The Characteristic Properties of Acids & Bases (Cambridge (CIE) IGCSE Combined Science: Chemistry): Flashcards
Exam code: 0653
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What is an acid?
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What is an acid?
An acid is a substance that produces hydrogen ions (H+) when dissolved in water. Acids have a pH below 7 and can neutralise bases to form a salt and water.
True or False?
Acids turn litmus red and methyl orange red.
True.
Both litmus and methyl orange turn red in acidic conditions. Litmus turns blue in alkalis, and methyl orange turns yellow in alkalis.
What are the three types of substance that acids react with, and what are the products in each case?
Acids react with metals (forming a salt and hydrogen), bases (forming a salt and water) and metal carbonates (forming a salt, carbon dioxide and water).
acid + metal carbonate → .......... + .......... + water
acid + metal carbonate → salt + carbon dioxide + water
The acid provides the anion for the salt name, and CO2 is always released when a carbonate reacts with an acid.
What is an alkali?
An alkali is a soluble base that produces hydroxide ions (OH-) when dissolved in water. Alkalis have a pH above 7.
Why do bases have a pH above 7?
Bases release hydroxide ions (OH-) in aqueous solution. The presence of OH- ions raises the pH above 7, making the solution alkaline.
True or False?
All bases are alkalis.
False.
An alkali is a soluble base. Many bases are insoluble in water (e.g. copper oxide) and are therefore not alkalis. All alkalis are bases, but not all bases are alkalis.
Bases are typically oxides or .......... of metals.
Bases are typically oxides or hydroxides of metals.
Examples include copper(II) oxide (CuO) and sodium hydroxide (NaOH).
What products form when a base reacts with an ammonium salt?
A base reacting with an ammonium salt produces a salt, water and ammonia gas. The base displaces ammonia from the ammonium compound.
What does the pH scale measure?
The pH scale measures the relative acidity or alkalinity of a solution. It runs from 0 to 14: pH below 7 is acidic, pH 7 is neutral and pH above 7 is alkaline.
A neutral solution has a pH of .......... and gives a .......... colour with universal indicator.
A neutral solution has a pH of 7 and gives a green colour with universal indicator.
True or False?
A lower pH value means a more acidic solution.
True.
The pH scale runs from 0 to 14. The lower the pH, the higher the concentration of H+ ions and the more acidic the solution.
How is universal indicator paper used to measure the pH of a solution?
A strip of universal indicator paper is dipped into the solution. The colour it turns is matched to a colour chart, which gives the corresponding pH value.
Why is universal indicator more useful than litmus for comparing the acidity of two solutions?
Universal indicator gives a range of colours across the pH scale, allowing the pH to be estimated and solutions to be compared numerically. Litmus only distinguishes between acid and alkali, with no information on strength.
As a solution becomes more alkaline, its universal indicator colour moves towards .......... and its pH moves towards 14.
As a solution becomes more alkaline, its universal indicator colour moves towards blue/purple and its pH moves towards 14.
True or False?
A solution with pH 2 has a higher hydrogen ion concentration than a solution with pH 5.
True.
A lower pH indicates a higher concentration of H+ ions. pH 2 is more acidic than pH 5, so it contains more hydrogen ions per unit volume.
State the approximate pH range for a strong acid and for a strong alkali.
A strong acid has a pH of approximately 0–2. A strong alkali has a pH of approximately 12–14.
What is an acidic oxide?
An acidic oxide is an oxide formed from a non-metal element. Examples include carbon dioxide (CO2) and sulfur dioxide (SO2).
Oxides of metals are .......... oxides, while oxides of non-metals are .......... oxides.
Oxides of metals are basic oxides, while oxides of non-metals are acidic oxides.
True or False?
Carbon dioxide (CO2) is a basic oxide.
False.
CO2 is formed from carbon, a non-metal. Oxides of non-metals are acidic oxides, not basic oxides.
Give two examples of basic oxides.
Two examples of basic oxides:
Copper(II) oxide (CuO)
Calcium oxide (CaO)
Both are formed from metals.
Why do metals form basic oxides while non-metals form acidic oxides?
The acid-base character of an oxide is determined by the metallic character of the element. Metals tend to lose electrons and form oxides that react with acids, making them basic. Non-metals tend to share or gain electrons and form oxides that react with bases, making them acidic.
True or False?
Sulfur dioxide (SO2) is an acidic oxide.
True.
Sulfur is a non-metal, so SO2 is an acidic oxide. It dissolves in water to form sulfurous acid.
CO2 is an .......... oxide because carbon is a non-metal.
CaO is a .......... oxide because calcium is a metal.
CO2 is an acidic oxide because carbon is a non-metal.
CaO is a basic oxide because calcium is a metal.
What type of oxide is formed by elements on the left-hand side of the Periodic Table?
Elements on the left-hand side of the Periodic Table are metals, so they form basic oxides. Examples include Na2O and MgO.
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