Acids and metals
Salt
A compound formed when the hydrogen in an acid is replaced by a metal ion or an ammonium ion.
Effervescence
The rapid release of bubbles of gas from a liquid during a chemical reaction.
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A dilute acid reacts with many metals to form a salt and hydrogen gas.
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The general word equation for this reaction is shown below.
metal+acid→salt+hydrogen \text{metal} + \text{acid} \rightarrow \text{salt} + \text{hydrogen} metal+acid→salt+hydrogen -
Hydrogen is a gas, so it escapes from the mixture as bubbles and you see effervescence around the metal.
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The piece of metal gets smaller and finally disappears, because its atoms are used up to make the salt.
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The salts formed in these reactions are usually soluble, so the salt stays dissolved and the mixture ends as a clear solution.
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The reaction gives out thermal energy, so the test tube feels warm.
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The name of the salt has two parts: the metal gives the first word and the acid gives the second.
- Hydrochloric acid, HCl\text{HCl}HCl, forms salts called chlorides.
- Sulfuric acid, H2SO4\text{H}_2\text{SO}_4H2SO4, forms salts called sulfates.
- Nitric acid, HNO3\text{HNO}_3HNO3, forms salts called nitrates.
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Not every metal reacts, because metals of low reactivity such as copper, silver and gold do not react with dilute acids at all.
Magnesium and hydrochloric acid
- Magnesium is the metal and hydrochloric acid makes chlorides, so the salt formed is magnesium chloride.
- Magnesium forms Mg2+\text{Mg}^{2+}Mg2+ ions and chloride ions are Cl−\text{Cl}^{-}Cl−, so the formula of the salt is MgCl2\text{MgCl}_2MgCl2.
- Two HCl\text{HCl}HCl units are needed to supply the two chloride ions, and their two hydrogen atoms make one H2\text{H}_2H2 molecule.
- In the same way, zinc and sulfuric acid give zinc sulfate and hydrogen.
Acids and bases
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An acid reacts with a base to form a salt and water only.
base+acid→salt+water \text{base} + \text{acid} \rightarrow \text{salt} + \text{water} base+acid→salt+water -
Bases include metal oxides, such as copper(II) oxide, and metal hydroxides, such as sodium hydroxide.
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No gas is made in this reaction, so there is no fizzing.
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When black copper(II) oxide is warmed with dilute sulfuric acid, the black solid disappears and the colourless acid turns blue as copper(II) sulfate forms.
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When sodium hydroxide solution is added to hydrochloric acid, both solutions and the products are colourless, so the only sign of reaction is a small rise in temperature.
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The salt is named in the same way as before, with the metal from the base and the ending from the acid.
Copper(II) oxide and sulfuric acid
- Copper(II) oxide supplies the metal and sulfuric acid makes sulfates, so the salt is copper(II) sulfate.
- Sodium hydroxide and hydrochloric acid react in the same pattern to give sodium chloride and water.
Acids and carbonates
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An acid reacts with a carbonate to form a salt, water and carbon dioxide.
carbonate+acid→salt+water+carbon dioxide \text{carbonate} + \text{acid} \rightarrow \text{salt} + \text{water} + \text{carbon dioxide} carbonate+acid→salt+water+carbon dioxide -
Carbon dioxide is a gas, so the mixture shows effervescence.
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A solid carbonate, such as calcium carbonate in the form of marble chips, gets smaller and disappears if enough acid is present.
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The fizzing stops when either the carbonate or the acid has been completely used up.
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Compared with a metal, a carbonate gives carbon dioxide rather than hydrogen, and it also produces water.
Calcium carbonate and hydrochloric acid
- Calcium carbonate supplies the metal and hydrochloric acid makes chlorides, so the salt is calcium chloride.
- Sodium carbonate and sulfuric acid follow the same pattern to give sodium sulfate.
Test-tube reactions of a dilute acid
- Aim: to observe how dilute hydrochloric acid reacts with a metal, a base and a carbonate, and to identify any gas given off.
- Apparatus: test tubes in a rack, test-tube holder, dilute hydrochloric acid, 10 cm310\ \text{cm}^310 cm3 measuring cylinder, magnesium ribbon cut into 2 cm2\ \text{cm}2 cm strips, copper(II) oxide powder, marble chips, spatula, bung fitted with a delivery tube, limewater, wooden splints, Bunsen burner, heat-resistant mat and eye protection.
- Method, acid and metal:
- Pour 3 cm33\ \text{cm}^33 cm3 of dilute hydrochloric acid into a test tube standing in the rack.
- Drop in one strip of magnesium ribbon and watch the surface of the metal closely.
- Feel the outside of the tube near the bottom to check for any temperature change.
- Hold a lighted splint at the mouth of the tube while the bubbling is steady.
- Method, acid and base:
- Add half a spatula of black copper(II) oxide to 3 cm33\ \text{cm}^33 cm3 of the acid in a clean tube.
- Hold the tube in a test-tube holder and warm it gently in a Bunsen flame, pointing the mouth away from everyone and shaking it gently.
- Stop heating before the liquid boils and note the colour of the solution.
- Method, acid and carbonate:
- Pour 3 cm33\ \text{cm}^33 cm3 of limewater into a second tube and stand it in the rack.
- Place two small marble chips in a clean tube and add 5 cm35\ \text{cm}^35 cm3 of the acid.
- Fit the bung straight away and let the gas bubble through the limewater.
- Results:
- Magnesium fizzes quickly, gets smaller and disappears, the tube feels warm, and the gas burns with a squeaky pop.
- Copper(II) oxide disappears and the solution turns blue, with no bubbles at any stage.
- The marble chips fizz and shrink, and the limewater turns milky.
- Conclusion: the metal gives hydrogen, the base gives no gas, and the carbonate gives carbon dioxide, while every reaction forms a salt.
- Safety: wear eye protection, treat dilute acid as an irritant to skin and eyes, keep the splint flame away from the acid bottle, and never point a heated tube at anyone.
Acids and indicators
Indicator
A substance that shows one colour in an acidic solution and a different colour in an alkaline solution.
- An indicator is a dye that has one colour in an acid and a different colour in an alkali.
- Litmus is used as paper, while methyl orange and thymolphthalein are added as a few drops of solution.
- Litmus
- An acid turns blue litmus paper red.
- Red litmus paper stays red in an acid, so only blue litmus shows a change.
- Methyl orange
- Methyl orange is red in an acidic solution.
- A few drops added to an acid turn the whole solution red.
- Thymolphthalein
- Thymolphthalein is colourless in an acidic solution.
- A colourless result does not prove that a solution is acidic, because thymolphthalein is also colourless in a neutral solution.
- Do not describe acidic litmus as "pink", because the correct colour is red.
- Do not mix up the gases, because metals give hydrogen and carbonates give carbon dioxide.
- Do not leave water out of the products when an acid reacts with a base or a carbonate.
Describing observations
- Write what you would see, such as fizzing or bubbles, rather than naming the gas, when a question asks for observations.
- Say that a solid disappears or gets smaller, and give a colour change with both the starting and final colours.
- Check the acid before naming a salt, so hydrochloric acid gives a chloride, sulfuric acid a sulfate and nitric acid a nitrate.
- Give the indicator colour in the acid itself, such as "methyl orange turns red", not just "it changes colour".
- Which two products form when a metal reacts with a dilute acid?
- Which three products form when a carbonate reacts with a dilute acid?
- Name the salt formed when zinc reacts with sulfuric acid.
- What do you see when black copper(II) oxide is warmed with dilute sulfuric acid?
- What colours are blue litmus, methyl orange and thymolphthalein in an acidic solution?
