Acids and Bases
Sydney Water tested 1.2 billion litres of tap water in 2022–23 and adjusted its pH every day, here is the chemistry behind that.
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You've probably heard that acids are dangerous. Name two substances you use or encounter every day that are acidic and two that are basic (alkaline). What tells you whether something is an acid or a base?
A pH scale runs from 0 to 14. Why do you think the scale has a middle point (pH 7) that is called "neutral"? What do you think happens chemically when an acid and a base are mixed together in equal amounts?
● Know
- That acids and bases are classes of substances with characteristic properties
- The pH scale and what acidic, neutral and alkaline mean
- Common acids and bases found in everyday life
● Understand
- How pH indicators detect acidity and alkalinity through colour change
- That pH meters give a numerical reading of acidity/alkalinity
- Why universal indicator is more useful than single-colour indicators
● Can do
- Classify common substances as acidic, neutral or basic using pH
- Interpret indicator colour changes to estimate pH
- Select the appropriate tool (indicator paper or pH meter) for a given situation
Pour vinegar onto baking soda and the mixture fizzes violently, two common household chemicals reacting because one is an acid and the other is a base. Acids and bases are two fundamental classes of chemicals with opposite properties. Understanding their behaviour is essential for chemistry, biology, medicine, and environmental science.
An acid is a substance that releases hydrogen ions (H+) when dissolved in water. Acids taste sour, turn blue litmus paper red, and react with metals to produce hydrogen gas. Common acids include hydrochloric acid (HCl) in stomach acid, citric acid in lemons, and acetic acid in vinegar.
A base is a substance that releases hydroxide ions (OH-) when dissolved in water. Bases taste bitter, feel slippery, and turn red litmus paper blue. Common bases include sodium hydroxide (NaOH) in drain cleaner, calcium hydroxide in mortar, and ammonia in cleaning products.
The pH scale measures the concentration of H+ ions in a solution. pH ranges from 0 (strongly acidic) to 14 (strongly basic), with 7 being neutral. Each step on the pH scale represents a tenfold change in H+ concentration. A solution with pH 3 has 10 times more H+ than a solution with pH 4, and 100 times more than pH 5.
Your stomach maintains a pH of 1-3 through hydrochloric acid secretion. This strongly acidic environment kills most ingested bacteria and activates digestive enzymes. When you have heartburn, stomach acid flows back into the oesophagus, causing pain because the oesophagus lacks the protective lining of the stomach. Antacids contain bases like magnesium hydroxide or calcium carbonate that neutralise excess stomach acid, raising the pH and relieving symptoms.
Australian acid-base chemistry: The mining industry uses vast quantities of sulfuric acid for ore processing. At the same time, managing acid mine drainage - acidic water that flows from mining sites - is a major environmental challenge. The Australian Government Office of the Chief Scientist has identified acid sulfate soils in coastal Australia as a significant issue; when disturbed, these soils produce sulfuric acid that damages ecosystems and infrastructure.
Strong acids are always more dangerous than weak acids. This is generally true at equal concentrations, but concentration matters enormously. A dilute solution of a strong acid like HCl at pH 5 is less dangerous than a concentrated solution of a weak acid like acetic acid at pH 2. Furthermore, some weak acids are extremely toxic. Hydrofluoric acid (HF) is a weak acid but is deadly because fluoride ions penetrate tissue and disrupt calcium metabolism. Never assume safety based solely on strong versus weak classification.
Classify each substance as acid, base, or neutral.
The pH scale is logarithmic, which means it compresses a huge range of concentrations into a manageable scale. The H+ concentration in stomach acid (pH 1) is 10 million times higher than in blood (pH 7.4). Without the logarithmic scale, we would need to work with extremely small numbers like 0.000000025 mol/L.
Indicators are substances that change colour depending on pH. Litmus is a simple indicator: red in acid, blue in base. Universal indicator is a mixture of several indicators that produces a spectrum of colours: red (strong acid), orange (weak acid), yellow (slightly acid), green (neutral), blue (slightly basic), indigo (weak base), violet (strong base).
For precise pH measurement, a pH meter is used. It measures the electrical potential across a specialised glass electrode that responds to H+ concentration. pH meters are calibrated using buffer solutions of known pH.
Swimming pools in Australia are typically maintained at pH 7.2-7.6. If pH drops below 7, the water becomes acidic, causing eye irritation and corroding metal fittings. If pH rises above 8, chlorine disinfectant becomes less effective and scale forms on pool surfaces. Pool managers test pH daily using test strips or digital meters, then add acid (hydrochloric acid) or base (sodium carbonate) to adjust. This simple acid-base management prevents health problems and extends pool equipment life.
Australian water quality: The Australian Drinking Water Guidelines specify pH should be between 6.5 and 8.5. Water outside this range can corrode pipes (too acidic) or deposit scale (too basic). Water treatment plants across Australia monitor and adjust pH continuously. In some areas, naturally acidic water from peatlands or acid sulfate soils requires neutralisation with lime (calcium oxide, a base) before distribution.
pH can be negative or greater than 14. This is theoretically possible for extremely concentrated strong acids or bases, but practically irrelevant for most chemistry. A pH of -1 would require an H+ concentration of 10 mol/L, which is physically impossible in aqueous solution because water itself limits the concentration. For all practical purposes in school chemistry, pH ranges from 0 to 14.
Neutralisation is the reaction between an acid and a base to produce a salt and water. It is one of the most important reactions in chemistry, with applications in medicine, agriculture, industry, and environmental protection.
The general equation is:
Acid + Base -> Salt + Water
For example:
HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)
The H+ from the acid combines with the OH- from the base to form water (H2O). The remaining ions (Na+ and Cl-) form the salt (sodium chloride).
Neutralisation reactions are exothermic - they release heat. When you mix a strong acid and strong base, the temperature rise can be significant. This heat comes from the energy released when H+ and OH- ions bond to form water molecules.
The equivalence point is reached when the moles of H+ exactly equal the moles of OH-. At this point, the solution is neutral (pH 7 for strong acid + strong base). Indicators or pH meters are used to detect the equivalence point in titrations.
When you get an ant sting, the insect injects formic acid (methanoic acid) under your skin, causing pain and inflammation. Applying a paste of baking soda (sodium hydrogen carbonate, a weak base) neutralises the acid, relieving pain. The reaction produces sodium formate, water, and carbon dioxide gas. This is a practical application of acid-base neutralisation that many Australians have experienced during summer barbecues or beach outings.
Australian agriculture: Many Australian soils are naturally acidic due to leaching of bases by rainfall. Acidic soils reduce crop yields because aluminium becomes soluble at low pH and toxic to plant roots. Farmers apply lime (calcium carbonate, a base) to neutralise soil acidity. The neutralisation reaction raises soil pH, locks up toxic aluminium, and makes nutrients more available to plants. This is one of the largest-scale acid-base chemistry applications in Australia.
Neutralisation means the products are neutral and harmless. This is false. Neutralisation produces a salt, and some salts are toxic or corrosive. Neutralising hydrochloric acid with sodium hydroxide produces sodium chloride solution - essentially salty water, which is harmless. But neutralising sulfuric acid with sodium hydroxide produces sodium sulfate, which can cause diarrhoea if consumed in large amounts. And neutralising a strong acid with a weak base may not reach pH 7. Always consider what salt is produced, not just that neutralisation has occurred.
- Hydrochloric acid
- Citric acid
- Sodium hydroxide
- Acetic acid
- Ammonia
- Window cleaner and fertiliser
- Drain cleaner and soap making
- Vinegar
- Lemons and citrus fruits
- Stomach acid
Wrong: "Acids are dangerous and bases are safe." No, both strong acids and strong bases can be corrosive and cause serious burns. Concentrated drain cleaner (a strong base) is just as dangerous as concentrated acid.
Right: Both strong acids and strong bases are hazardous, danger depends on concentration and strength, not whether a substance is an acid or a base. Always handle both with equal care.
Wrong: "pH measures strength." No, pH measures acidity or alkalinity. A substance can be a weak acid at a low pH (like vinegar) or a strong acid that has been diluted to a higher pH. Concentration and pH are related but different concepts.
Right: pH measures how acidic or alkaline a solution is, not the strength of the acid or base. Diluting a strong acid raises its pH, but it is still a strong acid, concentration and acid strength are separate ideas.
Wrong: "All acids are liquids." No, acids can be solids (citric acid crystals), liquids (vinegar), or gases dissolved in water (carbonic acid in fizzy drinks). Bases can also be solids (baking soda) or liquids (ammonia solution).
Right: Acids and bases exist in all three states of matter. Citric acid is a solid crystal, acetic acid is a liquid, and carbon dioxide dissolved in water forms a weak acid. The state depends on the substance, not whether it is acidic or basic.
pH in Australian Industries
Australia's wine industry relies heavily on pH measurement. Wine grapes contain natural acids (mainly tartaric acid) that give wine its crispness and help preserve it. Winemakers monitor pH throughout fermentation because it affects taste, colour stability, and microbial safety. Australian Shiraz typically has a pH between 3.3 and 3.6.
Swimming pools across Australia are also tested regularly for pH. The ideal pool pH is between 7.2 and 7.6, slightly alkaline. If the pH is too low, the water corrodes metal fittings and irritates swimmers' eyes. If the pH is too high, chlorine becomes less effective at killing bacteria.
✍ Copy Into Your Books
▾Acids
- Taste sour, turn blue litmus red
- pH less than 7
- Examples: lemon juice, vinegar, stomach acid
Bases (Alkalis)
- Bitter taste, slippery feel, turn red litmus blue
- pH greater than 7
- Examples: baking soda, soap, ammonia
The pH Scale
- 0–6 = acidic; 7 = neutral; 8–14 = alkaline
- Universal indicator: red (acid) → green (neutral) → purple (alkaline)
Everyday pH Detective
Indicator Match-Up
At the start of this lesson, the hook told you that your stomach produces acid strong enough to dissolve a razor blade, yet it doesn't dissolve you. Think back to what you thought then about how that could be possible.
Now that you understand the pH scale, indicators, and how Sydney Water monitors tap water, can you explain what keeps your stomach lining safe? How has your thinking about acids and pH shifted from your first guess?
Q1. 1. Describe three characteristic properties of acids and three characteristic properties of bases. Include at least one property that can be safely tested at home. 4 MARKS
Q2. 2. A farmer wants to test whether the water in their dam is suitable for irrigation. Explain why a pH meter would be more useful than litmus paper for this purpose, and describe what pH range would be considered acceptable for most crops. 4 MARKS
Q3. 3. Compare and contrast litmus paper, universal indicator and phenolphthalein as tools for measuring pH. In what situations would each be most appropriate? 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain why some substances taste sour while others feel slippery?
- What is the difference between an acid and a base in terms of pH?
Model answers (click to reveal)
Answers
▾MCQ 1
CBaking soda (sodium bicarbonate) is a base. Lemon juice, vinegar and cola are all acidic.
MCQ 2
BUniversal indicator turns green at pH 7 (neutral).
MCQ 3
DBlue litmus turning red tells you the solution is acidic, but litmus cannot tell you the exact pH. You would need universal indicator or a pH meter for that.
MCQ 4
AA pH meter gives a precise numerical reading, which is needed to ensure pool water is in the safe range (7.2–7.6). Litmus paper only shows acidic vs basic, not the exact pH.
MCQ 5
CBoth results show the solution is alkaline, but neither gives an exact pH value. Universal indicator purple covers pH 11–14, and phenolphthalein turns pink at any pH above ~8.3. A pH meter would be needed for an exact reading.
Short Answer 1
Model answer: Acids taste sour, turn blue litmus red, and have a pH less than 7. Bases taste bitter, feel slippery, turn red litmus blue, and have a pH greater than 7. A safe test at home is using red cabbage juice (a natural indicator) to test kitchen substances: it turns pink in acid and green/yellow in base. (Any safe test described appropriately is acceptable.)
Short Answer 2
Model answer: A pH meter is more useful than litmus paper because it gives a precise numerical pH reading, whereas litmus paper can only tell you if the water is acidic or basic. For irrigation, most crops grow best in slightly acidic to neutral water with a pH between 6.0 and 7.5. If the pH is too low (acidic), it can damage roots and leach toxic metals. If the pH is too high (alkaline), nutrients become less available to plants.
Short Answer 3
Model answer: Litmus paper is simple and cheap but only shows whether a substance is acidic or basic, not the degree. It is best for quick go/no-go tests. Universal indicator shows a range of colours across the full pH scale, making it useful for estimating approximate pH in classroom investigations. Phenolphthalein only changes in alkaline conditions (colourless to pink), so it is best for detecting when a solution has become basic, such as in neutralisation reactions.