Unit 7E

Acids and Alkalis

Name: ……………………………..

Year 7: …………


Unit 7 E: Acids and Alkalis

______

Acids, bases, alkalis and metals are found in the laboratory and at home. They can be irritant or corrosive and must be handled carefully.

How acid or alkaline a chemical is can be measured on the pH scale, using indicators like litmus and universal indicator. Acids and bases react together to form salts and other products too.

Acids:

Acids in the laboratory:

Dilute acids:

You will have used some dilute acids at school,

such as hydrochloric acid, sulphuric acid and nitric acid.

Their bottles are labelled with the warning symbol for 'irritant'.

This means that if any of them makes contact with your skin,

it will become red or blistered. You must wash off any spills

with plenty of water, otherwise your skin will soon feel as

if it is burning.

Concentrated acids:

You are unlikely to have used concentrated acids but your teacher might have shown you some experiments with them. This is because concentrated acids are corrosive. They can attack metals and destroy skin if spilled.

Acids in the home:

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.

Other acids you will find at home are carbonic acid in fizzy drinks, tannic acid in tea and ascorbic acid which is vitamin C, found in fruit and vegetables.

Bases v alkalis:

Bases are substances that react with acids and neutralise them. They are usually metal oxides, metal hydroxides, metal carbonates or metal hydrogen carbonates. Many bases are insoluble - they do not dissolve in water.

If a base does dissolve in water, we call it an alkali.

Here are two examples:

·  Copper oxide is a base because it will react with acids and neutralise them, but it is not an alkali because it does not dissolve in water.

·  Sodium hydroxide is a base because it will react with acids and neutralise them. It's also an alkali because it dissolves in water.

All alkalis are bases, but only soluble bases are alkalis

Bases in the laboratory:

You will have used some strong bases and alkalis at school, such as sodium hydroxide solution. Like acids, their bottles are labelled with the warning symbol for 'irritant'. This means that they will 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.

Concentrated alkalis are corrosive. They can attack metals and destroy skin if spilled. They are just as dangerous as concentrated acids, but many people do not realise this.

Bases in the home:

Bases react with oils and fats, so they are often used in strong household cleaners. Drain cleaners and oven cleaners usually contain sodium hydroxide for example. And ammonia is also commonly used in cleaners. Ammonia 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.

Indicators and the pH scale:

When an acid is dissolved in water we get an acidic solution, and alkalis make alkaline solutions. If a solution is neither acidic nor alkaline we call it neutral. Pure water is neutral, and so is paraffin.

Indicators are substances that change colour when they are added to acidic or alkaline solutions. You can prepare homemade indicators from red cabbage or beetroot juice - these will help you see if a solution is acidic or alkaline.

Litmus and universal indicator are two indicators that are commonly used in the laboratory.

Litmus indicator:

Litmus indicator solution turns red in acidic solutions and blue in alkaline solutions - and it turns purple in neutral solutions.

Litmus paper is usually more reliable, and comes as red litmus paper and blue litmus paper. The table shows the colour changes it can make.

Red litmus / Blue litmus
Acidic solution / Stays red / Turns red
Neutral solution / Turns blue / Stays blue
Alkaline solution / Turns blue / Stays blue

Universal indicator and the pH scale:

Universal indicator is a mixture of several different indicators. Unlike litmus, universal indicator can show us exactly how strongly acidic or alkaline a solution is. This is measured using the pH scale. The pH scale runs from pH 0 to pH 14.

Universal indicator has many different colour changes, from red for strong acids to dark purple for strong bases. In the middle, neutral pH 7 is indicated by green.

These are the important points about the pH scale:

·  neutral solutions are pH 7 exactly

·  acidic solutions have pH values less than 7

·  alkaline solutions have pH values more than 7

·  the closer to pH 0 you go, the more strongly acidic a solution is

·  the closer to pH 14 you go, the more strongly alkaline a solution is

Reactions of acids with bases:

A chemical reaction happens if you mix together an acid and a base. The reaction is called neutralisation, and a neutral solution is made if you add just the right amount of acid and base together.

Metal oxides and metal hydroxides:

Metal oxides and metal hydroxides are two types of bases. For example copper oxide and sodium hydroxide.

Here are general word equations for what happens in their neutralisation reactions with acids.

metal oxide + acid → a salt + water

metal hydroxide + acid → a salt + water

Notice that a salt and water are always produced. The mixture usually warms up a little during the reaction, too. The exact salt made depends upon which acid and base were used.

Carbonates and hydrogen carbonates:

Carbonates and hydrogen carbonates are two other types of base. They also make a salt and water when we neutralise them with acid. But this time we get carbon dioxide gas too.

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.

These are the general word equations for what happens:

acid + metal carbonate → a salt + water + carbon dioxide

acid + metal hydrogen carbonate → a salt + water + carbon dioxide

Using neutralisation:

·  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.

Naming salts:

A salt is always made when an acid is neutralised by a base. But the exact salt made depends upon which acid and base were used

The name of a salt has two parts:

·  the first part comes from the metal in the base used

·  the second part comes from the acid that was used

Example: Where does the name potassium nitrate come from?

These are the rules for the second part of the name of a salt:

Acid used / Second part of salts’ name
Hydrochloric acid / Chloride
Sulphuric acid / Sulphate
Nitric acid / Nitrate

Preparing salts:

Example: Copper sulphate

How can we make copper sulphate?

The first part of the name is 'copper', so we need a base containing copper. We could use copper oxide or copper carbonate, for example. The second part of the name is 'sulphate', so we need to use sulphuric acid.

Here are word equations for those reactions.

copper oxide + sulfuric acid → copper sulfate + water

copper carbonate + sulfuric acid → copper sulfate + water + carbon dioxide

Example: Sodium chloride

How can we make sodium chloride?

The first part of the name is 'sodium', so we need a base containing sodium. We could use sodium hydroxide or sodium hydrogen carbonate, for example. The second part of the name is 'chloride', so we need to use hydrochloric acid.

Here are word equations for those reactions.

sodium hydroxide + hydrochloric acid → sodium chloride + water

sodium hydrogen carbonate + hydrochloric acid → sodium chloride + water + carbon dioxide

It would be very difficult to neutralise the acid in these reactions perfectly exactly. Some acid or base would be left over. So it would not be safe to taste the sodium chloride solution produced.

Q1:

Q2:

Q3:

Q4:

  • Match the boxes below to the correct part of the pH scale.

Q5:

Activity 1: Household substances

Activity 2: Using indicators

Q6:

Activity 3: Making a salt

Q7:

Complete the sentences.

Activity 4: Making and naming salts

Q8:

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Q10:

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