Introduction:

The smell of cooked food or the sight of fire smoke is evidence of a chemical reaction. While some signs of chemical reactions may not always be clear, there are indicators that confirm their occurrence. Chemical phenomena are an integral part of our daily lives, with chemical reactions constantly occurring around us. The ability to recognize these reactions is essential for understanding the world we live in. In this article, we will discuss the Law of Conservation of Mass.

Physical and Chemical Changes:

The Law of Conservation of Mass applies to two types of changes: physical changes and chemical changes. Physical changes affect the physical properties of matter only, such as volume, shape, and state (solid, liquid, or gas). For example, when water freezes, its physical state changes from liquid to solid, but it remains water. On the other hand, chemical changes result in the formation of a substance with properties different from the original substance. Rust on iron products is an example of a chemical change, as is the solid precipitate formed by mixing two liquid substances like silver nitrate and sodium chloride. The process that produces a chemical change is called a chemical reaction.

Distinguishing between physical and chemical changes can be illustrated using a folded sheet of paper. Folding the paper changes its size and shape, a physical change, while burning the paper results in a chemical change because it produces a new substance. The Figure 2 in the article illustrates this concept.

Chemical Equations:

Expressing chemical equations involves identifying the reactants, the substances undergoing the reaction, and the products, the substances formed as a result. Reactants are written to the right of the arrow, separated by a plus sign (+), while products are written to the left of the arrow, also separated by a plus sign. The arrow between them represents the changes occurring during the chemical reaction. Reading the equation is indicated by the word "yields."

Using Words:

Chemical equations can be written using the names of the reacting substances and the products. The verbal equation for the reaction between vinegar and baking soda can be expressed as acetic acid + sodium hydrogen carbonate yields sodium acetate + water + carbon dioxide.

Using Chemical Names:

Common household chemicals, like vinegar and baking soda, have chemical names such as acetic acid and sodium hydrogen carbonate. Chemical names are often used in chemical equations instead of common names. The chemical equation for the reaction can be written as follows:

Using Chemical Formulas:

Chemical equations can be written more concisely using chemical formulas. The chemical formula for acetic acid is and for sodium hydrogen carbonate, it isThe chemical equation becomes:

Subscripts:

Subscripts, written to the lower right of atoms, indicate the number of atoms of each element in a compound. For example, the "2" in CO₂ signifies that one molecule of carbon dioxide contains two oxygen atoms. If no number is written next to an element, it means there is only one atom of that element in the compound.

This translation covers the main concepts discussed in the original Arabic text about the Law of Conservation of Mass, physical and chemical changes, chemical equations, and the use of words, chemical names, and formulas in expressing chemical reactions.

Mass Conservation:

What happens to the atoms of the reacting substances when they transform into other substances (products)? According to the law of mass conservation, the mass of the resulting substances must be equal to the mass of the reacting (or entering) substances in the chemical reaction. This law was formulated by the French chemist Antoine Lavoisier (1743-1794), one of the first modern chemists who used logic and scientific methods to study chemical reactions. Lavoisier demonstrated through his experiments that nothing is created or destroyed in chemical reactions except by the will of God.

He explained that chemical reactions closely resemble mathematical equations in which the right side is equal to the left side. Similarly, in a chemical equation, the number and type of atoms on both sides of the equation are equal. Each atom in the reactants also appears in the products, as illustrated in the figure. Atoms are neither created nor destroyed in chemical reactions; instead, they are rearranged.

Chemical Equation Balancing:

When you write a chemical equation for a reaction, you must not neglect the law of mass conservation. Look again at Figure 4, which shows that the numbers of carbon, oxygen, hydrogen, and sodium atoms on both sides of the arrow are equal. This means that the equation is balanced, and the law of mass conservation has been applied.

However, not all equations can be balanced as easily. Consider, for example, the black silver formed in the reaction of silver with hydrogen sulfide in the air (hydrogen sulfide). The unbalanced equation is as follows:

Ag+H2S →Ag2S + H2

If you calculate the number of atoms for each element in the reactants and products, you'll find that the number of hydrogen and sulfur atoms is equal on both sides. Still, there is one silver atom in the reactants, while there are two in the products. This cannot be correct because, in a chemical reaction, a silver atom cannot be created from nothing. Therefore, the law of mass conservation is not represented correctly in this equation! Place the number 2 in front of the silver atom in the reactants and check the equation's balance by calculating the number of atoms for each element.

2Ag+H2S→Ag2S+H2

The equation is now balanced, with equal numbers of silver atoms on both sides of the arrow. Remember that when balancing a chemical equation, coefficients are placed before the formulas, as done for the silver atom. These coefficients should not change the subscripts to the right of the atoms in the chemical compound formula; changing them alters the compound type.

Energy in Chemical Reactions:

Chemical reactions often release or absorb energy. In the energy released from a welding flame, for example, when hydrogen and oxygen unite to produce water, the equation is as follows:

2H2 + O2 → 2H2O + energy 

Energy Release:

Where does this energy come from? To answer this question, consider the chemical bonds that are broken or formed when atoms gain, lose, or share electrons. In such reactions, bonds in the reactants break to form new bonds in the products. In reactions that release energy, the products are more stable than the reactants, and their bonds have lower energy than the bonds in the reactants. The excess energy is released in various forms, including light, sound, and heat.

Many types of reactions release heat, such as combustion, where a substance combines with oxygen to produce heat, light, carbon dioxide, and water.

Quick Release:

Energy is released rapidly sometimes, as seen in a charcoal lighter, where a liquid combines with the oxygen in the air, producing enough heat to ignite the charcoal in a few minutes.

Slow Release:

There are materials that also combine with oxygen but release heat slowly, making it invisible or imperceptible. For example, when iron reacts with oxygen in the air to form rust, heat is released slowly. This slow release of heat is used in warm compresses that are applied to certain body parts for several hours. Figure 7 illustrates the difference between the quick and slow release of heat.

Energy Absorption:

But what happens when the reaction is reversed? In reactions where energy is absorbed, the reactants are more stable than the products, and the bonds between them have less energy than the bonds in the products.

2H2O+energy →2H2 +O2

The above reaction shows that the additional energy needed to supply the reactants to form the products can be in the form of electricity, as shown in Figure 8. The energy (released or absorbed) accompanying chemical reactions can take various forms, including electrical, light, sound, and thermal energy.

When energy is released or absorbed in chemical reactions, specific terms are used to indicate this. A reaction that absorbs heat is called an endothermic reaction, where heat is absorbed. Conversely, an exothermic reaction releases heat. The word "therm" refers to heat, as seen in the term "thermos," a heat insulator, and the temperature measuring device, thermometer.

Some chemical reactions and physical processes require heat energy before they occur. Cold packs, for example, are examples of heat-absorbing physical processes. Figure 9 shows a cold pack containing water and ammonium nitrate. When this pack is broken, the ammonium nitrate dissolves in water, absorbing heat from the surrounding environment (air or the skin of the affected person) after applying the cold pack to the injured area.

Energy in the Chemical Equation:

The word "energy" is written in the chemical equation with the reactants or the products. If the word energy is written with the reacting substances, it indicates that it is an essential component for the reaction to occur. For example, we need electrical energy to break water molecules into hydrogen and oxygen. It is crucial to know that energy is necessary for this reaction to take place. Similarly, in exothermic reactions that release heat, the word "energy" is added to the products to indicate the liberation of energy. The word "energy" is added, for example, in the reaction between oxygen and methane when the burner flame ignites, as shown in Figure 10.

Conclusion:

1. We recognize the importance of understanding chemical reactions in our daily lives. The ability to distinguish between physical and chemical changes enables us to understand how reactions occur and form new substances.

2. Through the concepts presented about chemical equations and the use of chemical names and formulas, we gain a deeper insight into how reactants are transformed into new products.

3. The law of mass conservation shows us that nothing is destroyed or created in chemical processes, but rather, there is rearrangement and change in the distribution of atoms. This understanding helps us explain ordinary phenomena and changes we observe in our lives.

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