Question

When calcium oxide, \(\mathrm{CaO}(\mathrm{s})\), dissolves in water the water becomes hot. Write a chemical equation for this process and indicate whether it is exothermic or endothermic.

Short answer

The reaction is \( \text{CaO (s) + H}_2\text{O (l) } \rightarrow \text{Ca(OH)}_2\text{ (aq)} \) and it is exothermic.

Step-by-step solution

Understand the reaction components

Calcium oxide (CaO) reacts with water (H₂O) to form calcium hydroxide, Ca(OH)₂. This reaction is a combination of a solid and liquid to form an aqueous solution.

Write the chemical equation

The chemical reaction of calcium oxide with water is as follows: \[ \text{CaO (s) + H}_2\text{O (l) } \rightarrow \text{Ca(OH)}_2\text{ (aq)} \]. This equation represents the transformation of solid calcium oxide and liquid water into aqueous calcium hydroxide.

Determine the reaction type

Since the water becomes hot during this process, the reaction releases heat. Therefore, the reaction is classified as exothermic, as heat energy is released into the surroundings.

Key concepts

Exothermic Reactions

An exothermic reaction is a type of chemical reaction where energy, usually in the form of heat, is released into the surroundings. This happens because the energy needed to break bonds in the reactants is less than the energy released when new bonds are formed in the products. In simpler terms, the process gives off more energy than it absorbs.
Imagine you are striking a match against a matchbox. The friction of the match head creates enough energy to cause a chemical change, producing heat and light. Similarly, when calcium oxide reacts with water, the reaction is exothermic because it releases heat into the water, making it feel hot.

• Exothermic reactions can be seen in everyday processes like burning wood, respiration, and even hand warmers.
• They are characterized by a temperature increase in the surroundings as the reaction proceeds.

This is a clear indication that energy is being emitted from the reaction itself.

Calcium Hydroxide Formation

Calcium hydroxide formation involves combining calcium oxide, a solid, with water, a liquid, to form an aqueous solution of calcium hydroxide. This process transforms basic substances into a new compound.
When calcium oxide, often known as quicklime, is mixed with water, the result is a highly alkaline solution of calcium hydroxide, commonly called slaked lime, which can dissolve partially in water. This simple yet intriguing reaction has important practical applications.

• Calcium hydroxide is widely used for soil stabilization, where it helps to alter the properties of the soil for better construction support.
• It is also used in treating drinking water and wastewater, where it helps to remove impurities.
• In the production of corn tortillas, calcium hydroxide aids in the process of nixtamalization, which improves the nutritional quality of the corn.

This reaction not only changes physical states but also the chemical identity of the substances involved.

Chemical Equations

Chemical equations are a way to represent chemical reactions using symbols and formulas for the reactants and products. They provide a concise and informative method to convey the details of chemical processes.
The chemical equation for the reaction of calcium oxide with water is \[ \text{CaO (s) + H}_2\text{O (l) } \rightarrow \text{Ca(OH)}_2\text{ (aq)} \]. This equation tells us:

• "CaO (s)" represents solid calcium oxide.
• "H₂O (l)" stands for liquid water.
• "Ca(OH)₂ (aq)" is aqueous calcium hydroxide, meaning it is dissolved in water.

The arrow in the equation points from the reactants to the products, visually indicating the transformation.
Understanding chemical equations allows us to predict the products of a reaction, calculate quantities of reactants needed, and comprehend the conditions under which reactions occur. In summary, chemical equations are crucial tools in chemistry that offer insight into the nature of reactions and help communicate complex processes in a manageable format.