--- title: "Acids, Bases and pH" book: "School Chemistry — Grades 1 to 12" subject: chemistry language: en chapter: 18 exercises: 12 source: https://one-course.com/books/chemistry/1/en/chapter/18-acids-bases-and-ph license: CC-BY-NC-SA-4.0 credit: "One Chemistry Book, One Course (one-course.com)" --- # Chapter 18 — Acids, Bases and pH On a kitchen shelf stand a lemon, a bottle of vinegar, a bar of soap and a box of baking soda; under the sink, a bottle of drain cleaner with a black-and-red warning label. Some of these taste sour, some feel slippery, some can burn the skin. One single scale, numbered from 0 to 14, sorts them all — and the same scale is used for swimming pools, garden soil, the blood and the oceans. **You already know.** [Ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) are [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) or groups of [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) carrying a charge; a [cation](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) such as $\ce{Na+}$ is positive, an [anion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) such as $\ce{Cl-}$ or $\ce{OH-}$ negative. Ionic solutions contain [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) marked (aq), which move freely ([Chapter 17](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#ch-g9-ions)). ![Lemon, vinegar, soap and baking soda: acids and bases in the kitchen.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-acids-bases-ph/img-e824346caec9.jpg) *Lemon, vinegar, soap and baking soda: acids and bases in the kitchen.* ## 18.1 Acidic, basic and neutral solutions **Definition 18.1 (Acidic, basic and neutral solutions).** Every [aqueous solution](https://one-course.com/books/chemistry/1/en/chapter/9-solutions-and-solubility#def-g6-solutions-and-solubility-solute) contains hydrogen [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) $\ce{H+(aq)}$ and hydroxide [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) $\ce{OH-(aq)}$. A solution is an *acidic solution* when it contains more $\ce{H+}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) than $\ce{OH-}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion), a *basic solution* when it contains more $\ce{OH-}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) than $\ce{H+}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion), and a *neutral solution* when it contains as many of each. **Example 18.2 (Three solutions).** Hydrochloric acid is an [acidic solution](#def-g9-acids-bases-ph-acidic): it holds $\ce{H+(aq)}$ and $\ce{Cl-(aq)}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion), with very few $\ce{OH-}$. A sodium hydroxide solution is basic: $\ce{Na+(aq)}$ and $\ce{OH-(aq)}$, with very few $\ce{H+}$. Pure water and salt water are neutral. ## 18.2 The pH scale **Definition 18.3 (pH).** The *pH* of an [aqueous solution](https://one-course.com/books/chemistry/1/en/chapter/9-solutions-and-solubility#def-g6-solutions-and-solubility-solute) is a number, usually between 0 and 14, that tells how acidic or basic it is: at $25\,{}^{\circ}\mathrm{C}$, a [neutral solution](#def-g9-acids-bases-ph-acidic) has pH 7, an [acidic solution](#def-g9-acids-bases-ph-acidic) a pH below 7, a [basic solution](#def-g9-acids-bases-ph-acidic) a pH above 7. The more $\ce{H+}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) a solution holds, the lower its pH. ![The pH scale, with the pH of some common solutions.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-acids-bases-ph/fig-070bc416398c.svg) *The [pH](#def-g9-acids-bases-ph-ph) scale, with the [pH](#def-g9-acids-bases-ph-ph) of some common solutions.* **Definition 18.4 (pH paper and pH meter).** *pH paper* is a strip of paper soaked in a [mixture](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-mixture) of dyes that takes a different colour at each [pH](#def-g9-acids-bases-ph-ph); a drop of the solution is put on it and the colour is compared with a chart printed on the box. A *pH meter* measures the [pH](#def-g9-acids-bases-ph-ph) with a glass probe dipped in the solution and shows it as a number, to one or two decimal places. **Method 18.5 (Measuring a pH with pH paper).** 1. Put a short strip of [pH paper](#def-g9-acids-bases-ph-ph-paper) on a clean, dry dish. 2. With a clean glass rod, touch the strip with a drop of the solution (never dip the strip in the bottle). 3. Compare the colour at once with the chart, and read the [pH](#def-g9-acids-bases-ph-ph) , usually to the nearest whole number. ![Measuring a pH with a pH meter: the glass probe dips in the solution, which is gently stirred.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-acids-bases-ph/fig-9cc47429e119.svg) *Measuring a [pH](#def-g9-acids-bases-ph-ph) with a [pH meter](#def-g9-acids-bases-ph-ph-paper): the glass probe dips in the solution, which is gently stirred.* ## 18.3 Diluting an acid **Proposition 18.6 (Diluting brings the pH closer to 7).** Adding water to an [acidic solution](#def-g9-acids-bases-ph-acidic) spreads its $\ce{H+}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) through a larger volume: the [pH](#def-g9-acids-bases-ph-ph) goes up, towards 7, but never beyond. For an acid such as hydrochloric acid, diluting ten times raises the [pH](#def-g9-acids-bases-ph-ph) by about one unit: one volume of acid at [pH](#def-g9-acids-bases-ph-ph) 2 plus nine volumes of water gives a solution at [pH](#def-g9-acids-bases-ph-ph) 3. Diluting a [basic solution](#def-g9-acids-bases-ph-acidic) likewise lowers its [pH](#def-g9-acids-bases-ph-ph) towards 7. ![Diluting an acid ten times, then ten times again: the pH goes from 2 to 3 to 4 (each beaker is a tenth of the previous solution topped up with water).](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-acids-bases-ph/fig-7e7f28db9497.svg) *Diluting an acid ten times, then ten times again: the [pH](#def-g9-acids-bases-ph-ph) goes from 2 to 3 to 4 (each beaker is a tenth of the previous solution topped up with water).* **In the lab — Diluting an acid.** To dilute an acid, the teacher always pours the acid slowly into the water, never the water into the acid: the mixing gives out heat, and water poured onto a concentrated acid can boil at once and spit acid out of the container. ## 18.4 Everyday acids and bases **Proposition 18.7 (Acids and bases around us).** Acidic: lemon and lime juice, vinegar, wine, fizzy drinks, coffee, gastric juices in the stomach. Basic: sea water (slightly), baking soda dissolved in water, soap, milk of magnesia (a remedy for stomach acidity), household ammonia, bleach, drain cleaner (very strongly). **Example 18.8 (The pH of the ocean).** The average [pH](#def-g9-acids-bases-ph-ph) of the ocean is about 8.1: slightly basic. Since the industrial era began, carbon dioxide dissolving from the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) has lowered the [pH](#def-g9-acids-bases-ph-ph) of surface waters by about 0.1: the oceans are slowly becoming less basic, which harms shellfish and corals. ![Red cabbage juice, prepared by a teacher, changes colour with the pH: red in acids, violet near neutral, blue, green then yellow in more and more basic solutions.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-acids-bases-ph/img-722c4154b46b.jpg) *Red cabbage juice, prepared by a teacher, changes colour with the [pH](#def-g9-acids-bases-ph-ph): red in acids, violet near neutral, blue, green then yellow in more and more [basic solutions](#def-g9-acids-bases-ph-acidic).* ## 18.5 Safety with corrosive products **Safety.** ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-acids-bases-ph/fig-20f476ac910d.svg) ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-acids-bases-ph/fig-def5b4083f5d.svg) Concentrated hydrochloric acid and sodium hydroxide (the base of drain cleaners) carry the corrosion pictogram GHS05: they cause severe burns to skin and eyes and attack metals. GHS07: irritant. Goggles and gloves; a splash is rinsed at once with plenty of water. **Remark 18.9 (Never mix cleaning products).** Acidic and basic products react together, sometimes violently, and some [mixtures](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-mixture) release toxic gases: bleach mixed with an acidic descaler gives off chlorine. Cleaning products are never mixed. ## 18.6 Exercises **Exercise 18.1 ★.** Acidic, basic or neutral? A solution of [pH](#def-g9-acids-bases-ph-ph) 3, one of [pH](#def-g9-acids-bases-ph-ph) 7, one of [pH](#def-g9-acids-bases-ph-ph) 11. **Solution of Exercise 18.1.** [pH](#def-g9-acids-bases-ph-ph) 3: acidic. [pH](#def-g9-acids-bases-ph-ph) 7: neutral. [pH](#def-g9-acids-bases-ph-ph) 11: basic. **Exercise 18.2 ★.** Which [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) are more numerous in an [acidic solution](#def-g9-acids-bases-ph-acidic)? In a basic one? **Solution of Exercise 18.2.** Acidic: $\ce{H+}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion). Basic: $\ce{OH-}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion). **Exercise 18.3 ★.** Using the [pH](#def-g9-acids-bases-ph-ph) scale of this chapter, put in order from the most acidic to the most basic: coffee, sea water, lime juice, household ammonia, pure water. **Solution of Exercise 18.3.** Lime juice (2), coffee (5), pure water (7), sea water (8.1), household ammonia (12). **Exercise 18.4 ★.** Describe how to measure the [pH](#def-g9-acids-bases-ph-ph) of a solution with [pH paper](#def-g9-acids-bases-ph-ph-paper). **Solution of Exercise 18.4.** A strip on a clean dry dish is touched with a drop of the solution carried by a clean glass rod; the colour is compared at once with the chart. **Exercise 18.5 ★★.** A solution of [pH](#def-g9-acids-bases-ph-ph) 4 is diluted ten times with water. What is its new [pH](#def-g9-acids-bases-ph-ph), about? And if it is diluted a hundred times? **Solution of Exercise 18.5.** About 5 after a tenfold dilution; about 6 after a hundredfold one. **Exercise 18.6 ★★.** Can an [acidic solution](#def-g9-acids-bases-ph-acidic) become basic by adding water? Explain. **Solution of Exercise 18.6.** No. Diluting brings the [pH](#def-g9-acids-bases-ph-ph) closer to 7, but never past it: water itself is neutral, and adding it cannot make $\ce{OH-}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) outnumber $\ce{H+}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion). **Exercise 18.7 ★★.** Why must an acid be poured into water, and not the other way round? **Solution of Exercise 18.7.** Mixing gives out heat. Water poured onto a concentrated acid could boil at once and throw acid out; acid poured slowly into water is spread at once in a large volume. **Exercise 18.8 ★★.** A bottle carries the GHS05 pictogram. What does it mean? Give two precautions. **Solution of Exercise 18.8.** Corrosive: burns skin and eyes, attacks metals. Wear goggles and gloves; rinse any splash with plenty of water. **Exercise 18.9 ★★.** How many tenfold dilutions are needed to take an acid from [pH](#def-g9-acids-bases-ph-ph) 1 to [pH](#def-g9-acids-bases-ph-ph) 4? What total dilution is that? **Solution of Exercise 18.9.** Three dilutions (1 to 2, 2 to 3, 3 to 4): $10 \times 10 \times 10 = 1000$ times. **Exercise 18.10 ★★.** Milk of magnesia is taken for stomach acidity. Using the [pH](#def-g9-acids-bases-ph-ph) scale, explain why. **Solution of Exercise 18.10.** Milk of magnesia is basic ([pH](#def-g9-acids-bases-ph-ph) about 10.5): it reacts with part of the excess acid of the stomach. **Exercise 18.11 ★★★.** The ocean’s [pH](#def-g9-acids-bases-ph-ph) has fallen by about 0.1 since the industrial era. Is the ocean acidic today? In which direction is it moving, and why? **Solution of Exercise 18.11.** No: with a [pH](#def-g9-acids-bases-ph-ph) of 8.1 it is still slightly basic. It is moving towards lower [pH](#def-g9-acids-bases-ph-ph), less basic, because carbon dioxide from the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) [dissolves](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-dissolve) in it. **Exercise 18.12 ★★★.** A student measures the [pH](#def-g9-acids-bases-ph-ph) of the same vinegar with [pH paper](#def-g9-acids-bases-ph-ph-paper) (3) and with a [pH meter](#def-g9-acids-bases-ph-ph-paper) (2.6). Explain why the two results differ and which is more precise. **Solution of Exercise 18.12.** [pH paper](#def-g9-acids-bases-ph-ph-paper) is read by eye against a colour chart, to about one unit; the [pH meter](#def-g9-acids-bases-ph-ph-paper) gives a number to one or two decimals. The meter is more precise; the two agree within the precision of the paper. ## 18.7 Problem: The Acid Spill **Problem 18.1.** Weekend problem — a litre of acid spilt in a laboratory sink, and why diluting it is not the answer A bottle containing $1\,\mathrm{L}$ of dilute hydrochloric acid, of [pH](#def-g9-acids-bases-ph-ph) 2, breaks in a laboratory sink. The drain must not receive any solution of [pH](#def-g9-acids-bases-ph-ph) below 5. Take the rule of this chapter: for this acid, each tenfold dilution raises the [pH](#def-g9-acids-bases-ph-ph) by one unit. **Part I — The acid.** 1. Is the solution acidic, neutral or basic? Which [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) does it contain most? 2. Which pictogram should the bottle carry, and why? 3. The teacher measures the [pH](#def-g9-acids-bases-ph-ph) with a [pH meter](#def-g9-acids-bases-ph-ph-paper) rather than [pH paper](#def-g9-acids-bases-ph-ph-paper) . Give one advantage. 4. How many more $\ce{H+}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) are there in a litre at [pH](#def-g9-acids-bases-ph-ph) 2 than in a litre at [pH](#def-g9-acids-bases-ph-ph) 3 of the same acid? **Part II — Diluting.** 5. What volume of solution is obtained by diluting the litre of acid ten times? What is its [pH](#def-g9-acids-bases-ph-ph) ? 6. How many tenfold dilutions are needed to reach [pH](#def-g9-acids-bases-ph-ph) 5? 7. What total volume of solution would that make? 8. Could any amount of water make the solution basic? Explain. **Part III — A better way.** The teacher instead adds, little by little, a solution of a base until the [pH paper](#def-g9-acids-bases-ph-ph-paper) shows 7: the $\ce{H+}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) of the acid react with the $\ce{OH-}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) of the base. 9. Write the equation of the reaction between $\ce{H+}$ and $\ce{OH-}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) . 10. Why is this better than diluting? 11. Name an everyday basic product that could be used in a kitchen to clean up a small spill of vinegar. 12. Compute the volume of water that would have to be added to the litre of acid to bring it to [pH](#def-g9-acids-bases-ph-ph) 5. **Solution of Problem 18.1.** **1.** Acidic: it contains mostly $\ce{H+(aq)}$ and $\ce{Cl-(aq)}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion). **2.** GHS05, corrosive: an acid attacks skin, eyes and metals. **3.** It gives a precise number instead of a colour to compare by eye. **4.** Ten times more: one [pH](#def-g9-acids-bases-ph-ph) unit corresponds to a tenfold dilution. **5.** $10\,\mathrm{L}$, of [pH](#def-g9-acids-bases-ph-ph) 3. **6.** Three: [pH](#def-g9-acids-bases-ph-ph) 2 to 3, 3 to 4, 4 to 5. **7.** $1 \times 10 \times 10 \times 10 = 1000\,\mathrm{L}$. **8.** No: diluting only brings the [pH](#def-g9-acids-bases-ph-ph) closer to 7. **9.** $\ce{H+ + OH- -> H2O}$. **10.** The acid is destroyed, turned into water, instead of being spread through a huge volume; only a little base is needed and no thousand litres of water. **11.** Baking soda dissolved in water (or soapy water). **12.** $1000 - 1 = 999\,\mathrm{L}$ of water.