Question

Write chemical formulas for (a) quicklime and (b) slaked lime.

Short answer

Quicklime: CaO; Slaked lime: Ca(OH)_2.

Step-by-step solution

Identify Quicklime

Quicklime is the common name for calcium oxide. In chemistry, the term "calcium oxide" suggests that the compound consists of one calcium ion and one oxide ion. Calcium, being in group 2 of the periodic table, forms a 2+ ion ( Ca^{2+} ), and oxide, being a non-metal from group 16, forms a 2- ion ( O^{2-} ). When these ions combine, their charges balance out to zero, giving the chemical formula for quicklime, CaO.

Understand Slaked Lime

Slaked lime is the term used for calcium hydroxide. It is formed when water is added to quicklime (calcium oxide). The chemical reaction between calcium oxide ( CaO ) and water ( H_2O ) produces calcium hydroxide ( Ca(OH)_2 ). The hydroxide part means that each calcium ion pairs with two hydroxide ions ( OH^- ) to balance the +2 charge of calcium.

Write the Chemical Formula for Quicklime

Based on the identification (calcium ions and oxide ions), write the chemical formula for quicklime as CaO .

Write the Chemical Formula for Slaked Lime

Using the reaction understanding, write the chemical formula for slaked lime as Ca(OH)_2 , showing that calcium combines with two hydroxide ions.

Key concepts

Calcium Oxide

Calcium oxide, commonly known as quicklime, is a widely used chemical compound in industries and laboratories. It is represented by the chemical formula \(CaO\) and is composed of calcium ions \(Ca^{2+}\) and oxide ions \(O^{2-}\). Calcium is an alkaline earth metal from group 2 on the periodic table, while oxygen is a non-metal from group 16. When these two elements combine, their respective charges neutralize each other, forming an electrically neutral compound.

Calcium oxide is a white, caustic, alkaline crystal. It is important to handle it safely as it reacts exothermically with water, releasing heat. This reaction makes it a vital component in industries for producing cement, limewash, and agricultural treatments.

Calcium Hydroxide

Calcium hydroxide, or slaked lime, is a compound formed by the reaction of calcium oxide with water. The chemical process can be written as:
\[CaO + H_2O \rightarrow Ca(OH)_2\]
Here, quicklime (calcium oxide) reacts with water to form calcium hydroxide \(Ca(OH)_2\), which is a less hazardous compound compared to quicklime. Calcium hydroxide consists of a calcium ion \(Ca^{2+}\) and two hydroxide ions \(OH^-\). The hydroxide ions balance the calcium ion's charge, creating this stable ionic compound.

This reaction is important in many applications, such as in the preparation of mortar and plaster, and it is also used in water treatment and food preparation processes, like traditional nixtamalization.

Ionic Compounds

Ionic compounds are formed from the chemical bonding of a metal and a non-metal. They consist of positive ions (cations) and negative ions (anions) that are held together by strong electrostatic forces. These forces result from the attractions between oppositely charged ions, forming a stable structure.

For example, in calcium oxide \(CaO\), calcium forms a cation \(Ca^{2+}\) and oxygen forms an anion \(O^{2-}\). The charges are balanced in the compound, giving it its neutral charge.

• Typically crystalline solids at room temperature
• High melting and boiling points
• Conduct electricity in liquid form or when dissolved in water
• Generally soluble in water

These properties make them widely useful in everyday life, from salt to mineral supplements.

Chemical Reactions

Chemical reactions involve the transformation of one or more substances into different substances. They are crucial for understanding how matter interacts under different conditions. In the context of calcium oxide and calcium hydroxide, reactions demonstrate how energetic and transformative these processes can be.

For example, the conversion of calcium oxide to calcium hydroxide is an exothermic reaction, meaning it releases heat:
\[CaO + H_2O \rightarrow Ca(OH)_2 + ext{heat}\]
This release of energy makes it an effective and widely used reaction in various industrial applications.

Recognizing the signs of a chemical reaction includes understanding changes such as heat production, gas formation, or precipitate formation. These reactions are fundamental to the fields of chemistry and are applied in areas like manufacturing, agriculture, and environmental management.