Acids, Bases & Salts (WJEC GCSE Science (Double Award): Chemistry): Flashcards
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- FrontThe pH Scale
Define pH.
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Cards in this collection (90)
Define pH.
pH is a numerical scale from 1 to 14 used to measure how acidic or alkaline a solution is.
True or False?
A solution with a pH of 3 is more acidic than a solution with a pH of 6.
True.
The lower the pH, the more acidic the solution. pH 3 is more acidic than pH 6.
A solution with a pH of 7 is described as ........... Solutions with pH values above 7 are ...........
A solution with a pH of 7 is described as neutral. Solutions with pH values above 7 are alkaline.
What pH range describes a strong acid?
A strong acid has a pH in the range 0–2.
Define universal indicator.
Universal indicator is a mixture of plant indicators that changes through a range of colours across the pH scale, used to estimate the pH of an unknown solution.
On the pH scale, weak acids have pH values of ........... Strong alkalis have pH values of ...........
On the pH scale, weak acids have pH values of 3–6. Strong alkalis have pH values of 12–14.
True or False?
Sodium hydroxide (NaOH) is a common acid.
False.
Sodium hydroxide is a common alkali, not an acid. It has a pH above 7.
Why is universal indicator only able to give an approximate value for pH?
Universal indicator gives only an approximate pH because it produces a colour that must be matched against a colour chart, rather than giving an exact numerical reading.
A student tests a solution and finds it turns universal indicator orange. What can the student conclude about the solution?
The student can conclude that the solution is a weak acid, with a pH of approximately 3–6.
What are hydrogen ions?
Hydrogen ions are H+ ions released by an acid when dissolved in water. Their presence is what makes a solution acidic.
When hydrochloric acid dissolves in water it produces H+ ions and .......... ions, making the solution .......... .
When hydrochloric acid dissolves in water it produces H+ ions and Cl– ions, making the solution acidic.
True or False?
Alkalis contain hydroxide ions (OH–) which are responsible for making a solution alkaline.
True.
The presence of OH– ions is what makes an aqueous solution alkaline.
When sodium hydroxide dissolves in water, what two ions are produced?
Sodium hydroxide produces Na+ (aq) and OH– (aq) ions when dissolved in water.
Define alkali.
An alkali is a base that dissolves in water to produce hydroxide ions (OH–), giving a solution with a pH above 7.
Acids contain .......... ions. Alkalis contain .......... ions.
Acids contain H+ ions. Alkalis contain OH– ions.
Why does wearing gloves and eye protection become necessary when handling moderately concentrated acids?
Moderately concentrated acids are corrosive and can cause serious damage to skin and eyes on contact.
True or False?
All acids have a sour taste and a pH above 7.
False.
Acids have a sour taste but a pH below 7, not above 7.
What is the general word equation for the reaction of a metal with an acid?
Metal + acid → salt + hydrogen
True or False?
Copper reacts with dilute hydrochloric acid to produce copper chloride and hydrogen gas.
False.
Copper is below hydrogen in the reactivity series and does not react with dilute acids.
Magnesium reacts with sulfuric acid to form magnesium sulfate and .......... . The equation is: Mg (s) + H2SO4 (aq) → .......... (aq) + H2 (g).
Magnesium reacts with sulfuric acid to form magnesium sulfate and hydrogen. The equation is: Mg (s) + H2SO4 (aq) → MgSO4 (aq) + H2 (g).
How can you test that hydrogen gas is produced when a metal reacts with an acid?
Hydrogen gas is identified because it burns with a squeaky pop when a lit splint is held near the mouth of the test tube.
Why is it dangerous to react potassium or sodium with dilute acids?
Potassium and sodium are very high in the reactivity series and react explosively with acids because of how vigorously they react.
Only metals .......... hydrogen in the reactivity series will react with dilute acids. The more reactive the metal, the more .......... the reaction.
Only metals above hydrogen in the reactivity series will react with dilute acids. The more reactive the metal, the more vigorous the reaction.
True or False?
When zinc reacts with hydrochloric acid, the balanced equation is: Zn (s) + 2HCl (aq) → ZnCl2 (aq) + H2 (g).
True.
Two moles of HCl are needed to react with one mole of Zn because zinc forms the Zn2+ ion, requiring 2 Cl– ions.
When making zinc sulfate crystals from zinc and sulfuric acid, why is the zinc added in excess?
Zinc is added in excess to ensure that all the acid has reacted, so no unreacted acid remains in the final product.
Write the word equation for the reaction between iron and sulfuric acid.
Iron + sulfuric acid → iron sulfate + hydrogen
Define neutralisation.
Neutralisation is the reaction between an acid and a base that produces a salt and water. It is an exothermic reaction.
In a neutralisation reaction: acid + base → .......... + .......... . If the base is a carbonate, .......... is also produced.
In a neutralisation reaction: acid + base → salt + water. If the base is a carbonate, carbon dioxide is also produced.
True or False?
All bases dissolve in water to form alkalis.
False.
Many bases are insoluble in water. Only bases that dissolve in water are called alkalis.
Write a balanced symbol equation for the reaction of sulfuric acid with sodium hydroxide.
H2SO4 + 2NaOH → Na2SO4 + 2H2O
The salt produced is sodium sulfate. The equation shows a 2:1 ratio of NaOH to H2SO4 because sulfuric acid is diprotic (produces two H+ ions).
How can you tell by observation that an acid is reacting with a metal carbonate rather than a metal oxide?
When an acid reacts with a metal carbonate, effervescence (fizzing) is observed because carbon dioxide gas is produced. No fizzing occurs with a metal oxide.
Hydrochloric acid produces .......... salts. Sulfuric acid produces .......... salts. Nitric acid produces .......... salts.
Hydrochloric acid produces chloride salts. Sulfuric acid produces sulfate salts. Nitric acid produces nitrate salts.
True or False?
The reaction between a metal and an acid is a neutralisation reaction.
False.
The reaction between a metal and an acid is a redox (displacement) reaction, not neutralisation, because neutralisation specifically involves an acid reacting with a base or alkali — metals are not bases.
Write the net ionic equation for all acid-base neutralisation reactions. Include state symbols. (Higher Tier Only)
H+ (aq) + OH– (aq) → H2O (l)
This is the net ionic equation for all acid-base neutralisations. The H+ and OH– ions combine to form water.
Give one real-world application of neutralisation in agriculture.
Farmers add lime (calcium hydroxide or calcium oxide) to soil to neutralise overly acidic conditions, because most crops grow poorly if the pH drops too low (typically below about 5.5–6).
Describe how a titration is carried out to find the volume of acid needed to neutralise an alkali. (Higher Tier Only)
Use a burette to add acid drop by drop to a known volume of alkali containing a few drops of indicator. Add acid slowly near the endpoint until the indicator permanently changes colour. Record the volume of acid added from the burette.
What is the carbonate ion?
The carbonate ion is CO32–. It is present in all carbonates and reacts with acids to produce carbon dioxide gas.
When an acid reacts with a carbonate, the gas produced is .......... . This can be confirmed by bubbling the gas through .......... which turns .......... .
When an acid reacts with a carbonate, the gas produced is carbon dioxide. This can be confirmed by bubbling the gas through limewater which turns milky (cloudy).
True or False?
The observation when an acid reacts with a carbonate is fizzing.
True.
Fizzing (effervescence) is the correct observation. Stating that carbon dioxide is formed is a conclusion, not an observation.
Write the ionic equation for the reaction of a carbonate ion with an acid. Include state symbols.
CO32– (aq) + 2H+ (aq) → CO2 (g) + H2O (l)
This is the net ionic equation for any acid reacting with a carbonate ion.
What is the chemical equation for limewater turning milky in the presence of carbon dioxide?
CO2 (g) + Ca(OH)2 (aq) → CaCO3 (s) + H2O (l)
A white precipitate of calcium carbonate forms, making the limewater appear milky.
The ionic equation for the acid-carbonate reaction is: CO32– (aq) + .......... → CO2 (g) + H2O (l).
The ionic equation for the acid-carbonate reaction is: CO32– (aq) + 2H+ (aq) → CO2 (g) + H2O (l).
True or False?
You must connect the test tube containing the suspected carbonate to limewater quickly so that no CO2 gas escapes.
True.
If the connection is slow, CO2 escapes before reaching the limewater, giving a false negative result.
Define a salt in chemistry.
A salt is a compound formed when the hydrogen atom in an acid is replaced by a metal ion.
To make a soluble salt, an acid is reacted with an insoluble base added in .......... . The excess solid is then removed by ...........
To make a soluble salt, an acid is reacted with an insoluble base added in excess. The excess solid is then removed by filtration.
True or False?
The insoluble base is added in excess to ensure all the acid has reacted.
True.
Adding the base in excess ensures all the acid is used up, preventing unreacted acid from concentrating dangerously during evaporation.
Write the balanced symbol equation for the reaction of copper(II) oxide with sulfuric acid to make copper(II) sulfate. Include state symbols.
CuO (s) + H2SO4 (aq) → CuSO4 (aq) + H2O (l)
This reaction produces copper(II) sulfate and water.
Why is it dangerous to leave unreacted acid in a solution being evaporated to form crystals?
Any unreacted acid becomes dangerously concentrated during evaporation and crystallisation, creating a hazard.
After filtering the salt solution, it is heated until .......... begin to appear. Allowing slow evaporation over days produces .......... crystals.
After filtering the salt solution, it is heated until small crystals begin to appear. Allowing slow evaporation over days produces larger crystals.
True or False?
If a carbonate is used as the solid base to make a soluble salt, carbon dioxide gas is produced and released.
True.
When a carbonate reacts with an acid, CO2 gas is produced and escapes into the atmosphere, unlike with metal oxides or hydroxides.
Which metals can be used to react directly with an acid to form a soluble salt?
Only metals that are above hydrogen in the reactivity series can be used, and the metal must not be so reactive that the reaction becomes dangerous.
In the specified practical for making copper(II) sulfate, which acid and insoluble base are used?
Dilute sulfuric acid (H2SO4) is used with copper(II) oxide (CuO) as the insoluble base.
In the preparation of a soluble salt, the insoluble base is added until it .......... (stops disappearing). This shows the acid is in ...........
In the preparation of a soluble salt, the insoluble base is added until it stops disappearing and a suspension forms. This shows the acid is fully neutralised (base is in excess).
True or False?
The indicator is left in the solution when making crystals of a soluble salt by the acid-base method.
False.
The indicator is not added when making the final salt by titration. It is used only to find the exact volume of acid required, then the experiment is repeated without indicator.
How does a student check whether the salt solution is saturated during the preparation of copper(II) sulfate crystals?
A cold glass rod is dipped into the heated solution. If crystals form on the rod when it is removed, the solution is saturated.
After filtering the copper(II) sulfate solution, the student heats it to evaporate roughly .......... of the water, then leaves it in a warm place to .......... .
After filtering the copper(II) sulfate solution, the student heats it to evaporate roughly half of the water, then leaves it in a warm place to crystallise.
Why is the mixture filtered during the preparation of copper(II) sulfate crystals?
The mixture is filtered to remove excess unreacted copper(II) oxide, so that only the dissolved copper(II) sulfate salt remains in solution.
True or False?
Heating the filtered solution strongly will result in the formation of larger crystals.
False.
Heating strongly produces smaller crystals. Larger crystals form when the solution is left to evaporate slowly over about a week.
In the specified practical, how is indicator paper used and what does it tell the student?
A glass rod is touched onto indicator paper after adding the base. A neutral colour reading confirms that the acid has been fully neutralised.
What are the two parts that make up the name of a salt?
The first part comes from the metal, metal oxide or metal carbonate used. The second part comes from the acid used in the reaction.
Hydrochloric acid always produces .......... salts. Sulfuric acid always produces .......... salts. Nitric acid always produces .......... salts.
Hydrochloric acid always produces chloride salts. Sulfuric acid always produces sulfate salts. Nitric acid always produces nitrate salts.
True or False?
The reaction of sulfuric acid with calcium carbonate produces calcium sulfate.
True.
Sulfuric acid produces sulfate salts. The metal here is calcium, so the salt is calcium sulfate (CaSO4).
Name the salt produced when hydrochloric acid reacts with magnesium oxide.
The salt produced is magnesium chloride (MgCl2). Hydrochloric acid produces chloride salts and the metal is magnesium.
Write the balanced symbol equation for the reaction between nitric acid and calcium hydroxide.
2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
The salt produced is calcium nitrate.
Potassium hydroxide reacts with nitric acid to form the salt .......... and water. The formula of this salt is .......... .
Potassium hydroxide reacts with nitric acid to form the salt potassium nitrate and water. The formula of this salt is KNO3.
What is the correct formula and charge for the nitrate ion, NO3-?
True.
Nitric acid (HNO3) produces the nitrate ion (NO3–) in solution, which is why all salts from nitric acid are nitrates.
Name the salt formed when sulfuric acid reacts with magnesium oxide.
The salt formed is magnesium sulfate (MgSO4). Sulfuric acid produces sulfate salts and the metal is magnesium.
What reagents are added to test for sulfate ions in solution?
First add dilute hydrochloric acid, then add a few drops of barium chloride solution. A white precipitate confirms the presence of sulfate ions.
If sulfate ions are present, adding barium chloride solution produces a .......... precipitate of ...........
If sulfate ions are present, adding barium chloride solution produces a white precipitate of barium sulfate (BaSO4).
True or False?
The ionic equation for the test for sulfate ions is: Ba2+ (aq) + SO42– (aq) → BaSO4 (s).
True.
This is the correct ionic equation. The barium ion combines with the sulfate ion to form the insoluble white precipitate barium sulfate.
Why is dilute hydrochloric acid added before barium chloride when testing for sulfate ions?
The hydrochloric acid is added first to remove any carbonates that may be present, because they would also produce a white precipitate and give a false positive result.
The test for sulfate ions can also be carried out using .......... acid and .......... solution instead of HCl and BaCl2.
The test for sulfate ions can also be carried out using nitric acid and barium nitrate solution instead of HCl and BaCl2.
A student adds dilute HCl and then barium chloride solution to a sample and observes no precipitate. What does this tell the student?
The absence of a precipitate shows that no sulfate ions are present in the sample.
True or False?
The full symbol equation Ba2+ (aq) + SO42– (aq) → BaSO4 (s) is the same as the ionic equation for this test.
True.
Because only the barium ion and sulfate ion take part in forming the precipitate, the ionic equation and the full symbol equation are the same for this reaction.
Define concordant results in a titration. (Higher Tier Only)
(Higher Tier Only)
Concordant results are two or more titration readings that are within 0.1 cm3 of each other. Only concordant results are used to calculate the mean titre.
(Higher Tier Only)
In a titration, if 30 cm3 of acid neutralises 15 cm3 of an unknown alkali, the alkali must be .......... as concentrated as the acid, because .......... the volume of alkali is needed.
(Higher Tier Only)
In a titration, if 30 cm3 of acid neutralises 15 cm3 of an unknown alkali, the alkali must be twice as concentrated as the acid, because half the volume of alkali is needed.
True or False?
(Higher Tier Only)
If a greater volume of acid is used than alkali in a titration, the acid is more concentrated than the alkali.
False.
(Higher Tier Only)
If more acid is needed to neutralise the alkali, the acid is less concentrated than the alkali, not more concentrated.
Why is a rough titration performed first?
(Higher Tier Only)
A rough titration establishes approximately where the end-point is, so that subsequent accurate titrations can be slowed to drop-by-drop addition near that volume.
(Higher Tier Only)
To prepare a soluble salt by titration, the experiment is repeated using the same volume of acid and alkali but without adding a .......... to avoid contaminating the product.
(Higher Tier Only)
To prepare a soluble salt by titration, the experiment is repeated using the same volume of acid and alkali but without adding an indicator to avoid contaminating the product.
How is the actual concentration of an acid calculated from titration data? (Higher Tier Only)
(Higher Tier Only)
Calculate moles of the alkali: moles = concentration x volume (dm3).
Use the molar ratio from the equation to find moles of acid.
Calculate concentration of acid: concentration = moles / volume (dm3).
True or False?
(Higher Tier Only)
The rough titration result is included when calculating the mean titre.
False.
(Higher Tier Only)
The rough titre is excluded from the mean calculation. Only concordant results (within 0.1 cm3 of each other) are used.
25 cm3 of 0.5 mol dm–3 KOH is neutralised by 0.50 mol dm–3 nitric acid. Calculate the volume of acid required in cm3. (Higher Tier Only)
(Higher Tier Only)
Moles KOH = 0.5 x 0.025 = 0.0125 mol. Ratio 1:1, so moles HNO3 = 0.0125 mol. Volume = 0.0125 / 0.50 = 0.025 dm3 = 25 cm3.
What pieces of apparatus are needed to carry out an acid-base titration? (Higher Tier Only)
(Higher Tier Only)
A 25 cm3 volumetric pipette, pipette filler, 50 cm3 burette, conical flask, small funnel, clamp stand, clamp and white tile.
(Higher Tier Only)
Burette readings are recorded to the nearest .......... cm3. The titre is calculated by subtracting the .......... reading from the .......... reading.
(Higher Tier Only)
Burette readings are recorded to the nearest 0.05 cm3. The titre is calculated by subtracting the initial reading from the final reading.
True or False?
(Higher Tier Only)
A funnel should be left in the burette during the titration to prevent spills.
False.
(Higher Tier Only)
The funnel must be removed before starting. If left in place, it can drip liquid into the burette and make the initial reading false.
Why is a white tile placed under the conical flask during a titration?
(Higher Tier Only)
A white tile is used so that the colour change at the end-point of the indicator is easier to see clearly against a pale background.
(Higher Tier Only)
The titration is repeated until two .......... results are obtained, which are within .......... cm3 of each other.
(Higher Tier Only)
The titration is repeated until two concordant results are obtained, which are within 0.1 cm3 of each other.
Near the end-point of a titration, why should acid be added one drop at a time?
(Higher Tier Only)
Near the end-point, acid is added drop by drop to avoid overshooting the end-point, ensuring an accurate result as the indicator colour change is permanent after just one extra drop.
True or False?
(Higher Tier Only)
In the worked example, Titre 1 = 29.10 cm3 and Titre 2 = 29.00 cm3. These are concordant, so the mean titre is 29.05 cm3.
True.
(Higher Tier Only)
The two titres differ by 0.10 cm3, which is within the 0.1 cm3 concordance limit. Mean = (29.10 + 29.00) / 2 = 29.05 cm3.
In a titration, 14.95 cm3 of HCl was needed to neutralise 25 cm3 of 0.1 mol dm–3 NaOH. Is the acid more or less concentrated than the alkali? (Higher Tier Only)
(Higher Tier Only)
The acid is more concentrated than the alkali because a smaller volume of acid (14.95 cm3) was needed to neutralise the larger volume of alkali (25 cm3).
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