Chapter 3: Chemical Reactions part 1 Introducing chemical Reactions SABIS Grade 9 (Level K)

3.1 Introducing Chemical Reactions

Over the thousands of years since they were built, the temples at the Athenian acropolis have been exposed to many different destructive forces that have each caused different changes in the buildings. These changes can be classified into two main categories—physical changes and chemical changes.

Physical Change versus Chemical Change
How does a chemical change differ from a physical change? A chemical change is a change in which new substances are formed. For example, burning wood involves chemical changes because the chemical compounds in the wood are changed to produce new compounds such as carbon dioxide and water vapor. A physical change is a change in the form of a substance. Chopping wood involves physical changes because the chemical compounds in the wood are still present after chopping. Only the size of the wood pieces has changed.


Figure 1 Burning wood is an example of a chemical change. Chopping wood is an example of a physical change.

Physical changes can be distinguished from chemical changes by analyzing whether or not the changing matter undergoes a change in its chemical identity. Chemical changes result in a change in the chemical identity of the substance, while physical changes involve a change in the form of the substance but not in its identity. An example of a physical change is a change in state: melting, evaporation, and solidification are physical changes. As ice melts, it does not change its chemical identity; it is water in both states, solid and liquid. Another physical change is crushing a sugar cube into smaller sugar crystals. During a physical change, the size, shape, and/or state of a substance may change, but its chemical identity remains intact.
Which of the following are examples of a physical change?

freezing liquid water into ice

sculpting clay

tearing a piece of paper

burning wood

cutting wood





Physical Change versus Chemical Change - Cont.

Consider the changes in the figure below. If you look closely, you can see that parts of the roof and walls have crumbled away and that some of the statues have missing arms and facial features.

Figure 2 Two different types of change are evident in this photo.
One is a chemical change, which can be observed by the damaged faces of the statues.
The other is a physical change, which can be observed by the crumbling roof and walls.

The fracture in the marble wall represents a physical change. Some force was exerted on this marble block and caused the crack. The marble changed form as it cracked and broke. However, most importantly, the marble did not undergo any change in its chemical identity as it changed form; it remained marble after the change. This change was therefore a physical change.
The same cannot be said for the changes in the facial features of the statues. These changes are chemical changes mainly caused by acid rain—a form of pollution. Acid rain occurs when gaseous pollutants, such as sulfur dioxide gas emitted by coal-burning power plants, mix with water droplets in clouds. This mixing results in the formation of acids, such as sulfuric acid. When the rain falls to the ground, it carries the acid with it. When exposed to acid rain, the marble making up the faces of the statues undergoes a chemical change to form substances that are chemically different from marble and sulfuric acid. One of the new substances formed is carbon dioxide gas. Consequently, the marble appears to be fading away, since carbon dioxide escapes into the atmosphere.

Fill in the blanks by choosing the correct answer.

_________ change is a change where compounds are altered into new substances.

_________ changes involve a change in the form of the substance but not in its identity.

Your answer :

_________ change is a change where compounds are altered into new substances.
= Chemical
_________ changes involve a change in the form of the substance but not in its identity.
= Physical
Response:
That's the correct answer

Evidence of a Chemical Change

During a chemical change, one or more substances are replaced by one or more different substances. It is not always easy to tell when a chemical change has occurred. This is especially true for very slow changes, such as the reaction of acid rain with marble or for changes that are difficult to analyze directly, such as fireworks exploding. However, there are certain observations that tend to indicate evidence of chemical changes taking place. You may observe one or more indicators that a new substance with new properties has been formed, such as a change in color, or formation of a gas and/or a solid. Indicators that energy changes might have occurred can also be detected. These include a change in temperature or emission of light.

Formation of a Gas Many liquid and solid substances undergo chemical changes producing a gas. The formation of bubbles of gas might be a sign of a chemical change. An example of a chemical change that results in gas formation is the change that occurs when you place an antacid tablet in water. Some substances of the tablet undergo a chemical change when they mix together in water. Bubbles of carbon dioxide gas are formed as a result of this change.
Figure 3 Rapid bubbling can be an indication that a gas
is produced by a chemical reaction.  

Formation of a Solid Just as the formation of a gas can signal the formation of a new substance, so can the formation of a solid. An example of a chemical change that results in formation of a solid is rusting of iron. Although this is a slow change, rust is a new solid that appears on iron surfaces undergoing the chemical change known as rusting.

Figure 4 During rusting, a chemical change occurs,
producing a reddish brown solid.

Change in Color A change in color might be an indication that a new substance has formed. An example is the color change you observe when treating stained fabric with a stain remover. The stain appears to vanish, but in reality, it has changed chemically so that it is no longer visible.

Figure 5 Some colored substances undergo a chemical change when treated
with stain removers, forming new, colorless substances.

Change in Temperature or Emission of Light 

Chemical changes involve a release or absorption of energy as heat or light.
However, the amount of energy involved may or may not be detectable. Hot packs and cold packs that are sold with first aid kits are examples of substances that undergo a chemical change detectable through observable temperature changes. Fluorescent light sticks that glow when you activate them are examples of substances that release light during a chemical change.
The above indicators cannot be exclusively attributed to chemical changes; some of them may accompany physical changes as well. For example, mixing two paint colors together is a physical change that is definitely accompanied by a change in color; boiling is a physical change accompanied by the formation of gas bubbles; and spraying paint from a spray can is a physical change accompanied by a drop in the temperature of the can and its contents.
Observing the formation of a solid (a precipitate) in a liquid can be either a physical change or a chemical change. For example, evaporating the water from salt water causes the formation of solid salt crystals, which are the same material as dissolved salt; hence, it is a physical change. However, when solutions of sodium chloride and silver nitrate are mixed, silver chloride, which is a new material, precipitates, giving evidence of a chemical change taking place. 
Moreover, chemical changes might also be accompanied by physical changes. In this case, the distinction in changes is, in reality, between a physical change only and both a change and a physical change.
The best approach to distinguish changes is to make as many observations as possible of any substance undergoing change. The more indicators of chemical change you observe, the more likely it is that you are witnessing a chemical change rather than a physical change. 

What are the evidence for a chemical change?

change in shape

change in temperature

formation of a gas

change in color

formation of a solid

Which of the following are examples of a chemical change?

baking a cake

evaporating water

rusting

digesting a meal

exploding fireworks





answer :
exploding fireworks
Response:
That's the correct answer

Chemical Reactions

Every chemical change involves a chemical reaction. A chemical reaction is a process by which one or more substances are transformed into one or more new substances. The starting substances in a chemical reaction are called reactants and the substances produced are called products.
When iron rusts, for example, it reacts with oxygen in the air to produce rust. If you analyze the chemical composition of rust, you would find that it consists of a compound called iron oxide. In this case, iron and oxygen are chemically transformed via chemical reaction into a new substance, iron oxide.
Each chemical reaction involves changes in the chemical identities of reactants as they form new substances (the products). How does this happen? Reactants are composed of atoms. The atoms are associated with one another in specific bonding arrangements. The properties and identity of chemical compounds are determined by these bonding arrangements. When a chemical reaction occurs, the bonds holding atoms together in the reactants are broken and the atoms rearrange to form new bonds. The resulting products differ chemically from the reactants because of the changes in bonding between the atoms.
Consider the reaction that occurs between hydrogen and chlorine. Before this reaction takes place, each hydrogen atom is bonded to another hydrogen atom through a covalent bond. Similarly, each chlorine atom is covalently bonded to another chlorine atom. When these two pure substances are brought together and react, the covalent bonds between hydrogen atoms and the covalent bonds between chlorine atoms break, and then the hydrogen and chlorine atoms form covalent bonds with each other.
Figure 7 In this chemical reaction, bonds between hydrogen atoms and bonds between chlorine atoms break.
New bonds between hydrogen and chlorine atoms form to make the new product.

Water and sodium chloride are formed when sodium hydroxide and hydrogen chloride are combined. 
What are the reactants and what are the products of this chemical reaction?

Sodium hydroxide and hydrogen chloride are the products.

Water and sodium chloride are the reactants.

Water and sodium chloride are the products.

Sodium hydroxide and hydrogen chloride are the reactants.



Representing Chemical Reactions Using Word Equations

A chemical reaction can be represented by a chemical equation in which reactants and products are separated by an arrow. Reactants are represented to the left of the arrow, and products are represented to the right of the arrow. When the reaction involves two or more reactants, a plus sign is added in between the reactants. This is also applied for the products. In a word chemical equation, the reactants and products are represented by their names. For example, if you take a strip of magnesium metal and place it in hydrochloric acid solution, you would observe a vigorous rush of bubbles. The bubbles contain hydrogen gas, one of the products of the reaction. The other product is magnesium chloride, which is dissolved in water. You can represent this reaction using the following word equation:

Notice that water does not undergo any chemical change in this reaction. Water acts as a solvent. A solvent is a substance in which other substances are dissolved. A solution is what results when a substance dissolves in a solvent.





A World of Chemical Reactions You may not be aware of it, but you encounter chemical reactions constantly in your daily life. Whenever you cook an egg or bake a pie, you are observing chemical change. A chemical reaction is happening inside the batteries you use to power a cell phone or an MP3 player whenever they produce electricity. The gasoline that you put into a car reacts chemically when it is burned in the car’s engine.



One of the most important groups of chemical reactions you encounter every day happens inside your body. Cells take in substances from the environment and transform them into new substances through different series of chemical reactions, such as during cellular respiration. These new substances help maintain existing cells and build new ones. Cellular respiration is a chemical process, carried out by living cells, that transforms glucose and oxygen into carbon dioxide and water with the release of energy. Even though this process involves many chemical reactions, the overall process can be represented by the following chemical equation:
The reactants are quite different from the products. Glucose is a white powdery solid and oxygen is a colorless, odorless gas that has a tendency to react explosively with other substances. Carbon dioxide is also a colorless, odorless gas like oxygen, but, unlike oxygen, it is not very reactive. As for water, it is a colorless, odorless liquid.
These differences are expected because the reactants and products represent different substances. This is due to changes in the bonding between the atoms during cellular respiration.
Hydrogen, H2, reacts with bromine, Br2, to form hydrogen bromide, HBr. 
How are the atoms rearranged during this chemical reaction?

A new bond between hydrogen and bromine atoms is formed to make a new product.

The bond between hydrogen atoms and the bond between bromine atoms are broken.

Only the bond between bromine atoms is broken.

Only the bond between hydrogen atoms is broken.


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