Andhra Pradesh SSC (Class 10) Acids, Bases and Salts — practice questions — page 2

To identify the type of reaction, let's analyze the reactants and products in the chemical equation: $$\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$$ 1. **Analyze Reactants**: * $\text{NaOH}$ (Sodium hydroxide) is a strong base because it provides hydroxide ions ($\text{OH}^-$). * $\text{HCl}$ (Hydrochloric acid) is a strong acid because it provides hydrogen ions ($\text{H}^+$). 2. **Analyze Products**: * $\text{NaCl}$ (Sodium chloride) is a salt. * $\text{H}_2\text{O}$ is water. 3. **Define Reaction Type**: A chemical reaction in which an acid reacts with a base to form a salt and water is known as a **Neutralisation reaction**. In this process, the $\text{H}^+$ ions from the acid combine with the $\text{OH}^-$ ions from the base to form neutral water: $$\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$$ Since the given reaction follows the pattern $\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$, it is a neutralisation reaction.

Baking soda is the common name for the chemical compound **sodium bicarbonate** (also known as sodium hydrogen carbonate). ### Step-by-Step Identification: 1. **Chemical Name**: Baking soda's IUPAC name is Sodium bicarbonate. 2. **Chemical Components**: It consists of a sodium cation ($Na^+$) and a bicarbonate anion ($HCO_3^-$). 3. **Chemical Formula**: When these ions combine to form a neutral compound, the formula is $NaHCO_3$. ### Comparison with other options: - $Na_2CO_3$ is **Washing Soda** (Sodium Carbonate). - $NaCl$ is **Common Salt** (Sodium Chloride). - $CaSO_4$ is **Calcium Sulfate** (often found in Gypsum or Plaster of Paris).

### Step-by-Step Solution **1. Definition and Composition:** Plaster of Paris (PoP) is a quick-setting gypsum plaster consisting of a fine white powder (calcium sulphate hemihydrate). Its chemical name is **Calcium Sulphate Hemihydrate**. **2. Chemical Formula:** The term 'hemihydrate' refers to the presence of half a molecule of water of crystallization for every molecule of calcium sulphate. Therefore, the formula is written as: $$CaSO_4 \cdot \frac{1}{2}H_2O$$ **3. Formation from Gypsum:** It is prepared by heating Gypsum ($CaSO_4 \cdot 2H_2O$) to $373\text{ K}$ ($100^\circ\text{C}$). At this temperature, gypsum loses water molecules to become Plaster of Paris: $$CaSO_4 \cdot 2H_2O \xrightarrow{373\text{ K}} CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O$$ **4. Analysis of Options:** * $Ca(OH)_2$ is Calcium Hydroxide (Slaked Lime). * $CaSO_4 \cdot \frac{1}{2}H_2O$ is Calcium Sulphate Hemihydrate (Plaster of Paris). * $CaCO_3$ is Calcium Carbonate (Limestone). * $CaO$ is Calcium Oxide (Quicklime). **Conclusion:** The correct formula for Plaster of Paris is **$CaSO_4 \cdot \frac{1}{2}H_2O$**.