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L2 Plumbing Unit 4Apply scientific principles within mechanical services engineering

Unit 4: Activity Sheet 4.3

Pressure, water and corrosionTutor guidance notes

Aims and objectives

At the end of activities 4.3A and 4.3B, learners should be able to:

describe what is meant by pressure

describe what is meant by atmospheric pressure

state the main properties of water

state the meaning and importance of pH value

explain the main effects of corrosion and recognise the main types of corrosionaffecting plumbing systems and components

explain what is meant by electrolytic action.

The activity is divided into two parts: a set of questions (activity 4.3A) followed by amultiple-choice exercise (4.3B). They are not intended as formal assessment.However, the answers should be discussed with learners as a group to ensure theyfully understand the topic.

The activity can be used in different ways, for example:

during the session, either individually or in pairs, as a to-the-point learning tool

at the end of a session to recap learning as homework.

Activity 4.3B could be used at the beginning of a subsequent session to link withprior learning.

Whichever teaching method is chosen, it is advisable to set and agree a completiondate for each activity.

Pearson Education Ltd 2011. Copying permitted for purchasing institution only. This material is not copyright free.1

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Unit 4: Activity Sheet 4.3

Activity 4.3A Pressure, water and corrosionAnswer the following questions.

1 What is the definition of pressure?

Pressure is defined as force applied per unit area and is measured in

newtons per square metre (N/m), a unit also known as a Pascal (Pa).

2 What does the term atmospheric pressure mean?

Atmospheric pressure is exerted by the weight of the E arths

atmosphere pressing down on the ground and varies depending

on the height above sea level.

Workshop-based investigation: Set up the simple experiment below to show theeffect of atmospheric pressure.

Step 1: Put a small amount of water in a resealable metal can.

Step 2: Boil the water until the can contains only steam.

Step 3: Seal the neck of the can with a bung or screw-top cap.

Step 4: Place the can under cold water watch what happens.


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3 Write a brief report about what you observed, suggesting reasons for whatoccurred.

Learners own answer.

4 a Which two chemicals make up water? What is the chemical symbol forwater?

Water is a compound of two gases, hydrogen and oxygen (H2O).

b As rainwater is naturally solvent to begin with, how does it become moresolvent?

Rainwater contains nitrogen, oxygen and carbon dioxide from the air.

In towns, rainwater also contains dissolved soot and other pollutants.

5 Explain the term soft water.

Water is said to be soft when it is easy to produce lather with soap.

6 Explain the term hard water.

Water is classified as hard if it is difficult to produce lather with soap.

Hard water is created when rain falls on ground containing calcium

carbonate or sulf ates.

7 Complete the table below with the type of water hardness being described.

Type of hardness Description

Temporary Responsible for hard scale whichaccumulates on the inside of boilersand circulating pipes, restricting theflow of water, reducing the efficiency ofappliances and components andultimately causing damage and systemfailure.

Permanent Occurs as a result of the naturalsolvency of pure water which enablesit to dissolve the sulphates oflimestone. It causes the normalproblems associated with hard water(difficult to form lather with soap anddetergents).


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8 a Do you live and work in a hard-water area or a soft-water area?

Learners own answer. (Note: the information can usually be found on thelocal water utilities website.)

b Why is hard water a problem for plumbing systems?

The hard scale accumulates inside boilers and circulation pipes,

restricting the flow of water and reducing the efficiency of

appliances and components.

9 Workshop-based investigation: Set up the simple experiment below to showthe effect of pH values.

Select a range of everyday substances, e.g. vinegar, lemon, soap, heating-system inhibitor, etc. If possible, include a selection of variety of soil fromdifferent locations. Allow the soil to absorb some water. Use litmus paper tocheck the acidity and alkalinity of each substance. Explain the meaning andimportance of pH values.

Learners own answer.

10 Practical demonstration: Your tutor will show you examples of atmosphericcorrosion, including an example of attack on ferrous metals.

a What effect does living near the coast have on materials?

Sea salt (sodium chloride) corrodes metals such as aluminium.

b What is patina?

Patina is a protective layer that forms naturally on non-ferrous metals.

c Why do we get atmospheric corrosion inside systems such as heatingsystems and sealed systems?

Water naturally corrodes ferrous metals, which rust on contact

with water, forming black ferrous oxide and red rust.

11 Workshop-based investigation: Set up the simple experiment below to showanother form of corrosion.

Your tutor will show you a previously prepared glass container with a screw lid.Inside there is a 100 mm or 150 mm iron nail and a copper fitting or coin. If youlook closely, you will see rust on the nail and brown iron oxide deposits on thebase of the glass container.

a Name the type of corrosion seen on the nail.

Electrolytic corrosion.


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b A plumber found that similar corrosion in a heating system caused pinholes insome of the radiators. Why did the iron in the radiators break down and notthe copper pipework?

Copper is higher in the electromotives series than iron. Therefore,

iron corrodes much faster than copper.

c The system the plumber was working on was only a few years old. He foundthat the water in the system contained traces of flux. What effect might theflux have had on the problem?

Flux will increase the acidity of the electrolyte (water) causing

corrosion to speed up.

12 Practical demonstration: Your tutor will show you a selection of Type A and

Type B brass fittings, and explain to you how a process called dezincificationcan break down an incorrectly installed below-ground fitting. Dezincification is atype of electrolytic corrosion.

13 Certain building materials and underground conditions can be corrosive.Complete the sentences below by drawing a line between each buildingmaterial/underground condition and its effects.

Building material/underground conditions Effects


Some types of wood(such as oak)

Latex cement andfoamed concrete

Certain types of soil

Heavy clay soils

Ground containingash and cinders

is very corrosive as they are stronglyalkaline if pipes are to be laid in such

ground they should be wrapped inprotective material.

can damage underground pipe work.

have a corrosive effect on lead.

will adversely affect copper.

may contain sulfates which cancorrode lead, steel and copper.

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Activity 4.3B Pressure, water and corrosion

Circle the correct answer in each of the questions below.

1 If you know that the water in a domestic property contains a high percentage of

calcium carbonate, you would expect the water to be:

a soft.

b hard.

c neutral.

d mineral rich.

2 The pH scale is used to measure:

a pressure.

b acidity and alkalinity.

c tension.

d heat loss.

3 Exposed iron drainage pipework in an industrial town would be more at risk fromcorrosion because of higher atmospheric levels of:

a calcium carbonate.

b copper sulfate.

c potassium permanganate.

d sulfur dioxide.


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4 The protective sulfate coating that forms around non-ferrous metals and protectsthem from further corrosion is known as:

a patina.

b rust.

c galvanise.

d electrolyte.

5 The material that is destroyed through electrolytic corrosion is called the:

a anode.

b cathode.

c electrolyte.

d reductor.

6 What wood will corrode lead?

a Soft wood.

b Oak.

c Nettle wood.

d Solvent origana.


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