calcium carbide experiments explanation

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Exploding Pumpkin Mark as Favorite (16 Favorites)

DEMONSTRATION in Balancing Equations , Combustion , Classification of Reactions , Heat of Combustion . Last updated November 08, 2021.

In this demonstration, students will witness a chemical reaction create an explosion inside of a pumpkin. They will write balanced equations for the reactions that take place and will complete stoichiometry and thermochemistry calculations.

Grade Level

High School

NGSS Alignment

This demonstration will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-2 : Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

By the end of this demonstration, students should be able to:

• Write balanced chemical equations from the description of the reactions that take place.
• Calculate how much product is formed and how much energy is involved in the reactions that take place.

Chemistry Topics

This demonstration supports students’ understanding of:

• Chemical reactions
• Balancing equations
• Heat of combustion

Teacher Preparation : 30 minutes

Lesson : 30 minutes

• Knives for carving the pumpkin
• Calcium carbide , about 1 g
• 20 mL Water
• 100-mL plastic beaker
• Piezoelectric igniter
• Electronic balance
• Safety goggles
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
• An operational fire extinguisher should be in the outdoor work area for this demonstration.
• Keep acetylene away from open flames as it is highly flammable.

Teacher Notes

• This demonstration should be performed outside in a well-ventilated area.
• When it comes time to light the pumpkin, make sure students are at a safe distance away from the pumpkin(s) (at least 15 feet away) and are wearing goggles.
• You will have the best results using a medium- to large-sized pumpkin with simple shapes carved in it. Make sure that if the pumpkin is especially thick, remove excess pumpkin in the carved area until it is only about half an inch thick.
• This demonstration uses calcium carbide, CaC 2 , to produce acetylene gas, C 2 H 2 , when it is placed in water. This reaction follows the equation:

CaC 2 (s) + 2H 2 O (l) --> C 2 H 2 (g) + Ca(OH) 2 (aq)

When ignited, the acetylene gas then reacts with oxygen in the air according to the equation:

2C 2 H 2 (g) + 5O 2 (g) --> 4CO 2 (g) + 2H 2 O (g)

This reaction releases a lot of energy very quickly when the triple bond between the carbons in acetylene is broken.

• Since the calcium carbide produces the very flammable acetylene gas when exposed to water, be sure to seal the container tightly and store it in a cool, dry place. See the complete SDS .
• After the calcium carbide has completely reacted, you will be left with a basic solution of calcium hydroxide. Neutralize it with hydrochloric acid and test for a neutral pH with pH paper before pouring the liquid down the drain with lots of excess water. Calcium hydroxide is only slightly soluble in water, so if you are left with any precipitate in the beaker, refer to local waste disposal regulations for appropriate disposal methods.
• You may want to practice this demo before presenting it to students to get the timing right. You want to allow a reasonable amount of acetylene gas to build up in the pumpkin before igniting it or the resulting explosion will not be enough to blow the carved pumpkin pieces out, but you don’t want it to be too forceful an explosion. (Another problem that might occur with too much acetylene is that the interior of the pumpkin doesn’t have enough oxygen left and so the ratios of acetylene to oxygen are not ideal and the force of the explosion is not enough to blow out the carved pieces.) In general, the larger the pumpkin, the longer you should wait for it to fill with acetylene gas.
• DO NOT scale up this reaction – it could put demonstrator and students in danger! Keep a fire extinguisher on hand as well.
• Do not perform this experiment near sources of flame or flammable substances. Use a piezoelectric igniter to light the acetylene gas in the pumpkin. Piezoelectric igniters are preferable because they use materials that generate an electric spark in response to mechanical deformation, such as being struck by a small hammer-like mechanism. There is no fuel in these igniters that could catch fire if the acetylene explosion is larger than expected. Alternatively, you could light a long fuse , such as those used in model rockets, as they do in this video from the Royal Institution (which also explains some of the chemistry as well as other uses of acetylene as a fuel in mining lamps).
• The student document contains questions that require students to know how to write formulas from compound names and to predict products of reactions (particularly combustion reactions). There are also quantitative questions that require students to use stoichiometry to calculate the number of moles of acetylene gas produced by the first reaction (you could also ask for mass and/or volume), and then use the heat of combustion of acetylene (-1300 kJ/mol at 25°C) to calculate the amount of heat energy released by the explosion. You could also add questions about limiting reactants if you wanted. If you have not covered these topics yet, you can remove the questions.

Procedures for demonstration:

Preparation (Inside)

• Cut a circle around the stem of the pumpkin so you can remove it. If the inside is really thick, slice away the excess until you have about one half of an inch of inside remaining. Set the lid aside.
• Clean out the inside of the pumpkin and discard the seeds.
• Cut out the design that you want carved in the pumpkin. If the insides of these cut pieces are too thick, slice away the excess until you have about one half of an inch of inside remaining.
• Cut out a small 1-2” opening on the back of the pumpkin near the bottom and throw it away. This is where you will light the pumpkin.
• Replace all other pieces, including the top of the pumpkin.

Explosion (Outside)

• Measure out 1 g of calcium carbide on an electronic balance.
• Place a 100-mL plastic beaker containing 25 mL of water on the bottom of the pumpkin.
• Add the 1 g of calcium carbide to the beaker of water and quickly replace the lid on the pumpkin.
• If the pumpkin does not ignite, continue trying every 10 seconds or so up to two minutes of total elapsed time. If it does not light by then, remove the beaker and place it in a fume hood. Once it has finished reacting, dispose of it as described in the teacher notes above. Try steps 6-9 again with 2 g of calcium carbide in 50 mL of water. (DO NOT scale up beyond this!)

For the Student

Introduction

Many people celebrate Halloween by carving pumpkins. Today, we are going to do just that, with a catch. Instead of just a simple carve, we will create an explosion inside the pumpkin that will carve it for us!

This exploding pumpkin will depend on chemistry. Calcium carbide, CaC 2 (s), looks like small grey rocks. When water is added to it, very flammable acetylene gas, C 2 H 2 (g), is produced, along with aqueous calcium hydroxide. Then, your teacher will ignite the flammable acetylene gas, which undergoes a rapid combustion reaction and will carve the pumpkin!

• Wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.

Procedure Answer the following questions as the teacher prepares to explode the pumpkin.

• Based on the information provided in the introduction, write the balanced equations (including states of matter) for the two reactions that happen in this demonstration.
• Record the exact mass of calcium carbide added to water: __­­­­_______ Assuming that all the calcium carbide is used up in the first reaction, calculate how many moles of acetylene gas form.
• Acetylene has a heat of combustion of -1300 kJ/mol at 25°C. Assuming all the acetylene produced from the first reaction undergoes complete combustion, how much heat was released in this reaction?
• Acetylene has been used in several important industrial applications. Research at least one of them and describe it below in a paragraph or two. (Be sure you use reliable sources and list them below your answer!)

Chemical characteristics of calcium carbide and its reaction with water

Why so vigorous?

Cal­ci­um car­bide is a chem­i­cal com­pound of cal­ci­um and car­bon, and is a white crys­talline sub­stance when pure. It is ob­tained from the re­ac­tion

Ca + 2C → CaC₂

Cal­ci­um car­bide has great prac­ti­cal sig­nif­i­cance. It is also known as cal­ci­um acetylide.

The chem­i­cal char­ac­ter­is­tics of cal­ci­um car­bide

Cal­ci­um car­bide is not volatile and not sol­u­ble in any known sol­vent, and re­acts with wa­ter to yield acety­lene gas and cal­ci­um hy­drox­ide. Its den­si­ty is 2.22 g/cm³. Its melt­ing point is 2160 °C, and its boil­ing point is 2300 °C. Since the acety­lene that forms upon con­tact with wa­ter is flammable, the sub­stance is list­ed in haz­ard class 4.3.

Cal­ci­um acetylide was first ob­tained by Ger­man chemist Friedrich Wöh­ler in 1862 when he heat­ed an al­loy of zinc and cal­ci­um with coal. The sci­en­tist de­scribed the re­ac­tion of cal­ci­um car­bide with wa­ter. Cal­ci­um car­bide re­acts vig­or­ous­ly with even mere traces of Н₂O, re­leas­ing a large amount of heat. If there is an in­suf­fi­cient quan­ti­ty of wa­ter, the re­sult­ing acetylide spon­ta­neous­ly com­busts. Cal­ci­um acetylide re­acts vi­o­lent­ly with aque­ous so­lu­tions of al­ka­lis and di­lut­ed non-or­gan­ic acids. These re­ac­tions re­lease acetylide. With its strong re­duc­tive prop­er­ties, CaC₂ re­duces all met­al ox­ides to pure met­als or turns them into car­bides.

It is eas­i­er to ob­tain cal­ci­um car­bide from its ox­ide than from cal­ci­um it­self, as the ox­ide is re­duced at tem­per­a­tures above 2000 °C. The met­al and car­bon  com­bine:

CaO + 3C → CO↑ + CaC₂

The re­ac­tion takes place in an elec­tric arc fur­nace, where a mix­ture of un­slaked lime and coke or an­thracite is heat­ed. The tech­ni­cal prod­uct is grey due to the pres­ence of free car­bon, cal­ci­um ox­ide, phos­phide, sul­fide, and oth­er chem­i­cal com­pounds. CaC₂ com­pris­es 80-85% of the prod­uct by mass.

Use of cal­ci­um car­bide

In the past, cal­ci­um car­bide was used in car­bide lamps, where it served as a source of acety­lene flame. Nowa­days these lamps are still used to pow­er light­hous­es and bea­cons, and also in cave ex­plo­ration. CaC₂ also serves as a raw ma­te­ri­al in the de­vel­op­ment of chem­i­cal tech­nolo­gies, most no­tably syn­thet­ic rub­ber. Cal­ci­um car­bide is also used to make vinyl chlo­ride, acety­lene black, acry­loni­trile, acetic acid, ace­tone, eth­yl­ene, styrene, and syn­thet­ic resins.

In met­al­lur­gy, cal­ci­um car­bide is used to de­ox­i­dize met­als and re­duce their oxy­gen and sul­fur con­tent (desul­fu­ra­tion). Cal­ci­um car­bide is used to man­u­fac­ture pow­dered car­bide, a plant growth reg­u­la­tor. 3,000 kW/h of elec­tric­i­ty is re­quired to ob­tain one ton of CaC₂. For this rea­son, the man­u­fac­ture of the sub­stance is only prof­itable when elec­tric­i­ty costs are low. At the same time, the pro­duc­tion of cal­ci­um car­bide world­wide is con­stant­ly in­creas­ing.

Cal­ci­um car­bide – re­ac­tion with wa­ter

When cal­ci­um car­bide re­acts with wa­ter, acety­lene is re­leased:

2H₂O + CaC₂ → C₂H₂↑ + Ca(OH)₂

Acety­lene is an in­dus­tri­al sub­stance with an un­pleas­ant smell, which is caused by the im­pu­ri­ties it con­tains (NH₃, H₂S, PH₃, and oth­ers). In its pure form, acety­lene is a col­or­less gas with a char­ac­ter­is­tic faint smell, and it dis­solves in wa­ter.

A sim­ple ex­per­i­ment can be used to demon­strate the re­ac­tion of cal­ci­um car­bide with wa­ter: pour wa­ter into a 1.5 L bot­tle, quick­ly add sev­er­al pieces of cal­ci­um car­bide, and close the bot­tle with a stop­per. As a re­sult of the en­su­ing re­ac­tion be­tween cal­ci­um car­bide and wa­ter, acety­lene col­lects in the bot­tle as pres­sure builds. As soon as the re­ac­tion stops, place a burn­ing piece of pa­per in the bot­tle – this should trig­ger an ex­plo­sion ac­com­pa­nied by a fiery cloud. As the walls of the bot­tle can burst as a re­sult of the re­ac­tion, this ex­per­i­ment is dan­ger­ous, and should only be con­duct­ed with strict ob­ser­vance of safe­ty pre­cau­tions.

Warn­ing! Do not at­tempt these ex­per­i­ments with­out pro­fes­sion­al su­per­vi­sion! Look here for ex­per­i­ments with flame you can safe­ly do at home

To demon­strate the re­ac­tion of cal­ci­um car­bide with wa­ter, the ex­per­i­ment can be re­peat­ed in mod­i­fied form – us­ing a six-liter bot­tle. In this case, the com­po­nents must be weighed with pre­ci­sion, be­cause the greater the ra­dius of the bot­tle, the less the con­tain­er can with­stand high pres­sure (as­sum­ing iden­ti­cal ma­te­ri­al and wall thick­ness). A bot­tle with a large ca­pac­i­ty has a large ra­dius, but its walls are ap­prox­i­mate­ly the same – ac­cord­ing­ly, it is less re­sis­tant to pres­sure. To pre­vent it from ex­plod­ing, the amount of cal­ci­um car­bide must be cal­cu­lat­ed be­fore­hand. Cal­ci­um has a mo­lar mass of 40 g/mol, while car­bon’s is 12 g/mol, so the mo­lar mass of cal­ci­um car­bide is around 64 g/mol. Ac­cord­ing­ly, 64 g of car­bide will yield 22.4 L of acety­lene. The vol­ume of the bot­tle is 6 L, and the pres­sure has risen by ap­prox­i­mate­ly 4 at­mos­pheres.

The bot­tle must with­stand five at­mos­pheres: to con­duct the ex­per­i­ment, we take around 64 g of cal­ci­um car­bide and about 0.5 L of wa­ter. Place a piece of car­bide in­side a small bag. Push the bag into the bot­tle, then quick­ly close the bot­tle with the stop­per. The re­ac­tion of cal­ci­um car­bide with wa­ter con­tin­ues for sev­er­al min­utes, the bot­tle swells up and the process is ac­com­pa­nied by loud bangs, but the bot­tle should with­stand this.

Af­ter the re­lease of acety­lene is com­plete, place a hot rag soaked in hen­de­cane on the bot­tle stop­per, then move away to a max­i­mum safe dis­tance. You will soon see a bright yel­low flash, and a foun­tain of flame up to 4 me­ters high will rise out of the bot­tle. This will burn the stop­per and warp the bot­tle, but the bot­tle should re­main in­tact. This ex­per­i­ment must be con­duct­ed in the open air, far away from flammable and ex­plo­sive ob­jects. Be sure to ob­serve all rel­e­vant safe­ty pre­cau­tions.

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