Acids (Edexcel GCSE Combined Science: Chemistry): Flashcards
Exam code: 1SC0
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Which ion causes a substance to be acidic?
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Which ion causes a substance to be acidic?
The ion that causes a substance to be acidic is a hydrogen ion / H+.
Which ion causes a substance to be alkaline?
The ion that causes a substance to be alkaline is a hydroxide ion / OH-.
Define acid.
An acid is a substance that produces hydrogen ions (H+) when dissolved in water, giving a pH below 7.
What pH is classed as neutral?
pH 7 is classed as neutral.
What pH range is classed as alkaline?
The pH range for alkaline is 8–14.
What pH range is classed as acidic?
The pH range for an acid is 0–6.
True or False?
Acids produce hydrogen ions (H+) when dissolved in water.
True.
Acids produce hydrogen ions (H+) in solution. The higher the concentration of H+ ions, the lower the pH and the more acidic the solution.
What colour is litmus in acidic and alkaline conditions?
Litmus is red in acid and blue in alkaline conditions.
What colour is phenolphthalein when added to an acid?
Phenolphthalein is colourless in acid.
What colour is methyl orange in acidic and alkaline conditions?
Methyl orange is red in acid and yellow in alkaline conditions.
Substances with a pH below 7 are classified as .......... .
Substances with a pH below 7 are classified as acidic.
Describe how to use universal indicator to test the pH of a substance.
To measure the pH of a substance with universal indicator:
Add a few drops of universal indicator solution to the substance.
Check the colour against a colour chart to determine the pH value.
(Universal indicator paper can also be used).
Why is universal indicator not used to measure pH during titrations?
Universal indicator is not used for titrations as it does not give a sharp colour change, which is required to identify the end-point.
The pH scale starts at pH 0 and goes up to what pH?
The pH scale goes from pH 0–14.
What is the pH of distilled water?
The pH of distilled water is 7.
True or False?
The higher the concentration of hydrogen ions in solution, the lower the pH.
True.
The higher the concentration of hydrogen ions, the lower the pH.
Define the pH scale.
The pH scale is a numerical scale from 0 to 14 used to measure the acidity or alkalinity of a solution, based on the concentration of hydrogen ions (H+).
If the pH increases by 1, how much does the concentration of hydrogen ions decrease by?
If the pH increases by 1, the concentration of hydrogen ions decreases by a factor of 10.
True or False?
The higher the concentration of hydroxide ions in solution, the higher the pH.
True.
The higher the concentration of hydroxide ions in solution, the higher the pH.
Which acids of equal concentrations will have more hydrogen ions in solution, a weak acid or a strong acid?
A strong acid will have more hydrogen ions in solution than a weak acid of equal concentration.
True or False?
The lower the pH, the more acidic the solution.
True.
The lower the pH, the more acidic the solution is.
How many times greater is the concentration of hydrogen ions in a solution of pH 4 and pH 6.
The concentration of hydrogen ions in a solution of pH 4 is 100 times (10 x 10) greater than a solution with a pH of 6.
As the pH decreases by 1, the concentration of hydrogen ions in solution increases by a factor of .......... .
As the pH decreases by 1, the concentration of hydrogen ions in solution increases by a factor of 10.
If a hydrogen ion concentration in a solution of pH 5 increases by a factor 1000, what is the new pH?
The new pH is 2.
If the hydrogen ion concentration increases by a factor of 10, the pH decreases by 1. So an increase by a factor of 1000 will decrease the pH from 5 to 2.
What is the aim of the Core Practical: Investigating pH?
To investigate the changes in pH of a fixed volume of dilute HCl on addition of varying amounts of a solid base.
Which solid bases can be used to investigate how pH changes when a base is added to dilute hydrochloric acid?
Calcium oxide (CaO) and calcium hydroxide (Ca(OH)2) are both suitable.
Both are insoluble in water and react with hydrochloric acid, making it easy to add controlled amounts using a spatula.
Define neutralisation.
Neutralisation is a reaction between an acid and a base in which the acid and base cancel each other out, producing a salt and water, with a resulting pH closer to 7.
Name two methods that can be used to measure pH during a neutralisation experiment.
pH probe: gives precise, continuous numerical readings.
Universal Indicator paper: gives an approximate reading from a colour change matched to a pH chart.
When testing pH with Universal Indicator paper during a neutralisation experiment, how is a sample removed from the flask without contaminating it?
A glass rod is used to remove a small sample from the reaction mixture and transfer it to the indicator paper.
The glass rod should be rinsed between readings to prevent contamination of the flask.
When investigating neutralisation, the same .......... of acid is used each time, while the amount of solid base added is .......... .
When investigating neutralisation, the same volume of acid is used each time, while the amount of solid base added is varied.
When plotting results for a neutralisation experiment, what should be on each axis of the graph?
x-axis: amount of base added (e.g. number of spatulas)
y-axis: pH of the reaction mixture
The graph shows pH rising from acidic (below 7) to alkaline (above 7) as more base is added.
When plotting pH against amount of base added to an acid, what does a sudden vertical rise in the graph indicate?
A sudden vertical rise in pH indicates the equivalence point — the point at which the acid has been completely neutralised by the base. Beyond this point, excess base makes the solution alkaline.
As more solid base is added to an acid, the pH .......... , showing that the base is .......... the acid.
As more solid base is added to an acid, the pH increases, showing that the base is neutralising the acid.
State two safety precautions when working with solid bases and dilute acids in a neutralisation experiment.
Safety precautions when working with solid bases and dilute acids in a neutralisation experiment include:
Wear safety goggles to protect eyes from splashes of acid
Avoid skin contact with both the solid base and the dilute acid
If using a metal oxide powder: do not inhale it, as fine powders can irritate the respiratory system
What piece of apparatus is used to measure an accurate, fixed volume of acid at the start of a neutralisation investigation?
A volumetric pipette (e.g. 25 cm3 or 50 cm3) is used to measure a precise, fixed volume of acid.
This keeps the acid volume constant as a controlled variable throughout the experiment.
What does the ⇌ symbol indicate in the following dissociation of an acid?
CH3CH2COOH ⇌ H+ + CH3CH2COO-
The ⇌ symbol indicates that the acid is weak, an equilibrium is established and the acid only partially dissociates.
Define the term strong acid.
A strong acid is an acid that completely dissociates in water, producing a high concentration of H+ ions.
True or False?
Weak acids have a pH between 4–6.
True.
Weak acids have a pH between 4–6.
Give an example of a strong acid.
Hydrochloric acid (HCl), sulfuric acid (H2SO4) and nitric acid (HNO3) are examples of strong acids.
Define the meaning of solute.
A solute is a substance that is dissolved in a solvent to form a solution.
True or False?
A dilute strong acid can have a lower pH than a concentrated weak acid.
True.
A dilute strong acid can have a lower pH than a concentrated weak acid due to complete dissociation.
Define the term solvent.
A solvent is a substance, typically a liquid, that dissolves a solute to form a solution.
What is a concentrated solution?
A concentrated solution contains a large amount of solute in a given volume of solution.
A strong acid .......... dissociates in water, producing a high concentration of H+ ions.
A strong acid completely dissociates in water, producing a high concentration of H+ ions.
What is a dilute solution?
A dilute solution contains a small amount of solute in a given volume of solution.
What is the difference between a strong and weak acid?
A strong acid dissociates completely in water, while a weak acid only partially dissociates.
What is the difference between a base and an alkali?
A base that is water-soluble is referred to as an alkali. All alkalis are bases, but not all bases are alkalis.
What is the pH range for alkalis?
Alkalis have pH values above 7.
What are the common forms of bases?
Bases are usually oxides, hydroxides, or carbonates of metals.
What happens when ammonia reacts with water?
When ammonia reacts with water, it produces hydroxide ions.
How do basic (alkaline) conditions affect red litmus paper?
In basic (alkaline) conditions, red litmus paper turns blue.
True or False?
Aqueous ammonia and ammonium hydroxide are the same.
True.
Aqueous ammonia and ammonium hydroxide are the same thing.
Which ion is produced in alkaline solutions?
The hydroxide, OH-, ion is produced in alkaline solutions.
Give the general equation for the reaction between a base and acid.
The general equation for the reaction between a base and acid is:
acid + base ⟶ salt + water
Name the type of reaction that occurs when a base reacts with an acid.
The type of reaction that occurs when a base reacts with an acid is neutralisation.
Define base.
A base is a substance that neutralises an acid to form a salt and water. Bases are usually metal oxides, metal hydroxides, or metal carbonates.
A base that dissolves in water is called an .......... .
A base that dissolves in water is called an alkali.
Define the term neutralisation.
A neutralisation reaction is a reaction between an acid and a base to form a salt and water.
What is the general equation for the reaction of a metal with an acid?
The general equation for the reaction between a metal and acid is:
metal + acid ⟶ salt + hydrogen
Name the products formed from the reaction between sodium hydroxide and hydrochloric acid.
The products formed from the reaction between sodium hydroxide and hydrochloric acid are sodium chloride and water.
True or False?
All metals react with dilute acids.
False.
Only metals above hydrogen in the reactivity series will react with dilute acids.
State the general equation for the reaction of an acid with a base.
The general equation for the reaction between an acid and a base is:
acid + base ⟶ salt + water
What is the general equation for the reaction of a metal carbonate with an acid?
The general equation for the reaction of a metal carbonate with an acid is:
metal carbonate + acid ⟶ salt + carbon dioxide + water
True or False?
Effervescence is produced when an acid reacts with a metal oxide or hydroxide.
False.
Effervescence is produced when an acid reacts with a metal carbonate, due to the formation of carbon dioxide gas.
Write the word equation for the reaction between sodium carbonate and sulfuric acid.
The word equation for the reaction between sodium carbonate and sulfuric acid is:
sodium carbonate + sulfuric acid ⟶ sodium sulfate + water + carbon dioxide
When an acid reacts with a metal carbonate, the products are a salt, water, and .......... gas.
When an acid reacts with a metal carbonate, the products are a salt, water, and carbon dioxide gas.
What determines the identity of the salt produced in an acid-base neutralisation reaction?
The identity of the salt produced depends on the acid used and the positive ions in the base.
What substances act as bases in acid-base reactions?
Metal oxides, metal hydroxides and metal carbonates act as bases in acid-base reactions.
Give the test and result for hydrogen gas.
The test and result for hydrogen gas is that a burning splint gives a squeaky pop sound.
Give the test and result for carbon dioxide gas.
The test and result for carbon dioxide gas is that limewater turns cloudy (milky).
True or False?
To test for hydrogen gas, a burning splint is held near the gas, and a squeaky pop confirms its presence.
True.
The squeaky pop occurs because hydrogen ignites rapidly and reacts explosively with oxygen in the air, producing a characteristic sound.
What is the chemical name for limewater?
The chemical name for limewater is calcium hydroxide.
Give the formula of carbon dioxide.
The formula of carbon dioxide is CO2.
When carbon dioxide is passed through limewater, the solution turns .......... , showing that CO2 is present.
When carbon dioxide is passed through limewater, the solution turns cloudy (milky), showing that CO2 is present.
Why is it hard to identify hydrogen and carbon dioxide gas?
It is hard to identify these gases as they are colourless and odourless.
What is the limewater test used for?
The limewater test is used to identify carbon dioxide gas. When CO2 is bubbled through limewater (calcium hydroxide solution), the solution turns cloudy (milky).
What is the aim of the Core Practical: Preparing Copper Sulfate?
To prepare a pure, dry sample of hydrated copper(II) sulfate crystals.
What type of reaction is used to prepare copper(II) sulfate from dilute sulfuric acid?
Dilute sulfuric acid is reacted with an excess of copper(II) oxide (an insoluble base).
Using an excess base ensures all the acid is neutralised. This is called the excess insoluble base method.
.......... oxide is added to hot dilute .......... acid until the base is in excess.
Copper(II) oxide is added to hot dilute sulfuric acid until the base is in excess.
Why is copper(II) oxide added in excess when preparing copper(II) sulfate?
To ensure all the acid is used up. If acid remained, it would become dangerously concentrated during evaporation and crystallisation.
How can you tell when copper(II) oxide is in excess during the preparation of copper(II) sulfate?
The base stops dissolving and a suspension of solid base forms in the acid.
After filtering a salt solution, it is gently heated to make it .........., then left in a warm place to .......... .
After filtering a salt solution, it is gently heated to make it saturated, then left in a warm place to crystallise.
When preparing a soluble salt using an excess insoluble base, what is the purpose of the filtration step?
To remove the excess copper(II) oxide (insoluble base) from the copper sulfate solution.
How is the copper sulfate solution checked for saturation before crystallisation in the Preparing Copper Sulfate practical?
A cold glass rod is dipped into the solution; if crystals form on the end, the solution is saturated.
True or False?
Excess copper(II) oxide is added when preparing copper sulfate to ensure all the acid is used up.
True.
Excess copper(II) oxide ensures all the sulfuric acid is neutralised.
Describe the expected appearance of successfully prepared hydrated copper(II) sulfate crystals.
Hydrated copper(II) sulfate crystals are bright blue and regularly shaped.
State two safety precautions when preparing copper(II) sulfate using copper(II) oxide and dilute sulfuric acid.
Safety precautions when preparing copper(II) sulfate using copper(II) oxide and dilute sulfuric acid include:
Wear safety goggles
Avoid skin contact with copper(II) oxide and copper(II) sulfate
Do not inhale copper(II) oxide powder.
Define crystallisation.
Crystallisation is a separation technique used to obtain a pure solid solute from a solution by evaporating the solvent until the solution is saturated, then allowing it to cool so that crystals form.
Define titration.
A titration is a technique used to find the exact volume of one solution needed to completely react with another solution, used to prepare a soluble salt from an acid and an alkali.
Name the method used to prepare a soluble salt from an acid and alkali.
The method used to prepare a soluble salt from an acid and alkali is titration.
True or False?
The same volume of acid is added again to the alkali without indicator.
True.
The same volume of acid determined from the titration is added again to the same volume of alkali, but without the indicator this time.
Name the piece of equipment which could be used to measure a fixed volume of acid, when preparing a soluble salt.
The piece of equipment which could be used to measure a fixed volume of acid is a pipette.
Give the colour of phenolphthalein in acids and alkalis.
Phenolphthalein is colourless in acids and pink in alkalis.
How are crystals produced from a salt solution?
Heat to partially evaporate, leaving a saturated solution. Leave to crystallise, decant excess solution and allow crystals to dry.
In a titration, an .......... is added from a burette into a fixed volume of .......... in a conical flask, with an indicator to show the endpoint.
In a titration, an acid is added from a burette into a fixed volume of alkali in a conical flask, with an indicator to show the endpoint.
How is a soluble salt made?
A soluble salt can be made by the reaction of an acid with an alkali.
What is the purpose of adding an indicator?
The purpose of adding the indicator is to visually determine when the acid has neutralised the alkali by observing the colour change.
State what is meant by an ionic compound.
An ionic compound consists of a giant lattice of positively charged metal ions and negatively charged non-metal ions.
True or False?
All ionic compounds are soluble in water.
False.
Not all ionic compounds are soluble in water, some are insoluble.
Define the term insoluble.
Insoluble means the substance is unable to dissolve in a solvent, usually water.
True or False?
Sodium carbonate is soluble.
True.
Sodium carbonate is soluble.
True or False?
Compounds of potassium are soluble.
True.
Compounds of potassium are soluble.
Give three examples of insoluble sulfates.
Three examples of insoluble sulfates are:
barium sulfate
calcium sulfate
lead(II) sulfate
True or False?
Compounds of sodium are insoluble.
False.
Compounds of sodium are soluble.
Give two examples of soluble hydroxides.
Two examples of soluble hydroxides are:
Sodium hydroxide
Potassium hydroxide
True or False?
All nitrates are insoluble.
False.
All nitrates are soluble.
All compounds containing .........., .......... or .......... ions are soluble.
All compounds containing sodium, potassium or ammonium ions are soluble.
True or False?
Silver chloride is soluble.
False.
Silver chloride is insoluble.
What is an insoluble salt?
An insoluble salt is a salt that does not dissolve in water or other solvents.
Why is the precipitate washed with distilled water?
The precipitate is washed with distilled water to remove traces of the solutions it was made from.
True or False?
Insoluble salts can be prepared using a precipitation reaction.
True.
Insoluble salts can be prepared using a precipitation reaction.
Define precipitation reaction.
A precipitation reaction is a reaction where an insoluble solid (precipitate) is formed from two soluble reactants.
What is meant by a precipitate?
A precipitate is an insoluble solid that is formed from a reaction between two soluble reactants.
When preparing an insoluble salt, the precipitate is separated from the reaction mixture by .......... and then washed with .......... to remove any soluble impurities.
When preparing an insoluble salt, the precipitate is separated from the reaction mixture by filtration and then washed with distilled water to remove any soluble impurities.
What does the method for preparing an insoluble salt involve?
The method for preparing an insoluble salt involves measuring out a fixed volume of one solution and then adding the second salt solution until it is in a slight excess, to ensure the maximum amount of precipitate will be obtained.
Give an example of an insoluble salt often made by precipitation.
Examples of salts made by precipitation include silver and lead(II) salts e.g. lead(II) iodide, silver chloride.
True or False?
The precipitate is recovered by evaporation.
False.
The precipitate is recovered by filtration and then it must be washed with distilled water to remove reactants that are contaminating the residue (recovered solid).
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