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Chemical Reactions

Chemical Reactions

This lesson aligns with Next Generation Science Standards (NGSS) PS1.B

When a substance undergoes a chemical change, a new substance is formed. A Chemical reaction occurs when bonds among the atoms are broken or formed. Chemical changes involve color changes, temperature changes, gas production, or precipitate formation. When a piece of wood is burned, the matter is neither destroyed nor formed. Rather, the matter in the form of wood becomes ash, smoke, and gases. The chemical energy that was stored in the wood is now transformed into thermal energy and light. Matter and energy are conserved, which are redistributed during a chemical reaction. In this lesson, we will talk about chemical reactions, the difference between physical and chemical changes, the characteristics of chemical reactions, the conservation of matter, as well as energy consideration.

Chemical Reactions
A chemical reaction is a process in which bonds of the reactant molecules are broken, and new bonds are formed to create a new product. Examples of chemical reactions in everyday life involve cooking, digestion, photosynthesis, etc. 

Reactants are the starting material that chemically interacts to produce new substances.

The newly formed substances or compounds that are present at the end of the reaction are known as products.

Physical Changes vs. Chemical Changes 
A physical change involves a change of state, but the kind of matter remains the same. The nature of the substance and the number of particles present in it remain unchanged. Physical changes are reversible and do not produce a new product. When the ice melts, it becomes water, i.e., a liquid. If water is heated up, it is converted into steam. However, the chemical structure of the water in all these states remains the same. Boiling, melting, and freezing is examples of physical changes.

When a chemical change occurs, the new substance's properties and composition are different compared to the original substance. Chemical changes cannot be reverted easily. Rotting of food and rusting of iron are common examples of chemical changes.

Characteristics of Chemical Reactions 
Chemical changes are indicated by a change in color or temperature, the development of an odor, the formation of a gas, or the formation of a precipitate.

Change in Color
When an iron nail is left outside, it reacts with oxygen and forms iron oxide. The iron nail gets rusted, changing its color to reddish-brown. This color change indicates a chemical reaction. 

Change in Temperature 
Using a chemical ice pack on injuries reduces inflammation and swelling immediately. It absorbs heat and causes a cooling effect. 

Development of an Odor
When food reaches its expiration date, a chemical reaction begins. Bacteria, yeast, and fungi feed on the food and decompose it. As a result, the food spoils, producing a foul odor in its rotten state. 

Formation of a Gas 
Another common sign of a chemical reaction is the formation of gas. When two household chemicals, such as vinegar and baking soda, are mixed, it immediately starts bubbling and foaming. The bubbles are formed due to the release of carbon dioxide gas. It is a product of a chemical reaction between vinegar and baking soda.

Formation of Precipitate 
This happens when two liquids react chemically, and an insoluble solid called precipitate is formed. The production of a solid substance indicates that a chemical reaction has taken place which altered the original substance.

Conservation of Matter   
In chemical reactions, matter is neither created nor destroyed. It may seem that matter is disappearing during a chemical reaction, but it appears in a new form. So, the total mass of the reactants is always the same as the total mass of the products. When raw vegetables are cooked, the matter that makes up the vegetables goes through a chemical reaction, turning into delicious food but not destroyed at all. 

Energy Considerations 
Energy plays a key role in chemical reactions. Energy is needed to break bonds among the reactant molecules. It is released when new bonds are formed.

Endothermic Reactions 
The reactions in which reactants absorb energy from the surroundings to break their bonds and form new products are called endothermic reactions, e.g., the burning of wood.

Exothermic Reactions 
The reactions that release energy during a chemical change are called exothermic reactions. Rusting of iron is an example of an exothermic reaction.

  • A chemical reaction is a process in which reactants interact chemically, and new products are formed, e.g., the burning of a candle.
  • A physical change involves only a change of state, whereas a chemical change includes changes in the properties and composition of reactant molecules.
  • Chemical reactions can be indicated by a change in temperature or color, the development of an odor, the formation of a gas, or the formation of a precipitate. The matter is always conserved during a chemical reaction.  
  • Endothermic and Exothermic reactions are chemical reactions that absorb and release energy, respectively.

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