slowly and make foam in rivers.
Chemistry of soil and fertilisers
Most crops grow best at soil pH 6–7.5.
- Acid soil (pH below 6): add lime — quicklime CaO, slaked lime Ca(OH)₂ or powdered limestone CaCO₃ — to neutralise it.
- Alkaline or salty soil: add gypsum (CaSO₄·2H₂O) or organic matter and wash out salts with water.
- Organic matter (compost, manure) improves soil structure and holds water.
Plants need lots of N, P and K. Chemical fertilisers: urea CO(NH₂)₂ and ammonium nitrate (nitrogen → green leaves), superphosphate (phosphorus → roots), potash KCl (potassium → flowers and fruit), and NPK mixes. Overuse washes into lakes, feeds algae (eutrophication) and makes soil acidic, so farmers test soil and use the right amount.
Metal ions in our body, toxic metals and recycling
Needed metal ions: Fe²⁺ in haemoglobin carries oxygen (too little → anaemia); Ca²⁺ builds bones and teeth and helps muscles; Na⁺ and K⁺ carry nerve signals and keep water balance; Mg²⁺ is in chlorophyll and many enzymes; Zn²⁺ helps healing and immunity. Too much of even a needed ion can be harmful.
Toxic heavy metals harm even in tiny amounts and build up in the body: lead (old paint, batteries; harms the brain, especially in children), mercury (old thermometers, CFL bulbs; harms nerves and builds up in fish), cadmium (Ni–Cd batteries; harms kidneys and bones), arsenic (in some groundwater).
Recycling keeps these metals out of soil and water and saves ore and energy. Separate waste into dry (paper, plastic, metal, glass), wet (compost) and hazardous (batteries, e-waste, medicines). Aluminium, steel and copper can be recycled again and again without losing quality.
Try it at home
Red cabbage pH test: boil a few red-cabbage leaves in water and cool the purple liquid. Add a spoon of it to: lemon juice, vinegar, water, baking-soda water, and a crushed antacid tablet in water. Acid turns it pink-red; base turns it blue-green. In the 3D, step 1 shows the same colour change as the antacid works.
Key formulas and definitions
- Antacid: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
- Baking soda: NaHCO₃ + HCl → NaCl + H₂O + CO₂
- Saponification: fat + NaOH → soap + glycerol
- Liming acid soil: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂
- Key terms: analgesic, antipyretic, antibiotic, antiseptic, disinfectant, antihistamine, tranquiliser, preservative, antioxidant, sweetener, micelle
Worked examples
1. Why does a doctor not give sodium hydroxide for acidity?
NaOH is a strong, corrosive base. It would burn the mouth and stomach. A weak, insoluble base like Mg(OH)₂ neutralises just the extra acid safely.
2. A tablet has 0.29 g of Mg(OH)₂ (molar mass 58 g/mol). How many moles of HCl can it neutralise?
Moles of Mg(OH)₂ = 0.29 ÷ 58 = 0.005 mol. Each needs 2 mol HCl, so 0.010 mol HCl.
3. Phenol 0.2% and phenol 1%: which is the antiseptic and which the disinfectant?
0.2% is mild enough for skin, so it is an antiseptic. 1% is stronger and used on floors and drains, so it is a disinfectant.
4. A field has soil pH 5.0. What should the farmer add, and why not more urea?
Add lime (CaCO₃ or Ca(OH)₂) to raise the pH towards 6.5. Urea adds nitrogen but over time makes soil more acidic, so it would not fix the problem.
Common mistakes
- Taking antibiotics for a cold or flu. Colds are caused by viruses; antibiotics only work on bacteria.
- Using antiseptic and disinfectant as the same word. Antiseptics are for living tissue; disinfectants for objects and floors.
- Thinking a stronger base is a better antacid. Antacids must be weak bases; strong bases burn.
- Throwing old batteries or thermometers in the dustbin. They release lead, cadmium or mercury; they go to hazardous or e-waste collection.