Chemistry:
Explaining Chemical
Changes
In this topic, you will be completing some independent work. To
help you with this, this pack has some resources that will help you.
The same resources will help us with multiple lessons, so not
everything is relevant to each lesson. More resources will be added
to this pack when it is required.
It is your task to read through this information and decide what is
important to help you to answer the questions assigned on each
day.
Name: ……………………………………………..
Resources
Use the resources below to answer the questions for each lesson. When you write your answer, be
sure to write down which resource you got it from.
Resource 1:
• Blue Science textbook (page 126-129)
Resource 2:
• BBC KS3 Bitesize
Acids
Dilute acids
Strong acids such as hydrochloric acid, sulfuric acid and nitric acid are laboratory acids that have
been mixed with a lot of water before putting them out for use. Their bottles are labelled with a
warning symbol to show that they can irritate your skin or cause some other minor harm to health.
This means that if any of them makes contact with your skin, it may
become red or blistered. You must wash off any spills with plenty of water,
otherwise your skin may soon feel as if it is burning.
Concentrated acids
Concentrated acids are acids mixed with very little water. They are much
more dangerous than dilute acids. Concentrated acids are corrosive. They
can attack metals and destroy skin if spilled. Their bottles are labelled with
a warning symbol to show that they are corrosive.
Acids
Laboratory acids are far too dangerous to taste, but you will have swallowed some dilute weak acids.
Acids have a sour taste, like vinegar, which contains ethanoic acid, and lemons, which contain citric
acid. These are safe to use in food, but they can still hurt if they get into a cut or into your eyes.
The table shows these and other acids found in food:
Alkalis
A base is a substance that can react with acids and neutralise them. Bases are usually:
• metal oxides, such as copper oxide
• metal hydroxides, such as sodium hydroxide, or
• metal carbonates, such as calcium carbonate
Many bases are insoluble - they do not dissolve in water. However, if a base does dissolve in water, we
also call it an alkali. The table shows two examples of bases:
Bases in the laboratory
Household cleaning products contain strong bases, such as sodium hydroxide solution. Like acids, their
bottles are labelled with a symbol to warn that they may make your skin red or blistered unless you
wash off any spills with plenty of water.
Alkalis feel soapy when they get on your skin, so it is easy to tell when you have had an accident and
must wash your hands.
Just like concentrated acids, concentrated alkalis are corrosive. They can attack metals and destroy
skin if spilled, so their containers are labelled with a warning symbol. Concentrated alkalis are just as
dangerous as concentrated acids, sometimes more dangerous, but many people do not realise this.
Bases in the home
Bases react with oils and fats, so they are often used in household cleaners. For example, drain
cleaners and oven cleaners usually contain sodium hydroxide. Ammonia is also commonly used in
cleaners, and it can be recognised by its choking smell.
It is wise to wear gloves when using these substances, otherwise they will react with your skin and
burn it.
Weak bases and alkalis are found in toothpaste, antacid tablets (to help cure an upset stomach) and
baking powder.
Resource 3:
7. Acids contain hydrogen ions.
• S-cool: the revision website
8. Turn Universal Indicator from green to red, and
have a pH less than 7.
Properties of acids
1. They are liquids.
2. They are solutions of compounds in water.
3. If concentrated they can be corrosive.
4. Acids taste sour (for example, vinegar).
5. Turn blue litmus paper red - this is an easy test
for an acid!
6. Usually react with metals to form salts.
Examples of acids: are vinegar (ethanoic acid)
and lemon juice (citric acid)
Some common acids used in your laboratories
at school will be:
1. Hydrochloric acid, HCl(aq)
2. Nitric acid, HNO3(aq)
3. Sulphuric acid, H2SO4(aq)
Properties of alkalis
Properties of neutral substances
1. They feel soapy to touch.
1. Litmus paper is not affected by neutral paper.
2. They are soluble bases.
2. Tend to be harmless
3. Like acids, they can burn the skin.
3. Universal Indicator stays green.
4. They turn red litmus blue - this is how you test
for an alkali!
Common examples of neutral substances:
5. Alkalis contain hydroxide ions (OH-).
6. They taste bitter.
7. Turns Universal Indicator from green to blue or
purple.
1. Water
2. Sodium chloride solution, NaCl(aq)(common salt)
3. Sugar solution C6H12O6(aq)
Some common alkalis used in your laboratories
at school will be:
1. Sodium hydroxide, NaOH(aq)
2. Ammonia, NH3NH4OH(aq)
3. Calcium hydroxide, Ca(OH)2(aq
The pH scale
The Strength of an Acid
Acids and alkalis can be strong or weak!
So how can we measure their strength?
The strength of an acid or alkali is shown using a scale of numbers called the pH scale. The numbers
go from 0-14.
On the scale it follows that:
An acidic solution has a pH number less than 7
An alkaline solution has a pH number greater than 7
A neutral solution has a pH number of exactly 7.
You can find the pH of any solution using universal indicator. Universal indicator is a mixture of dyes.
It comes as a solution or in paper.
Universal indicator will change from green to a different colour depending on the pH of the solution
you place it in.
Note:
In a strong acid, nearly all the acid molecules form ions.
In a weak acid, only some of the acid molecules form ions.
The more OH- ions (hydroxide ions), the more alkaline an alkali will be.
In other words, the more OH- ions there are the higher the pH number.
Neutralisation resources
A chemical reaction happens if you mix together an acid and a base. The reaction is called
neutralisation. A neutral solution is made if you add just the right amount of acid and base together.
Neutralisation is an exothermic reaction, so the reaction mixture warms up during the reaction.
Metal oxides
Metal oxides act as bases. Here is the general word equation for what happens in their neutralisation
reactions with acids:
metal oxide + acid → a salt + water
The salt made depends on the metal oxide and the acid used. For example, copper chloride is made if
copper oxide and hydrochloric acid are used:
copper oxide + hydrochloric acid → copper chloride + water
CuO + 2HCl → CuCl2 + H2O
Metal hydroxides
Metal hydroxides act as bases. Some of them dissolve in water, so they form alkaline solutions. Here is
the general word equation for what happens in their neutralisation reactions with acids:
metal hydroxide + acid → a salt + water
As with metal oxides, the salt made depends on the metal hydroxide and the acid used. For example,
sodium sulfate is made if sodium hydroxide and sulfuric acid are used:
sodium hydroxide + sulfuric acid → sodium sulfate + water
2NaOH + H2SO4 → Na2SO4 + 2H2O
Notice that a salt plus water are always produced when metal oxides or metal hydroxides react with
acids.
Metal carbonates
Most carbonates are usually insoluble (they do not dissolve in water). They also neutralise acids,
making a salt and water, but this time we get carbon dioxide gas too.
Here is the general word equation for what happens:
metal carbonate + acid → a salt + water + carbon dioxide
The reaction fizzes as bubbles of carbon dioxide are given off. This is easy to remember because we
see the word 'carbonate' in the chemical names. For example, copper carbonate reacts with nitric
acid:
copper carbonate + nitric acid → copper nitrate + water + carbon dioxide
CuCO3 + 2HNO3 → Cu(NO3)2 + H2O + CO2
Using neutralisation
Here are some ways neutralisation is used:
• Farmers use lime (calcium oxide) to neutralise acid soils.
• Your stomach contains hydrochloric acid, and too much of this causes indigestion. Antacid tablets
contain bases such as magnesium hydroxide and magnesium carbonate to neutralise the extra acid.
• Bee stings are acidic. They can be neutralised using baking powder, which contains sodium
hydrogen carbonate.
Exploring acids and alkalis
Do now: what do you already know about acids and alkalis?
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Task 1: what do we use acids for in everyday life? What makes it an acid?
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Task 2: what do we use alkalis for in everyday life? What makes them alkali?
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Task 3: what is the difference between an acid and an alkali?
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Safe handling of acids and alkalis
Do now:
How do acids react?
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How do alkalis react?
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Extension: What do acids and alkalis make when you react them together?
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Concentrated acids
Dilute acids
What are concentrated acids?
What are dilute acids?
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What are the safety concerns of
concentrated acids?
What are the safety concerns of dilute
acids?
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Safety symbol on concentrated acids:
Safety symbol on dilute acids:
Safety of alkalis
What safety precautions do you need to take when you are working with alkalis?
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Concentrated alkalis can be considered to be more dangerous than acids. What property about
them makes people say this?
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Strong and weak acids and alkalis
Strong and weak are not the same as concentrated and dilute. Explain why…
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Using indicators
In this lesson, you are going to investigate whether liquids X or Y are acids or alkalis. To do this,
you need to make your own indicator.
To make the indicator:
1. Add approximately 100 ml of boiling water to a 250 ml baker.
2. Chop some red cabbage on a white tile and add a handful of pieces to the beaker of hot water.
3. Leave the cabbage in the boiling water for approximately 7 minutes.
5. Pour the mixture through a filter funnel with filter paper to remove the pieces of cabbage or
beetroot and collect the liquid in another beaker. This is the indicator.
6. Label four test tubes: acid, alkali, X and Y. Add 2 cm deep of the appropriate liquid to each.
7. Add an equal volume of indicator to each tube and record your observations.
Draw a results table here and record you observations here:
Analyse your results using these questions to help you:
1. Compare the colour of your indicator in acid and alkali.
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2.Explain what your results suggest about solution X.
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3.Explain what your results suggest about solution Y.
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4.Predict what colour your indicator would be in a neutral solution. Test this prediction if
possible.
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Summary question: using what you know about how acids and alkalis react with an indicator,
what happens when you mix acids and alkalis together?
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Using universal indicator
Do now:
What is the difference between an acid and an alkali?
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What do we use indicators for?
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Using indicator
1. Correctly label the universal indicator scale below:
(Think about: strong and weak, acids and alkalis, neutral substances)
2. What else does the pH scale show?
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3. Design an experiment to investigate the pH of the 4 different liquids on the benches:
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Results table
Draw your results table in the box below:
Conclusion
Based on your results, what conclusion can you make about the pH of the different liquids:
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Questions
1. Universal indicator is usually green to begin with. What does this mean?
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2. The solution in each of these test tubes has been tested with universal indicator.
State, for each solution:
1. What the pH value is
2. Whether it is an acid or an alkali and if it strong or weak
3.
A student is given some mixed-up bottles from the science laboratory and asked to find
out which are acids and alkalis and how strong each is. He decides to use litmus paper in
his investigation. Write a paragraph for the student to explain why he should choose
universal indicator, rather than litmus paper, for this task. (Use lined paper if you need)
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Exploring neutralisation
Do now:
Why is universal indicator the most useful indicator?
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Task 1
Design an experiment to test how acids and alkalis react together. Hint: try to make a neutral
solution.
Method
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Results
Task 2
Write down your observations of the titration demonstration.
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Task 3
Write a conclusion to explain the process of neutralisation. You can use these questions below to help
you (but you don’t have to!).
1) Describe the colour changes that the student may have observed.
2) Suggest why we swirled the flask after adding each drop of alkali.
3) State the pH of the solution when it was green.
4) i) Predict what we would observe if they continued to add alkali after the solution became green.
ii)Explain these observations.
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Plenary
In our experiment, the acid and the alkali had an equal concentration. How would the volume of acid
change if we used a higher concentration of acid but the same concentration of alkali?
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Explaining neutralisation
Do now:
Match the start of each sentence with its correct ending…
Task 1
One product that is made when acids and alkalis react is ... (water / hydroxide). Explain…
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When acids and alkalis react, the hydrogen of the acid reacts with the ... (hydroxide / oxygen) of the
alkali. Explain…
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If acid is added to alkali the pH of the alkali ... (increases / decreases). Explain…
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Pure water has a pH of ... (7 / 14).
Task 2
Write the general equation for the reaction between acid and alkali.
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Write an equation for the reaction between sodium hydroxide and:
i) hydrochloric acid
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ii) nitric acid
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iii) sulfuric acid
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Check your understanding…
Paul had four substances:
He dissolved 1 g of each substance in 20 cm3 of distilled water.
He used universal indicator to find the pH of each solution.
(a)
(i)
Sugar solution does not change the colour of green universal indicator.
What does this tell you about sugar solution?
Tick the correct box.
It is an acid.
It is an alkali.
It is neutral.
It is sweet.
1 mark
(ii)
Suggest the pH of citric acid.
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1 mark
(iii)
Indigestion tablets neutralise acid in the stomach.
What does this tell you about indigestion tablets?
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1 mark
(b) Complete the flow chart below with the names of the substances in the boxes at the top of the
page.
3 marks
Marks
/6