Calcium carbonate, hydrochloric acid, and their interaction

Let’s talk a lit­tle about the in­ter­ac­tion be­tween hy­drochlo­ric acid and cal­ci­um car­bon­ate, and the na­ture of these com­pounds them­selves.

Cal­ci­um car­bon­ate

Ca­CO₃ is a wide­spread com­pound found in chalk, lime, mar­ble, and more. This sub­stance is a cru­cial pil­lar of hu­man life – it is used in con­struc­tion, to man­u­fac­ture pa­per and plas­tic, and in many oth­er spheres. It is also pop­u­lar in the food in­dus­try as a nat­u­ral white col­orant.

Cal­ci­um car­bon­ate is a sol­id white sub­stance that won’t dis­solve in it com­plete­ly: the wa­ter will turn a mud­dy col­or and a white pre­cip­i­tate will ap­pear. But if the re­ac­tion with wa­ter takes place in the pres­ence of car­bon diox­ide, it yields cal­ci­um hy­dro­gen car­bon­ate, a sol­u­ble acidic salt:

Ca­CO₃ + CO₂ + H₂O → Ca(HCO₃)₂

Let’s see how cal­ci­um car­bon­ate is ob­tained

Most cal­ci­um car­bon­ate is ob­tained from nat­u­ral sources. A pure source, com­mon­ly mar­ble, is usu­al­ly used to ob­tain the cal­ci­um car­bon­ate used in food col­or­ing.

In the lab­o­ra­to­ry, pure cal­ci­um car­bon­ate can be ob­tained via cal­ci­um ox­ide slak­ing fol­lowed by car­bon diox­ide gas bub­bling. Wa­ter is mixed with cal­ci­um ox­ide to yield cal­ci­um hy­drox­ide. Car­bon diox­ide is then passed through the pre­vi­ous­ly-pre­pared so­lu­tion. Cal­ci­um car­bon­ate pre­cip­i­tates:

CaO + H₂O = Ca(OH)₂

Ca(OH)₂ + CO₂ = Ca­CO₃↓ + H₂O

Cal­ci­um ox­ide, also known as quick­lime or burnt lime, is com­mon­ly used in con­struc­tion. In in­dus­try, the cal­ci­um ox­ide need­ed for the above process is pro­duced via cal­ci­na­tion. Cal­ci­na­tion is the gen­er­al name for the chem­i­cal al­ter­ation of sub­stances via fir­ing or heat­ing to high tem­per­a­tures. If lime­stone or mar­ble is heat­ed to a tem­per­a­ture of 900-1000°C, the re­sult­ing chem­i­cal pro­cess­es yield car­bon diox­ide and the de­sired cal­ci­um ox­ide CaO:

СаСO₃ = СаО+СО₂↑

But if the tem­per­a­ture is even high­er (1500°C) and a car­bon source is present, the re­ac­tion will pro­duce cal­ci­um car­bide and car­bon monox­ide:

СаСO₃ + 4C = СаC₂ + 3СО↑

Hy­drochlo­ric acid

Hy­drochlo­ric acid is a strong monoba­sic acid and is ob­tained via the dis­so­lu­tion of hy­dro­gen chlo­ride HCl in wa­ter. It is a col­or­less liq­uid, al­though in­dus­tri­al acid can have a yel­low tint, of­ten due to a mix­ture of iron. The prop­er­ties of this so­lu­tion de­pend di­rect­ly on the con­cen­tra­tion of hy­dro­gen chlo­ride. The salts of hy­drochlo­ric acid are called chlo­rides.

This sub­stance is very caus­tic and re­quires care­ful han­dling: even a small drop on the skin will cause a se­vere chem­i­cal burn. When work­ing with strong acids, you should al­ways have neu­tral­iz­ers at hand – weak al­ka­line so­lu­tions, sodi­um bi­car­bon­ate (bak­ing soda), etc. You should re­mem­ber that open­ing a con­tain­er with con­cen­trat­ed acid re­leas­es HCl fumes ca­pa­ble of harm­ing the eyes and res­pi­ra­to­ry sys­tem. When con­duct­ing chem­i­cal ex­per­i­ments, it is rec­om­mend­ed to use a res­pi­ra­tor and pro­tec­tive eye­wear.

Ob­tain­ing hy­drochlo­ric acid

Hy­dro­gen chlo­ride gas dis­solves in wa­ter. Hy­dro­gen chlo­ride it­self is ob­tained by burn­ing hy­dro­gen in chlo­rine to yield a syn­thet­ic acid. Hy­drochlo­ric acid can also be ob­tained from the sec­ondary gas­es re­leased in a num­ber of chem­i­cal pro­cess­es, such as when hy­dro­car­bons are chlo­ri­nat­ed. Acid ob­tained via this method is of tech­ni­cal or in­dus­tri­al grade.

Hy­drochlo­ric acid is used in medicine, in­dus­try, and chem­i­cal re­ac­tions.

The col­or­less acid with the harsh smell of hy­dro­gen chlo­ride re­acts well with met­als; the met­al atoms re­duce the hy­dro­gen cations in an ox­i­da­tion-re­duc­tion re­ac­tion.

Chem­i­cal re­ac­tions with met­als are main­ly ac­com­pa­nied by the re­lease of hy­dro­gen, the in­ten­si­ty of which de­pends on the ac­tiv­i­ty of the met­al. For ex­am­ple, the al­ka­line met­al lithi­um re­acts vi­o­lent­ly, while alu­minum re­acts only weak­ly due to its durable ox­ide film.

Hy­drochlo­ric acid and zinc:

2HCl + Zn = Zn­Cl₂ + H₂↑

Hy­drochlo­ric acid and iron:

2HCl + Fe = Fe­Cl₂ + H₂↑

Hy­drochlo­ric acid and mag­ne­sium:

2HCl + Mg = Mg­Cl₂ + H₂↑

The re­ac­tion of the acid with met­al ox­ides forms salt and wa­ter:

CuO + 2HCl = Cu­Cl₂ + H₂O

Re­ac­tion of hy­drochlo­ric acid and cal­ci­um car­bon­ate

You will need:

• a test tube;

• a pipette;

• sol­id cal­ci­um car­bon­ate (mar­ble);

• hy­drochlo­ric acid;

• gloves;

• glass­es;

• a res­pi­ra­tor.

Warn­ing! Don’t try to per­form this ex­per­i­ment with­out pro­fes­sion­al su­per­vi­sion!

Look here for safe chem­istry ex­per­i­ments you can do at home

Per­form this ex­per­i­ment in a well-ven­ti­lat­ed room. Ob­serve cau­tion when work­ing with hy­drochlo­ric acid.

Add sev­er­al pieces of mar­ble to the ves­sel and drip hy­drochlo­ric acid on it us­ing the pipette. There will be an in­stan­ta­neous re­ac­tion in­volv­ing the for­ma­tion of bub­bles (the re­lease of car­bon diox­ide). This is an ex­change re­ac­tion that pro­duces a weak and un­sta­ble com­pound, car­bon­ic acid, which breaks down into car­bon diox­ide and wa­ter. The equa­tion of the dis­so­lu­tion re­ac­tion of cal­ci­um car­bon­ate in hy­drochlo­ric acid reads as fol­lows:

Ca­CO₃ + 2HCl(di­lut­ed) → Ca­Cl₂ + CO₂↑ + H₂O