ATI TEAS SCIENCE REVIEW – CHEMISTRY
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Questions related to chemistry test your knowledge of chemical properties and processes. You may be asked questions about states of matter, properties of matter, phase changes, chemical bonds, chemical solutions, chemical reactions, and acids and bases. You may be asked to balance chemical equations.
STATES OF MATTER
Matter is made up of microscopic particles that move different speeds depending on the energy they are exposed to. We measure this energy as temperature. The molecules can move either quickly and randomly or hardly at all.
When the energy is high, matter take the form of a gas, in which molecules are moving about quickly and are far apart. Gases have no fixed form. Molecules are free to move at random past each other, and they tend to fill any container that holds them. If a gas is not contained, its molecules will disperse.
Lower temperatures result in a liquid, in which molecules cohere but are fluid. Coherence means that the molecules remain close together, but they can change position by sliding over one another. In liquids, molecules move less freely than in a gaseous state, sliding past one another. They have a fixed volume but will flow freely unless they fill a portion of a container.
When the temperature is low, matter takes the form of a solid, in which molecules are packed closely together and retain their positions. Solid matter is rigid, and molecules retain a uniform spacing. A solid has a defined form, which is brittle. It can be broken into pieces but tends to stay together.
A somewhat unusual state of matter is plasma, which is like a gas in many of its properties but carries an electric charge. The TEAS focus on solids, liquids, and gases.
CHANGES IN STATE OF MATTER
The state of matter depends on temperature and pressure. Higher temperatures cause molecules to energize and move farther apart. Increasing pressure forces molecules closer together. Melting is the phase change from solid to liquid and boiling is the phase change from liquid to gas. There is also a direct change from solid to gas known as sublimation. The phase change from gas to liquid is condensation and the change from liquid to solid is freezing. A direct change from gas to solid is deposition.
PROPERTIES OF MATTER
All types of matter can be described in terms of the physical and chemical properties each substance has. Physical properties are observable and there is an extensive list of physical properties that one could observe about a substance. A few examples are density, the temperatures at which the substance undergoes phase changes, malleability, conductivity, specific heat capacity, mass, volume, color, and many other properties. Physical properties are further divided into intensive and extensive properties. An intensive property does not depend on the size or amount of matter in the object, while an extensive property does depend on the amount of matter in the object. For example, mass is extensive because the measurement would change the size of the sample. Boiling point is intensive because the temperature at which the object boils is not dependent on its volume.
Chemical properties describe how a substance behaves during and following a chemical reaction. For example, we can categorize wood as flammable because it becomes heat, ash, and carbon dioxide when heated in the presence of oxygen. Chemical properties include oxidation, flammability, radioactivity, toxicity, reactivity with other chemicals, chemical stability, and others.
The properties of an unknown substance can be used to identify it. You may be asked to do this using a chart showing various characteristics. You will also need to be familiar with the properties of water since water is such a common substance and has some unique properties.
Water is a polar inorganic compound that is transparent and nearly colorless. H2O is a covalent compound because oxygen and hydrogen are nonmetals. It has 8 total valence electrons (6 from oxygen and 1 from each hydrogen). Breaking the bonds requires a lot of energy, so water has a very high specific heat and heat of vaporization. The molar mass of water is 18.02 g/mol. It commonly exists as solid, liquid, and gas. The polarity of water allows it to exhibit both cohesive and adhesive properties. Cohesiveness allows water to travel through tiny capillaries and cohesiveness creates surface tension on the surface of a body of water. Adhesiveness allows water to stick to other molecules and dissolve them, making it known as the “universal solvent.” Water also has a unique property called osmosis, which is a specific type of diffusion. Diffusion is a term used to describe the process of a substance moving from an area of high concentration to an area of low concentration. Osmosis is a type of diffusion in which water moves passively through a semi-permeable membrane to equalize water concentration on both sides of the membrane. This is how water moves through cell walls in the body.
UNDERSTANDING CHEMICAL BONDS
A chemical compound is created when two or more atoms join to form a chemical bond that leaves the atoms in a less excited state than they were in before the bond. Such bonds form in two ways.
A covalent bond occurs when atoms share electrons between them. This type of bond is common between two atoms of the same element, as in hydrogen (H2) or in similar elements. When a molecule shares a pair of electrons in a stable state, it has formed a covalent bond. Alkanes, for example, share a single bond. In some compounds, one atom takes the shared electron for more time, due to its structure, forming a polar covalent bond. This molecule is partly negatively charged and partly positively charged. Some molecules form a double bond, sharing four electrons as opposed to two. These bonds are commonly represented in the alkenes, hydrocarbons with twice as many hydrogen molecules as carbon molecules. It is possible to form triple bonds as seen in a group of hydrocarbons called alkynes.
An ionic bond is created between atoms when one atom gives an electron to the other. These bonds typically take place between metals and nonmetals due to the unique electron configuration of metals, with the metal giving an electron to the nonmetal. This transfer creates a positive charge and a negative charge at the ends of the compound. The positive charge, or cation, is created by the giver of an electron. The negative charge, or anion, is located at the receiving end of the electron. The net charge of the compounds remains balanced at zero.
UNDERSTANDING CHEMICAL SOLUTIONS
A chemical solution is a group of chemical compounds evenly distributed in a state of matter. The solution is a homogenous mixture where one chemical compound is completely dissolved in the others. This is most easily achieved in a liquid state. There are mixtures that are not solutions. A heterogeneous mixture maintains separation between two substances, like oil and water.
The solute is the compound dissolved in the solvent. Liquids make excellent solvents. The solubility of a solvent depends on the nature of the liquid as well as external factors like temperature. The concentration of the solution is the amount of solute in the solution. The mole is the unit of measurement for chemical reactions and refers to a compound’s molecular mass.
UNDERSTANDING CHEMICAL REACTIONS
To create a new chemical compound from other elements and compounds, a chemical reaction is needed. Two or more reactants are added together, often with an input of energy, creating one or more products and by-products. Photosynthesis occurs, for instance, when a plant cell combines carbon dioxide and water. The sun’s rays provide the energy. The chemical reaction produces sugar and oxygen.
Chemical reactions are shown with equations and have a basic pattern: reactants go on the left and products go on the right, with the reaction sign (an arrow) showing the direction of the reaction in the middle. Here is an example showing the direction of the reaction in the middle. Here is an example showing the formation of water molecules:
Equations for chemical reactions must be balanced; there must be the same number of atoms of each element on both sides of the reaction. Notice in the equation above that there are four hydrogen atoms and two oxygen atoms on each side. Only their arrangement is changed.
There are five basic types of chemical reactions:
- Synthesis: two separate things joining together to form 1 compound
- Decomposition: 1 compound breaks down into 2 or more compounds
- Combustion: The use of fuel (combustible material) with oxygen to form carbon dioxide and water
- Single replacement: 1 element or compound replaces another element or compound in a compound
- For example: A + BC ⟹ AC + B
- Double replacement: 2 ionic compounds create two more iconic compounds
- For example: AB + CD ⟹ AD + CB
Reaction rates depend on the likelihood of collision between particles. The reaction rate can be altered by changing the following factors:
- Concentration – The more particles there are, the higher the chance of collisions.
- Temperature – Particles excite at higher temperatures, so more collisions are likely and will have more energy.
- Pressure – Increased pressure forces particles together, so collisions are more likely
- Surface area – In a solid, only particles at the surface can collide. The bigger the surface, the faster the reaction. Breaking up a sample into smaller particles provides more surface area for collisions.
- Catalysts – A catalyst is a substance that changes the rate of a chemical reaction but is chemically unchanged at the end of the reaction.
Chemical reactions occur in nature and in the laboratory. A catalyst will speed up the reaction by lowering the amount of energy needed to start the reaction. Enzymes act as catalysts in cellular processes. They quicken the chemical reaction, turning a molecule, known as a substrate, into a product without being altered themselves.
ACIDS AND BASES
Many acids and bases can be understood from the perspective of the theory developed by Arrhenius, a Swedish scientist. In this view, an acid is a substance that gives off hydrogen (H+) ions when it is dissolved in water. A base, or alkaline substance, is a substance that gives off hydroxide (OH-) ions when it is dissolved in water. Acidic solutions have higher concentrations of hydrogen ions, whereas alkaline solutions have lower concentrations of hydrogen ions.
The presence of acids and bases can be tested using tools known as indicators. One indicator in common use litmus paper. Litmus paper turns red in the presence of a base.
Here are some examples of acids and their chemical formulas.
Here are some examples of bases.
The acidity or alkalinity of a solution is measured using a scale known as the pH scale.
(Insert list of pH scale)
Each step of the pH scale has 10 times the difference in concentration of hydrogen () ions as the step before or after it. So, a solution with a pH of 7 will have 10 times more hydrogen ions than a solution with a pH of 8 and 10 times fewer hydrogen ions than a solution with a pH of 6.
BALANCING CHEMICAL EQUATIONS
The TEAS Science section may contain questions that ask you to balance chemical equations. We will outline the steps in this process later in this chapter.
One of the most important chemical equations for humans is the one that represents photosynthesis. Without the following equation, there would be no life on Earth:
This equation shows how green plant cells, with the help of the sun’s energy, convert carbo dioxide () and water () into glucose and oxygen (). The two reactants, carbon dioxide and water, are on the left side of the arrow. The arrow shows the direction of production. The two products, sugar and oxygen, are on the right side of the arrow.
According to the Law of the Conversation of Mass, in a chemical reaction, no energy is lost, but neither is mass destroyed. The amount of reactant must match the amount of products that are made, even if those products escape as a gas or a liquid.
In the photosynthesis equation, there is a difference in the number of atoms on the right and left sides:
To produce sugar and oxygen requires more reactants than we have on the left side. The solution is to balance the two sides.
We can multiply any molecule with a number, called a coefficient. We cannot change the subscript, however, without changing the nature of the molecule. By adding coefficients to the reactants and products, we can balance the equation in a few simple steps.
The best way to do this is by balancing each element in turn. Start with the carbon. The right side has 6 carbon atoms, so the left side needs 6:
Here we have multiplied the by 6 to result in 6 carbon atoms on the left side. When we multiply by 6, this also change the number of oxygen atoms on the left side from 3 to 13. There are now 12 oxygen atoms in the molecule, plus 1 in the molecule, for a total of 13. The 2 changed numbers are shown underlined in the following table.
Now the carbon is equal, but the hydrogen remains unequal, and the oxygen has changed in number. Next, fix the hydrogen:
Here we multiplied the molecule by 6 to result in 12 hydrogen 2 atoms on the left side. When we multiple by 6, the further changes the 2 number of the oxygen atoms on the left side from 13 to 18. There are now 12 oxygen atoms in the molecule, plus 6 in the molecule, for a total of 18.
The last remaining imbalance rests with the oxygen. At this point, there is plenty of oxygen in the reactants. We can balance the equation by producing more .
In this step, the molecule on the right side was multiplied by 6. This resulted in 18 oxygen atoms on the right side. The equation is now balanced. Each element has the same number of atoms on the left and right sides.
ATI TEAS Science Chemistry
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