Comprehensive School “M. Buonarroti“

Marina di Carrara

Secondary school

Presents

SCIENTIFIC WORKSHOP

Scientific project: Tiziana Perfetti

Text translated by : Maria Antonietta Caporale

LET’S DISCOVER THE CHEMICAL REACTIONS

We have a chemical reaction when we observe phenomena that testify the disappearance of the substances used and/or the formation of new ones.

In particular :

bubbles the formation and the disappearance of a solid spontaneous heating or cooling the colours changes

are evidence of chemical reactions

The substances that give rise to the transformation are called reagents and the substances that are formed are called chemicals (products)

Reagents Chemicals

Chemical reactions can be faster or less fast

However we may act to increase or decrease the speed of a reaction in a lot of ways , for example using catalysts. Almost all the reactions that occur in living organisms are activated by biological catalysts, the enzymes.

Even the stones breathe

( breathe out)

Marble and hydrochloric acid

You need:

a plastic spoon a glass rod marble powder

hydrochloric acid distilled water a beaker

Process:

o With the spatula take a small amount of marble powder, add water and a few drops of hydrochloric acid until all the solid has not disappeared.

Observations:

What do you observe? What do you think happened?

Does the acid dissolve the marble in the same manner in which water dissolves salt and sugar? Has a new substance formed or not? Can it be stated that there has been a chemical reaction? Give an explanation

What happened?

The effervescence observed in the test tube with the marble powder is due to the development of another gas, the carbon dioxide. The chemical reaction is:

hydrochloric acid + marble calcium chloride + carbon dioxide + water

Accadue

HYDROGEN PRODUCTION(be careful !)

The use of the match requires proper precautions and the completion of the experiment by the teacher.

TargetsTo recognize a very fast and effect chemical reaction.

You need:

Pieces of aluminum foil (for food)

Hydrochloric acid solution (muriatic)

Two test tubes A test tube rack

A match A wood pliers A glass rod

a pipette

Process:

o Put some pieces of aluminum in a test tubeo Add a few drops of hydrochloric acid

What do you observe?

In the test tube with aluminum effervescence is observed , the solution becomes darker and aluminum corrodes and disappears. You must wait a few minutes to allow reaction to develop, fragments are surrounded by bubbles of hydrogen that keep them afloat.o Place above the test tube the second inverted test tube, that is with the opening facing downwards, to collect the hydrogen which is lighter than air.o After a few minutes move away the second test tube, always keeping it inverted, from that in which hydrogen is generated and approach the entrance to a lighted match.

What happens?

We will hear a slight explosion and, if you look carefully, you can see a flame inside the test tube, while the blast of the explosion will turn off the match. The effervescence observed in the test tube with the aluminum is due to the development of a gas, the hydrogen. In the reaction we can also see that the hydrogen reacted with the oxygen in the air producing water vapor ( after the reaction we will find some condensation on the inner walls of the test tube).The chemical reaction is :

6HCI + 2AI 2 + 3

( Hydrochloric acid + aluminum aluminum chloride+ hydrogen)

Balloon Let’s inflate a balloon without blowing

You need :

an Erlenmeyer flask a balloon baking soda graduated cylinder vinegar a funnel a teaspoon

Process:

1. Using a graduated cylinder pour 30ml of vinegar in the Erlenmeyer flask.

2. Stick a balloon to the mouth of the funnel.3. Put a teaspoon of baking soda in the balloon . 4. Take off the funnel from the balloon.5. Adjust the opening of the balloon around the neck of the

flask, being careful not to fall into the baking soda.6. Keep the flask still, raise the balloon in such a way to allow

the baking soda to come into contact with the vinegar.

Reflections

What do you observe inside the flask? What happens to the balloon?What conclusions do you draw?

What happens?

After the baking soda has been dropped in vinegar, a chemical reaction starts for which we can note the development of foam and the balloon that is inflated. A gas called carbon dioxide develops. Since gases expand until they occupy all the space available , the carbon dioxide inflates the balloon. Touching it the bottle is cooled and at the end a deposit may remain.

Baking soda + acetic acid = sodium acetate + water+ carbon dioxide

The vinegar that remains ( if it is in excess of the amount of baking soda) is now diluted. If a white deposit remains on the bottom, it means that the vinegar was not sufficient to react all the baking soda.

A volcano of foam(be careful !)

TARGETS To recognize an exothermic reaction and the function of a catalyst

You need: An empty glass bottle (preferably with a narrow

neck)or a graduated cylinder 1/2 cup of hydrogen peroxide at 20 volumes

liquid detergent

3-4 drops of food coloring

Potassium iodide

A funnel

A large and low container

Process

1. Put the bottle in the centre of the container. Insert the funnel in the bottle and pour the peroxide.

2. Add the detergent to the peroxide in the bottle.

What happens?

3. Add 3-4 drops of food coloring. 4. Dissolve two teaspoons of sodium iodide in a few millimeters of

warm water in a beaker.5. Quickly pour the sodium iodide in the bottle ( and move away).

6. The students can touch the bottle to feel all the changes that occur.

What do you observe?

The reaction creates foam which is shot out of the bottle. After about a minute a moving flow is formed. The bottle will feel warm to the touch as it is an exothermic reaction.

To obtain the volcano of foam we have to use a catalyst, a substance that accelerates the rate of the reaction and that it is found unchanged at the end, without transformations.

How does it work?

The decomposition reaction of hydrogen peroxide into water and oxygen is the following:

2 2 O +

When we pour KI in the hydrogen peroxide / liquid detergent mixture, the volcano of foam starts. We will also recognize other phenomena such as the release of oxygen, the production of H2O in addition to changes in ph.

A lot of reactions of industrial importance take place with the aid of suitable catalysts, all the reactions of biological importance take place with the aid of enzymes, active catalysts in living organisms.

SECRET MESSAGESFirst message

You need:

Lemon juice A blank sheet of paper A glass stick A candle

Process:

o Take the stick and dunk it in the lemon juiceo Write on a paper what you want or make a small drawingo Let the ink dry and wait till the writing disappearso Now let the paper pass over the flame of a candle being

careful not to burn it.What do you observe?

The writing reappears and it is perfectly legible

What do you deduce? The heat makes the lemon juice react so that it darkens and makes visible the writing

SECRET MESSAGES

Second message

You need:

A wad of cotton Very diluted tincture of

iodine

A sheet of paper Lemon juice, brush

What to do

o Write a message on the white paper using a brush dipped in the lemon juice.

o Let the writing dry till it becomes invisible.o Rub a wad of cotton soaked in a dilute solution of iodine on the

sheet where it says.

What do you observe?

The writing reappears, white on a blue/purple background

What do you deduce?

The starch of the paper is combined with iodine by changing colour to the sheet of paper which becomes blue/purple. Also the vitamin c in lemon combines with the iodine but it forms a colourless molecule. So the area covered by the lemon juice stands on a colored background.

Let’s change the colours

The colour change may indicate the formation of new substances

You need: o Two test tubeso Two pipetteso Solution of ferric chlorideo Solution of potassium ferrocyanide

Process:

o Take a few ml of solution of ferric chloride ( a yellow-orange colour solution) by a pipette and put it into the two test tubes

o Add some drops of a reactive called potassium ferrocyanide to the first test tube

What do you observe? The second solution, light yellow, added to the solution of ferric chloride determines a marked change in colour : the content of the test tube becomes blue ( also called Prussian blue) due to the formation of a new substance ( the ferric ferrocyanide)

o Add some drops of potassium thiocyanate

( colourless solution) to the second test tube

What do you observe?

o The solution becomes dark red even if precipitate doesn’t form: the new formed red substance is soluble in water. Some chemical reactions occurred: the initial substances are transformed and they have given rise to new substances that there weren’t before, with their own evident characteristics ( a different coloration).

Rubber egg

How can an egg bounce like a ball without breaking

You need:

o A raw eggo A glass or a glass jaro Vinegaro (time)

Process:

1) Put the egg into the glass jar and cover the egg with vinegar2) After some hours look closely the egg. What do you observe? Do

you see the bubbles formation around the eggshell?3) Let the egg into the vinegar for 24-48 hours4) On the second day, carefully pour the old vinegar down the drain

and cover the egg with fresh vinegar5) Don’t disturb the egg but pay attention to the bubbles that are

formed on the surface of the eggshell ( or what’s left of it). 6) Pour the vinegar and rinse thoroughly the egg with water7) What do you observe?

The egg looks translucent because the outer shell has disappeared! The only thing that remains is the delicate membrane of the egg.

What happened?

The bubbles that cover the shell are carbon dioxide bubbles. Vinegar is an acid called acetic acid. ( White vinegar is usually made up of about 5% of acetic acid and 95% of water). The egg shells are composed of calcium carbonate. Vinegar reacts with the calcium carbonate giving origin to a substance called calcium acetate and carbon dioxide.

The egg shell is reduced to a very thin layer and the egg is flexible. The shelled membrane remains . The semi-impermeable membrane in contact with the external solution begins to flow solvent ( the vinegar) inside the egg from the outside so that the concentration of particles inside balances the outside one. This flow of a liquid through a semi-impermeable membrane from a less concentrated solution to a more concentrated one is called osmosis.

The volume of the egg has visibly increased showing the effect of osmotic pressure. If you shake the egg, the yolk can be seen splashing around the egg white. If the egg reacts with the carbon dioxide in the air it will harden.