The reaction of magnesium with silver nitrate
Description
A drop of water is added to a mixture of magnesium powder and solid silver nitrate.
The mixture ignites with a white flash.
Topic
Displacement reactions of metals.
Timing
Less than five minutes.
Level
Pre-16
Apparatus

Heat proof mat and a white tile.

Two sheets of filter paper

Safety screen

Burette

Retort stand with boss and clamp to hold the burette
Chemicals

1g of magnesium powder (highly flammable; contact with water releases an
extremely flammable gas - hydrogen)

2.4 g of finely ground solid silver nitrate (corrosive)
Method
Weigh out, on separate pieces of paper, 1 g of magnesium powder and 2.4 g of solid
silver nitrate powder. Do not exceed these quantities. The powders should be mixed
very gently by pouring repeatedly from one piece of filter paper to another. The
mixture must not be ground; if the silver nitrate needs to be powdered, do this before
mixing it with the magnesium. Carefully place the mixture on a clean, dry white tile.
Set up a burette or similar device containing water and adjust the tap so that it
produces one drip every 5 seconds. Let the water drip onto a heatproof mat to form a
damp patch.
When you are satisfied with the timing of the delivery of drips, place the white tile with
the silver nitrate / magnesium mixture, over the wet spot just after the delivery of a
drip and retire quickly behind a safety screen
After the next drop of water falls on to it, the mixture reacts vigorously and within a
second or so with a white flash. Should the mixture fail to react (because of badly
oxidised magnesium, for example) squirt the mixture with water from a wash bottle,
from behind the protection of a safety screen, until the silver nitrate has dissolved.
Teaching tips
It may be helpful to discus the role of the water in initiating the reaction.
Remind the audience of the relative positions of magnesium and silver in the
reactivity series of metals. The vigour of the reaction could usefully be compared with
that of other metal / metal compound displacement reactions that take place in the
solid phase. Examples include the reaction between zinc and copper oxide (see
Further details, below), where the two metals concerned are much closer in the
reactivity series. This reaction is much less vigorous than that between magnesium
and silver nitrate and requires heat from a Bunsen burner to start it. There are,
however a number of other factors that affect the vigour of the reaction. See also the
demonstration The reaction of aluminium with copper salts.
Theory
The reaction is:
Mg(s) + 2AgNO3(s)  Mg(NO3)2(s) + 2Ag(s)
Magnesium, much higher in the reactivity series than silver, displaces silver from its
salts exothermically. The reaction does not start until water is added because of poor
contact between the two powders and the inability of the Ag+ and NO3- ions to move.
The weights of reactants described give an excess of magnesium. This excess is
ignited by the heat given out by the reaction and produces a brighter flash.
Further details
The reaction of zinc and copper oxide is described in T. Lister, Classic Chemistry
Demonstrations, London: Royal Society of Chemistry, 1995, pp 22-23
Safety

The demonstrator should wear a face shield.

The audience should wear eye protection.

Protect the audience and demonstrator with a safety screen.

It is vital that both powders are absolutely dry when mixed and that they are
mixed with great care.

Mixtures of magnesium and silver nitrate should be made immediately before
the experiment and should not be stored for future use, even on the same
day.

Your employer’s risk assessment should be consulted before carrying out this
activity. This activity is unlikely to be covered by model (general) risk
assessments used in UK schools. CLEAPSS has prepared a special risk
assessment (below) that is likely to be acceptable to most employers. It is,
however, the responsibility of the teacher carrying out the activity to check
that this risk assessment is in fact acceptable to their employer. Bear in mind
also that this may need some modification to suit local conditions.
Acknowledgement
This demonstration was developed by Dr Colin Chambers, formerly of Bolton
Grammar School.
Model risk assessment
The model risk assessment below has been provided by CLEAPSS.
CLEAPSS SCHOOL SCIENCE SERVICE
Risk Assessment (to meet the COSHH and/or the Management of Health and Safety at
Work Regulations)
Applicant:
Royal Society of Chemistry
School/LEA:
This risk assessment is likely to be acceptable to most education employers in
England, Wales and Northern Ireland.
Operation:
The reaction of magnesium with silver nitrate
This risk assessment applies to the procedure as described in the accompanying RSC
document as outlined below.
Details of operation
A drop of water from burette is added to a mixture of 1 g of magnesium powder and 2.4 g of
silver nitrate. The mixture ignites with a white flash.
Substance(s) possibly hazardous to
health:
(a) magnesium powder
(b) magnesium oxide
(c) silver nitrate powder
(d) nitrogen dioxide (& other oxides?)
(e) silver metal
Classification under CHIP3
Regulations 2002
(a) highly flammable; contact with water liberates extremely
flammable gas
(b) –
(c) corrosive; causes burns
(d) very toxic by inhalation; irritating to the respiratory system
(e) -
Particular risks:
“An intimate mixture of dry powdered magnesium and silver
nitrate may ignite explosively with a drop of water” (Bretherick,
Handbook of Reactive Chemical Hazards, 6th edition, 1999,
Butterworth/Heinemann.)
“… A drop of distilled water was blown in from a wash bottle.
There was an instantaneous flash about 3 in high, accompanied by
a dull report. The eyebrows and eyelashes of the experimenter
were burnt off and reaction products blown into the eyes,
necessitating two eye operations, a week in hospital and the
inability to do serious reading for several weeks. …. The eyelids,
nose and face were all burned … Some bits of powder penetrated
the skin of the observer standing about 2 yds away …” (School
Science Review, November 1953, 35 (125), 138.)
“The science department arranged a demonstration, in a fume
cupboard, of the potentially very dangerous reaction between solid
silver nitrate and magnesium powder on the addition of a drop of
water. By some means, water entered a bottle of the reagents which
exploded, burning brightly. Unfortunately, a technician was
working at the fume cupboard at the time and received extensive
burns to both hands. It is reported that the heat was so intense that
her wedding ring melted. A light fitting was broken and glass
injured a Y6 pupil visitor.” CLEAPSS Bulletin 116 (Spring 2003)
Maximum exposure limits:
(a)
Occupational exposure standards:
(a) –
(b) 10 mg m-3 total inhalable dust (long-term)
(c) –
(d) 5 ppm (short-term)
(e) -
Risk assessment
1
The smell of nitrogen dioxide can usually be detected, but there is no significant risk
from exposure to magnesium oxide or nitrogen dioxide. Concentrations of both
nitrogen dioxide and magnesium oxide will not exceed limits even momentarily
provided that the experimenter and observers are at least 3 m away, and no more
than 2.4 g of silver nitrate or 1.0 g magnesium powder is used.
2
Because of the unexpected violence of the reaction, there is abundant advice against
carrying out this reaction in schools:

“Mixtures which should not be made: magnesium powder + silver nitrate”
(Safeguards in the School Laboratory, 10th edition, ASE, 1996)

“Silver nitrate: dangerous with magnesium. Mixture explodes in presence of a drop
of water” (Hazcards, CLEAPSS School Science Service, 1995)
3
There is no national ban on the activity – the above are only recommendations. The
demonstration can be carried out safely provided the following precautions are
rigidly adhered to:

The teacher-demonstrator to wear a face shield; pupils to wear normal eye protection

The demonstration to be carried out behind safety screens, to protect both teacher
and pupils; pupils to be at least 3 metres away.

Weigh out no more than 2.4 g silver nitrate and 1.0 g magnesium powder.

The silver nitrate must NOT be ground up with the magnesium powder. If the silver
nitrate needs pulverising, do so before mixing. Silver nitrate powder and magnesium
powder must be mixed only by pouring repeatedly from one piece of paper to
another; do NOT use a spatula, etc to mix the powders. Care, as silver nitrate is
corrosive.

The silver nitrate/magnesium powder mixture should be placed on a clean, DRY
white tile.

Set up a burette or similar device containing water and adjust the tap so that it
produces one drip every 5 seconds. Let the water drip onto heatproof mat to form a
damp patch.

When you are satisfied with the timing of the delivery of drips, place the white tile
with the silver nitrate/magnesium mixture, over the wet spot just after the delivery
of a drip and retire quickly behind a safety screen.

A very bright flash is observed within a second or two of the drop of water coming
into contact with the mixture.

If the mixture fails to ignite (eg because of badly oxidised magnesium), from behind
the protection of a safety screen squirt the mixture with water until the silver nitrate
has dissolved.

This activity is only suitable for experienced chemistry teachers, for example those
who have regularly passed hydrogen over hot copper oxide, burnt sodium in
chlorine, demonstrated the thermite reaction or similar activities where there is a
significant hazard.

The teacher must practice the demonstration in advance of the lesson.

The head of department must judge whether staff can be relied upon to follow these
instructions. S/he must also judge whether pupil behaviour might distract the
teacher’s attention such as to present a hazard. If either of these judgements is
unfavourable, the activity should not be attempted.
Assessor:
Dr T P Borrows, MA, PhD, CChem, FRSC
Director
If further clarification is required, contact:
The School Science Service, Brunel University, Uxbridge UB8 3PH
Tel: 01895 251496
Notes
COSHH stands for Control of Substances Hazardous to Health. The regulations require that
an assessment of risk should be made before substances hazardous to health are handled.
The substances covered are the reactants, the products and any intermediate or side products
that are very toxic, toxic, harmful corrosive or irritant. Just because a substance carries no
hazard label does not mean that it is completely safe.
The Management of Health and Safety at Work Regulations require a similar risk assessment
for substances with other hazard classifications or activities involving hazardous procedures.
MEL stands for the Maximum Exposure Limit. On no account should the level of vapour
exceed this value and as far as reasonably practicable the employer should see that exposure
is kept as low as possible.
OES stands for Occupation Exposure Standard. Exposure should be either at the standard or
preferably below. These values represent good practice. There are 2 limits. LTEL stands for
long term exposure limit and is averaged over an 8 hour time weighted average (TWA)
period. STEL is the short term exposure value and is averaged over a 15 minute TWA period.
It is the value more relevant to schools. If a STEL is not specifically prescribed then the STEL
for that substance is 3 times the LTEL value.