Question

When carbon dioxide is passed through lime water it turns milky due to the formation of
A. Calcium hydroxide
B. Calcium carbonate
C. Calcium oxide
D. Calcium

Answer 1

Hint: The above reaction is a precipitation reaction. One of the products will be water and the other one can be found out with the help of balancing the number of atoms on either side.

Complete step by step answer:
We have been given two reactants in question, one is carbon dioxide and another one is the lime water. First of all we need to understand these reactants, we know carbon dioxide is a gas whose molecular formula is \[{\text{C}}{{\text{O}}_{\text{2}}}\]and lime water is calcium hydroxide whose molecular formula is \[{\text{Ca(OH}}{{\text{)}}_{\text{2}}}\]. Now we have to make products with these two reactants.
Since we can see that, after the addition of carbon dioxide into calcium hydroxide the lime water (calcium hydroxide) turns milky. That means some new compound must have been formed. Also since the solution turns milky that indicates a new compound that formed is not soluble. Instead it precipitates due to which opaque milky solution forms. This confirms that it is a precipitation reaction. In general in precipitation reactions some side product is there along with the main product and most commonly it is water.
In this reaction also water will form, so taking into consideration all the above points we’ll now try to form and equation:
\[{\text{Ca(OH}}{{\text{)}}_{\text{2}}}{\text{ + C}}{{\text{O}}_{\text{2}}}\]\[ \to \]\[{\text{main product + }}{{\text{H}}_{\text{2}}}{\text{O}}\]
Now if we look on the left hand side of the equation, there are 4 oxygen atoms, 1 carbon atom and 2 hydrogen atoms. On the right hand side, we have water which has 2 hydrogen atoms and 1 oxygen atom. A chemical reaction is always the one which is balanced, so now we will use this property to make the final product. Now we have to just put the missing elements and we will get our final product. According to this the product must have 3 oxygen, 1 carbon and 1 calcium atom.
So balancing the number of atoms we will get the final product as: \[{\text{CaC}}{{\text{O}}_{\text{3}}}\]

Hence the correct option in B that is calcium carbonate .

Note:
Lime water is calcium hydroxide. So you can straight away neglect or eliminate option A, because some new compound will form after addition obviously. There are common names that are used for certain compounds in chemistry. For example: caustic soda is sodium hydroxide, lime water is calcium hydroxide, limestone is calcium carbonate and Baking soda sodium bicarbonate etc.