Question

Explain by writing a word equation.

a. Respiration is a chemical change.

b. Hard water gets softened on mixing with a solutions of washing soda.

c. Lime stone powder disappears on adding to dilute hydrochloric acids.

d. Bubbles are seen on adding lemon juice to baking soda.

Answer 1

a.Chemical change is a process in which a new substance is formed.

• Respiration is irreversible process
• During respiration, oxygen is convert into carbon-dioxide( a new substance is formed).
• During respiration, there is change in their chemical composition.
• During respiration, absorption and evolution of energy takes place.

These are characteristics of chemical reaction. Hence, respiration is a chemical reaction.

b.Sodium carbonate, Na₂CO₃, is also known as washing soda. It can remove temporary and permanent hardness from water. Sodium carbonate is soluble but calcium carbonate and magnesium carbonate are insoluble.
The carbonate ions from sodium carbonate reacts with calcium and magnesium ions in the water to produce insoluble precipitate.
For example:
$C{a}^{2+}\left(aq\right)+N{a}_{2}C{O}_3\to CaC{O}_3\left(s\right)+2N{a}^{+}\left(aq\right)$
The water is softened because it no longer contains dissolved calcium ions and magnesium ions.

c.Limestone is predominantly Calcium carbonate(CaCo₃). When calcium carbonate reacts with 10% solution of HCl. Then Calcium chloride is formed as a salt along with water and carbon dioxide gas (brisk effervescence).
$2HCl+CaC{O}_3\to CaC{l}_2+C{O}_2+H_{2}0$
In this reaction, we add hydrogen ions (H⁺), which will react with the carbonate ion to form hydrogen carbonate HCO₃- ions, which are soluble in water and the limestone will dissolve.

d.Bubbles are seen on adding lemon juice to baking soda because lemon juice is citric acid and baking soda is sodium bicarbonate. On adding lemon juice in baking soda, we add acid in base then neutralization reaction is taking place resulting in the formation of salt ,water and brisk effervesence of carbon-dioxide.This CO₂ is released in the form of bubbles.
$$\begin{gathered} {\mathrm{H}}_3{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{O}}_7\left(\mathrm{aq}\right)+3{\mathrm{NaHCO}}_3\left(\mathrm{aq}\right)\to {\mathrm{Na}}_3{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{O}}_7\left(\mathrm{aq}\right)+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+3{\mathrm{CO}}_2\left(\mathrm{g}\right) \\ citricacid+bakingsoda\to salt+water+carbondioxide \end{gathered}$$