Physical Sciences
Important Definitions:
Accelerationthe change in velocity of a body over an interval of time
Amplitudethe height of a wave
Atomsmallest particle of an element that retains all the chemical properties of that
element
Atomic Numberthe number of protons in an atom of an element
Avogadro’s Lawwhen pressure and temperature are constant, equal volumes of gas contain the same number of particles
Biomassany material from an organism that is used to make energy
Boyle’s Lawwhen temperature remains constant, the pressure that a fixed amount of gas exerts on its container is inversely proportional to its volume
Catalysta substance that changes the rate of a reaction, but is not a reactant or product in the reaction
Charles’ Lawwhen pressure remains constant, the volume of a fixed amount of gas increases as its temperature increases, and decreases as its temperature decreases
Chemical Propertya property that can be observed or measured only when a substance changes to another substance
Circuitclosed loop consisting of conducting materials and circuit elements
Compoundsubstance composed of two or more different elements that are chemically combined
Compound Machinedevice that is made up of two or more simple machines
Concentrationa measure of the amount of that substance in a given sample
Conservation of Momentumthe total momentum of an isolated system remains constant
Covalent Bondchemical bond that forms when an atom shares one or more electrons with another atom
Currentthe rate at which electric charge passes by a point in a circuit
Densitythe mass of a substance divided by its volume
Displacementthe change in position of a body
Elastic Collisionscollisions in which both linear momentum and total kinetic energy are conserved
Electromagnetic Inductionthe process in which relative motion between a magnet and an electrical conductor produces an electric current in a conductor
Electromagnetic Spectrumall possible wavelengths of electromagnetic radiation
Electromagnetic Wavesa range of waves with varying frequencies and energies that travel at the speed of light
Electronsubatomic particle with a negative charge
Elementsubstance that cannot be broken down into simpler substances through chemical reactions
Endothermicreaction or process in which energy is absorbed
Energy Resourcenatural resource that can be used to produce energy
Exothermicreaction or process in which energy is released
Fossil Fuelsenergy resources such as coal, oil and natural gas that form over millions of years from the remains of plants and microscopic organisms
Frame of Referencethe reference point form which motion of a body is observed
Frequencythe rate at which a wave oscillates
Gay-Lussac’s Lawwhen volume remains constant, the pressure that a fixed amount of gas exerts on its container increases as its temperature increases, and decreases as its temperature decreases
Ideal Gasgas whose particles collide only with each other and with the walls of their container and are otherwise unaffected by any forces
Ideal Gas Lawunder ideal conditions, PV = nRT where P is pressure, V is volume, n is moles, R is a
constant and T is temperature (specific units for each variable are required)
Inelastic Collisionscollisions in which linear momentum is conserved, but kinetic energy is not
Input Distancethe distance you move the device or object that the device is affecting
Input Forcethe force you apply to the device or to an object that the device is affecting
Ionic Bondchemical bond that forms when an atom transfers one or more electrons to another
atom
Law of Conservation of Energyenergy cannot be created or destroyed
Matteranything that has mass and takes up space
Mechanical Advantagea number that describes how much easier it is to do work with the device than
without it
Mechanical Efficiencythe ratio of the machine’s work output to its work input
Molethe unit of measurement for the amount of a substance
Avogadro’s Number6.02 X 1023
Momentumthe product of an object’s mass and its velocity
Motionthe change in position of a body, relative to a reference point, over time
Neutronsubatomic particle with no charge
Nonrenewable Energyenergy resource that cannot be replaced during a human lifetime
Ohm’s Lawrelates voltage, current and resistance in a DC circuit
current is directly proportional to voltage
current is inversely proportional to resistance
Output Distancethe distance the device moves an object, or the distance the object would have moved if you had not used the device
Output Forcethe force the device applies to an object, or the force you would have had to apply to the object if you had not used the device
Periodic Tablechart that displays all the known elements
Productsthe new substances that form during a reaction
Protonsubatomic particle with a positive charge
Physical Propertyproperty of a substance that can be observed without changing the chemical composition of the substance
Radiationwaves are radiated energy from a particular source
Reactantsthe substances that participate in the reaction
Reaction Ratea measure of the number of effective collisions (collisions that result in a reaction) that occur in a given amount of time
Renewable Resourceresource that can be replaced during a human lifetime
Resistancethe ratio of voltage applied to a resistor divided by the current that flows through
the resistor
Simple Machine(six) device that makes work easier by changing the size or direction of a force applied to them
Solubilitythe ability of one substance to dissolve in another substance
Valence Electronelectron in the outermost layer of an atom
Velocitythe change in position of a body over an interval of time
Voltagedifference in electrical potential between two points
Wavelengththe distance between one peak on a wave to the next peak
Workthe amount of energy needed to move an object
Points to Ponder
Everything is made of matter. Matter can be classified based on its properties and structure. One of those classifications is an element (substance that cannot be broken down further by chemical means). While we have found over 100 elements, most of the matter on Earth is composed of only a few elements. For example, by mass, the Earth’s crust is 98% oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium. Elements are arranged on the Periodic Table in order of increasing atomic number. The numbers increase going across the table in rows (periods). The elements in the table are arranged so that similar elements are grouped together. For example, metals are on the left side of the table, nonmetals are on the right side of the table, and metalloids are found in between the metals and nonmetals. In general, metals are solids at room temperature, have high melting points, are flexible (malleable – able to be pounded into a sheet, and ductile – able to be drawn into a wire), and conduct electricity and heat. Nonmetals can be solid, liquid or gas at room temperature, but usually have lower melting points than metals. They are also brittle and non-conductors. Metalloid characteristics are a combination of metal and nonmetal characteristics. NOTE: matter can also exist as plasma (in which the electrons are separated from the atoms). This occurs only under extreme conditions, and is therefore not seen often on Earth.
Elements are also arranged in groups (columns) on the periodic table. Each group has the same number of valence electrons (outermost electrons), which are the electrons involved in bonding. Therefore, these elements all have similar properties, including reactivity. The first group on the table is the Alkali Metals. They have one valence electron, are soft, and react readily (so they are not found alone in nature). The second group is the Alkaline Earth Metals. They have two valence electrons, are a little hard and less reactive than the Alkali metals (but are still usually found in compounds in nature). One Alkaline Earth Metal is Calcium, which is found in the bones of vertebrates and the shells of invertebrates. The middle block is called the Transition Metals. They have various numbers of valence electrons, which allows them to form many different compounds, with many different properties. They are much less reactive, and can be found alone in nature. The 17th group is called the Halogens. These have seven valence electrons and are very reactive. When found free, they are found as diatomic molecules (i.e. F2). The Halogens commonly form salts with Alkali Metals. The last group on the table is the Noble Gases. They are very stable and rarely react. They are always found in the pure form in nature. Helium, a Noble Gas, has replaced the use of Hydrogen in airships. This is much safer as hydrogen is very flammable.
A singular piece of an element is called an atom. The atom type determines the chemical identity and properties of a substance. Atoms are broken down into subatomic particles called protons, neutrons and electrons, which are positively, neutrally, and negatively charged, respectively. The number of protons in an atom defines its atomic number. When the number of protons equals the number of electrons, the atom is said to be neutral. Electrons are found in an electron cloud outside of the nucleus. Protons and neutrons are found in the nucleus. Both have much higher mass than electrons, therefore most of the mass of an atom is found in the nucleus.
The way that particles in a substance (i.e. atoms in an element, molecules in a compound, etc.) are grouped together determines the properties of that substance. Particles that are packed closely together will only be able to vibrate in place. Because they can’t move much, their volume and shape is unchanging. This is called the solid state of matter. Liquid particles are slightly further apart. This allows for a little bit more movement, and results in the shape being indefinite. They are still somewhat held in place, though, so their volume is still constant. Gas particles are very far apart and free to move around a lot, therefore they have changing volume and changing shape. Substances change from state to state by gaining (solid liquid gas) or losing (gas liquid solid) thermal energy (heat).
Matter can be described using properties. Physical properties can be observed without changing the nature of the substance (i.e. density, mass, boiling point, color). Some physical properties change as the state of matter changes. For example, in most substances, the density decreases as the particles spread further apart as they change from the solid to liquid to gas states. (Density will not change, however, if the mass of the sample changes). Water is a notable exception to this – water actually becomes less dense when it freezes because the particles are becoming more ordered, and push each other further apart. Another physical property is solubility. Substances that dissolve in another substance are called soluble. Those that do not are called insoluble. When substances dissolve in other substances, the freezing points go down, and the boiling points go up. This is why salt is put on the road in cold, wet weather: the salt interferes with the water trying to form its ordered solid pattern, and the water cannot freeze as readily.
Chemical properties can only be observed when a substance changes its identity (ability to burn, ability to corrode, ability to react with an active metal, etc.). Therefore, chemical properties are observed via chemical reactions. Reactions involve the forming of and breaking down of compounds (chemical combinations of two or more elements). The smallest piece of a compound that retains the characteristics of the compound is a molecule (chemical combination of two or more atoms). Molecules are held together by forces called chemical bonds. Chemical bonds form between the valence electrons of atoms. The electrons can be transferred or shared in the bonds. Usually, the goal is to get a stable valence shell (8 electrons). NOTE: compounds are chemical combinations of atoms, and result in new substances with new properties, while mixtures are physical combinations of atoms and /or molecules, and the individual components of the mixture retain their own properties.
Bonds in which the electrons are shared are called covalent bonds. In this case, the electrons of one atom are attracted to the nucleus of the other atom and vice versa. If they are shared between two of the same kind of atoms, the electrons are shared evenly. If they are shared between two different types of atoms, however, one atom often attracts the electrons better. Therefore, the electrons spend more time near that atom making it partially negative. The other atom then becomes partially positive. This is called a polar covalent bond. Elements with three, four, five or six valence electrons often form covalent bonds. Covalent compounds have low melting and boiling points, are often insoluble in water, and do not conduct electricity.
Bonds in which the electrons are transferred from one atom (which becomes a positive ion) to another atom (which becomes a negative ion) are called ionic bonds. The oppositely charged ions attract and hold each other together. Atoms with one or two valence electrons usually lose them, while atoms with seven valence electrons often gain one. Thus, many ionic compounds form between an Alkali or Alkaline Earth Metal and a Halogen. Ionic compounds have high melting and boiling points, are usually soluble in water, and conduct electricity when melted or dissolved in water.
Because the particles are so spread out, gases experience unique behaviors. These behaviors are dependent upon the amount of the gas present (moles), the volume of the container in which the gas exists (often L), the temperature of the gas (in Kelvin), and the pressure of the gas (often in atmospheres – atm). One of the special relationships is expressed in Boyle’s Law: if temperature and the amount of a gas are kept constant, then pressure and volume of the sample are inversely related. In other words, for a set sample of a gas at a set temperature, if volume increases, pressure will decrease. This makes sense, because as volume increases, there will be less collisions with the walls of the container, therefore pressure must go down. The equation for this is PoVo = Pn Vn where P is pressure, V is volume, o is original conditions, and n is new conditions.
A second special relationship is described by Charles’ Law. It states that if pressure is kept constant, then the volume and temperature of a sample of a gas are directly related. In other words, if temperature increases, volume will also. Again, this makes sense, because if the temperature is higher, the particles are moving faster. This causes them spread apart, taking up more volume. The equation for this is ToVn = Tn Vo. Remember, Temperature must be expressed in Kelvin!
A third relationship is described by Gay-Lussac’s Law. It states that if volume is kept constant, then the pressure and temperature of a sample of a gas are directly related. In other words, if temperature increases, pressure will also. Again, this makes sense, because if the temperature is higher, the particles are moving faster. Since volume is constant, they hit the walls of the container more often, thus increasing pressure. The equation for this is PnTo = PoTn. Remember, Temperature must be expressed in Kelvin!
Another relationship is expressed in Avogadro’s Law which states that equal volumes of gas at equal temperatures and pressures contain the same number of particles. Combining this idea with that of Boyle, Charles and Gay-Lussac results in the Ideal Gas Law which is expressed as PV = nRT. R is a constant, and you must be sure to pick the R value that matches the units in which P, V, and T are expressed. What makes a gas “ideal”? Its particles are only affected by the forces generated by the particles colliding with each other and the walls of the container. Most gases are ideal at ordinary temperatures and pressures. At extreme conditions, however, this equation no longer applies.
All particles, regardless of phase, are in constant motion. When particles move, they sometimes collide. If two particles collide with enough energy, and at the correct angle of orientation, they can react. In a reaction, the bonds in the reactants are broken, and the bonds in the products are formed. Remember, in an equation, reactants products. Also remember that matter cannot be created or destroyed in a chemical reaction, therefore the types and numbers of atoms must stay constant on both side of the equation. Reactions can occur quickly or slowly. Some, like the formation of petroleum, take hundreds of millions of years! To measure how quickly a reaction occurs, chemists measure how fast the concentration of the reactants goes down, and how quickly the concentration of the products increases. Reaction Rates will stay constant if the conditions stay constant. However, if a condition changes and it results in more collisions or higher energy, reaction rate will increase. These conditions include: increasing surface area, (so there is more available space to collide – often achieved by crushing a solid), increasing reactant concentration, (so more particles are available to collide), increasing temperature (which increases energy and makes the particles move faster, thus colliding more), and adding a catalyst (like enzymes in our bodies). Please note: most catalysts increase reaction rate. A negative catalyst will actually decrease reaction rate.