0620 · 7.1The characteristic properties of acids and bases

Acids, alkalis and bases

0:00The same acid can react with a metal, neutralise a metal oxide, or release gas from a carbonate. These reactions look different, but they follow a small set of connected patterns. Understanding the particles involved makes those patterns much easier to explain. By the end, you will connect reactions, indicators and acidity to what happens inside the solution. An ion is an atom or group of atoms with an electrical charge.

0:24Aqueous means dissolved in water, rather than simply being a liquid. Chemical equations show which substances react and what they produce, while their state symbols identify solids, liquids, gases and aqueous solutions. Aqueous solutions of acids contain hydrogen ions. Each hydrogen ion has a 1+ charge, and these particles account for the characteristic acidic behaviour. Aqueous solutions of alkalis contain hydroxide ions, each with a 1− charge. Those ions explain why different alkalis share characteristic properties, even when their names and formulae differ.

0:59Hydrochloric acid and sodium hydroxide solution make useful examples. The acid contains hydrogen ions alongside chloride ions, while the alkali contains hydroxide ions alongside sodium ions. The accompanying particles matter when identifying the salt produced in a reaction. However, hydrogen and hydroxide ions are central to understanding acidity, alkalinity and neutralisation. Bases are oxides or hydroxides of metals. Copper(II) oxide and sodium hydroxide are examples, although their behaviour in water differs.

1:30An alkali is a soluble base, so it dissolves in water to form an alkaline solution. Sodium hydroxide is an alkali, whereas copper(II) oxide is an insoluble base. This distinction prevents a common mistake: base and alkali do not mean exactly the same thing. Every alkali is a base, but not every base dissolves in water. An insoluble substance can still react with an acid and neutralise it. Solubility determines whether that base is also described as an alkali.

Indicators and the pH scale

2:00Indicators let us recognise acidic and alkaline solutions without seeing individual ions. These substances change colour depending on the solution they are in. Litmus is red in acid and blue in alkali. An acid therefore turns blue litmus red, while an alkali turns red litmus blue. Thymolphthalein is colourless in acid and blue in alkali. Methyl orange gives a different pair of colours: red in acid and yellow in alkali.

2:24The indicator name does not tell you its colour in every solution. In particular, methyl orange is not simply orange whenever you use it. Examiners often see the word clear used where colourless is meant. Clear means you can see through a liquid, and a blue solution can be perfectly clear. Write colourless for thymolphthalein in acid. Always give both colours when a question asks for a colour change, starting with the colour before the change.

2:47One indicator gives useful evidence, but its colour does not usually tell us an exact pH. Litmus turning blue shows that a solution is alkaline, but not how alkaline it is. We use universal indicator paper when we want a broader comparison. Its range of colours allows us to estimate pH by matching the result against a colour chart. At ordinary laboratory temperatures, a neutral solution has a pH of seven.

3:12Acidic solutions have values below seven, and alkaline solutions have values above seven. Universal indicator is green at neutrality. On this chart, pH one and two are red, three and four are orange, and five and six are yellow. Blue covers pH eight to eleven, while purple marks the strongest alkalis from twelve to fourteen. A lower pH indicates a higher hydrogen ion concentration and greater acidity. Concentration describes how much of something is present in a particular volume.

3:40Comparing equal volumes makes the particle idea easier to picture. The solution with the lower pH contains more hydrogen ions within that same volume of liquid. For alkaline solutions, a higher pH means greater alkalinity and a lower hydrogen ion concentration. Compare pH nine with pH twelve: both are alkaline, but the second is more alkaline. Likewise, pH two indicates greater acidity than pH five. Neutrality does not mean there are no ions; hydrogen and hydroxide ion concentrations are equal.

4:11In a practical task, use a fresh piece of universal indicator paper for each sample. Compare its colour with the supplied chart rather than relying on your memory of a particular shade. Record an estimated pH, then use that value to describe the solution. Indicator observations and their interpretation can be tested in the practical papers.

Acid reactions with metals and bases

4:31A suitable reactive metal reacts with a dilute acid to produce a salt and hydrogen. Magnesium provides a clear example with hydrochloric acid. The word equation is magnesium plus hydrochloric acid gives magnesium chloride plus hydrogen. As the reaction proceeds, the metal is used up and bubbles form because hydrogen gas escapes from the solution. The balanced symbol equation is on the board. One magnesium atom reacts with two units of hydrochloric acid, producing one unit of magnesium chloride and one hydrogen molecule.

5:02Two hydrogen atoms are needed because each hydrogen molecule contains two atoms. The chloride ions become part of the salt rather than forming the gas. Do not replace hydrogen with water in this reaction pattern. A metal reacting with acid produces hydrogen because the hydrogen ions are converted into the gas. Also, the pattern does not mean every metal reacts with every acid. For this topic, focus on familiar reactions between reactive metals and dilute acids, such as magnesium with hydrochloric acid.

5:31When an acid reacts with a base, the products are a salt and water. Hydrochloric acid plus copper(II) oxide gives copper(II) chloride plus water. Here, the oxide is the base even though it does not dissolve in water. Its reaction with acid demonstrates why being insoluble does not prevent a substance from acting as a base. The acid supplies hydrogen ions, which react with the oxide part of the base to form water.

5:54The metal becomes part of the salt produced. This reaction is neutralisation because the base removes the acid’s characteristic behaviour. Notice that no hydrogen gas is required: changing the reactant from a metal to its oxide changes the products. The acid also decides the name of the salt. Hydrochloric acid makes chlorides, sulfuric acid makes sulfates, and nitric acid makes nitrates. For example, sulfuric acid plus magnesium oxide gives magnesium sulfate plus water.

6:23The metal part of the name comes from the base, so you can predict any salt from its two reactants. Neutralisation between an acid and an alkali has an especially simple particle explanation. A hydrogen ion reacts with a hydroxide ion to produce water. The ionic equation shows only the particles directly involved in that change. One positive charge and one negative charge combine to form a neutral water molecule.

6:48Hydrochloric acid plus sodium hydroxide gives sodium chloride plus water. In this example, the sodium and chloride ions remain dissolved while the hydrogen and hydroxide ions react. They are omitted from the neutralisation equation because they do not undergo that chemical change. The state symbols show aqueous ions forming liquid water. Mixing acid and alkali does not automatically produce a neutral final solution. If hydrogen ions remain in excess, the mixture is still acidic.

7:16Excess hydroxide ions leave it alkaline instead. With exactly the right reacting amounts, neither ion is left over and the salt solution is neutral.

Carbonates and ammonium salts

7:25Carbonates react with acids to produce three substances: a salt, water and carbon dioxide. Hydrochloric acid plus calcium carbonate gives calcium chloride plus water plus carbon dioxide. The bubbles therefore contain a different gas from the acid and metal reaction. Recognising the starting material helps you predict which gas forms. The carbonate part reacts with hydrogen ions, forming water and carbon dioxide. Calcium becomes part of the salt, while the gas escapes from the liquid.

7:55The balanced equation shows two units of hydrochloric acid reacting with one unit of calcium carbonate. Keep water in the products even though the bubbles are the most noticeable evidence. A good description of this reaction gives what you would see. Bubbles of gas form, which examiners also accept as effervescence. The solid carbonate gets smaller and may disappear if the acid is in excess. Those are observations, while naming carbon dioxide identifies the gas they come from.

8:21These three reaction patterns are easier to remember when you connect them to the starting substance. A reactive metal produces hydrogen alongside a salt. With a base, neutralisation produces salt and water. A carbonate gives those products plus carbon dioxide, so the gas identifies a different chemical change. Bases also react with ammonium salts to produce ammonia. A familiar example uses sodium hydroxide, which is an alkali, and ammonium chloride.

8:48When the mixture is warmed, ammonia is released. The word equation is ammonium chloride plus sodium hydroxide gives sodium chloride plus water plus ammonia. Any ammonium salt behaves in the same way with a base. Warming ammonium sulfate with calcium hydroxide also releases ammonia, together with calcium sulfate and water. The anion of the ammonium salt ends up in the new salt, while the ammonium part leaves as ammonia gas.

9:16An ammonium ion contains nitrogen and hydrogen and carries a 1+ charge. Hydroxide removes a hydrogen ion from it, producing water. The remaining nitrogen and hydrogen form ammonia, which has no overall charge. This particle explanation connects the reaction to the ability of a base to take up hydrogen ions. The condition of warming is important when describing this practical reaction. State that ammonia is produced rather than simply saying that bubbles appear.

9:42An observation describes what happens, whereas identifying a substance explains what that evidence means. Detailed gas identification tests belong in the video on identification of ions and gases.

Proton transfer and acid strength

9:54This next part is for the Extended paper. An acid is a proton donor, and a base is a proton acceptor. A proton is a hydrogen ion in this acid and base context. An ordinary hydrogen atom has one proton and one electron, so losing its electron leaves the proton behind. The word donor describes what the acid does during a reaction. It transfers a proton to another particle, while the base accepts it.

10:20Hydroxide acts as a base because it accepts a proton to become water. This definition explains neutralisation as a transfer, not merely as two substances cancelling each other out. The ammonium reaction fits the same idea. Hydroxide accepts a proton from an ammonium ion, leaving ammonia behind. Water forms when that proton joins the hydroxide ion. So one definition links reactions that might otherwise seem unrelated. Metal oxides and hydroxides remain our main examples of bases.

10:49Acid strength describes how completely an acid dissociates in aqueous solution. Dissociation means that the acid forms ions when dissolved in water. A strong acid is an acid that is completely dissociated in aqueous solution. Hydrochloric acid is strong, so all of its dissolved molecules form hydrogen ions and chloride ions. The equation on the board shows aqueous hydrochloric acid giving aqueous hydrogen ions and aqueous chloride ions. The single forward arrow shows complete dissociation.

11:19Each unit of hydrochloric acid gives one hydrogen ion and one chloride ion. The word strong therefore describes the extent of ion formation, not simply the amount poured into a beaker. A weak acid is an acid that is partially dissociated in aqueous solution. Ethanoic acid is a weak acid, so only some of its molecules form ions. Most remain as unchanged acid molecules under typical conditions. The equation shows ethanoic acid reacting reversibly to give hydrogen ions and ethanoate ions.

11:49The reversible arrow matters because ion formation and recombination can both occur. Some acid molecules release hydrogen ions, while the reverse process reforms molecules. Partial dissociation does not mean the acid has stopped reacting altogether. It means the solution contains a mixture of unchanged molecules and the ions formed from them. Strength and concentration must be kept separate. Concentration describes the amount of acid dissolved in a particular volume, whereas strength describes how completely it dissociates.

12:17Diluting hydrochloric acid reduces its concentration without changing its classification as a strong acid. A concentrated ethanoic acid solution still contains a weak acid because its dissociation remains partial. At equal acid concentrations, hydrochloric acid produces more hydrogen ions than ethanoic acid because its dissociation is complete. It therefore has the lower pH in that comparison. Without knowing concentration, pH alone cannot reliably identify whether an acid is strong or weak.

12:48A dilute strong acid can contain fewer hydrogen ions than a more concentrated weak acid.

Worked pH and reaction questions

12:53In this worked example, solution X has pH two, solution Y has pH seven, and solution Z has pH twelve. First, classify them by comparing each value with seven. So X is acidic, Y is neutral, and Z is alkaline. Next, compare hydrogen ion concentration using the direction of the pH scale. Solution X has the highest concentration because it has the lowest pH. Solution Z has the lowest hydrogen ion concentration of the three, and it is also the most alkaline.

13:25This is why greater alkalinity must not be confused with more hydrogen ions. Now predict the methyl orange colours for the acidic and alkaline samples. It is red in X and yellow in Z. Finally, identify calcium carbonate as a carbonate reacting with acid. The products are a salt, water and carbon dioxide, but pH alone does not reveal which salt forms. An exam might ask you to define a weak acid and explain a comparison with hydrochloric acid.

13:51Give one creditworthy point in each sentence. A weak acid is partially dissociated in aqueous solution. Hydrochloric acid is completely dissociated, so at equal concentrations it produces more hydrogen ions and has a lower pH. If asked to describe reactions, identify the reactants before choosing the products. Do not give carbon dioxide for a metal reaction or omit water from a carbonate reaction. When asked to state indicator colours, name the specific indicator rather than writing only red or blue.

14:22Those details show that you understand which chemical behaviour the question is testing. The central connection is between particles and observable behaviour. Acids contain hydrogen ions, while alkalis contain hydroxide ions, and neutralisation combines these to form water. Indicators reveal acidity or alkalinity through colour changes, with universal indicator also estimating pH. Reaction patterns tell us which additional products form from metals, bases and carbonates. Remember the two distinctions that prevent many mistakes.

14:52An alkali is a soluble base, while acid strength is about dissociation rather than concentration. For Extended chemistry, acids donate protons and bases accept them. Together, these ideas explain the reactions rather than leaving you with a disconnected list of colours and equations.