Notes for Chemistry Unit

Section 1.1 Chemistry is the study of matter. Matter is made up of extremely tiny particles called atoms and molecules. Atoms and molecules make up the three common states of matter on Earth—solids, liquids, and gases. The particles of a liquid are attracted to one another, are in motion, and are able to move past one another. Being a solid, liquid, or gas is a property of a substance.

Section 1.2 and 1.3 Heating a liquid increases the speed of the molecules. An increase in the speed of the molecules competes with the attraction between molecules and causes molecules to move a little further apart. Cooling a liquid decreases the speed of the molecules. A decrease in the speed of the molecules allows the attractions between molecules to bring them a little closer together. The way a thermometer works is an example of heating and cooling a liquid.

Section 1.4 In a solid, the atoms are very attracted to one another. The atoms vibrate but stay in fixed positions because of their strong attractions for one another. Heating a solid increases the motion of the atoms. An increase in the motion of the atoms competes with the attraction between atoms and causes them to move a little further apart. Cooling a solid decreases the motion of the atoms. A decrease in the motion of the atoms allows the attractions between atoms to bring them a little close together.

Section 1.5 In a gas, the molecules have very weak attractions for one another. Molecules are able to move freely past each other with little interaction between them. The molecules of a gas are much more spread out and move independently compared to the molecules of liquids and solids. Whether a substance is a solid, liquid, or gas at a certain temperature depends on the balance between the motion of the atoms or molecules at that temperature and how strong their attractions are for one another. Heating a gas increases the speed of its molecules. Cooling a gas decreases the speed of its molecules.

Section 2.2 (evaporation) and Section 2.3 (condensation) Evaporation occurs when molecules in a liquid gain enough energy that they overcome attractions from other molecules and break away to become a gas. Adding energy increases the rate of evaporation. Condensation is the process in which molecules of a gas slow down, come together, and form a liquid. When gas molecules transfer their energy to something cooler, they slow down and their attractions cause them to bond to become a liquid.

Section 2.4 (Freezing) Section 2.5 (Melting) Freezing is the process that causes a substance to change from a liquid to a solid. Freezing occurs when the molecules of a liquid slow down enough that their attractions cause them to arrange themselves into fixed positions as a solid. Melting is a process that causes a substance to change from a solid to a liquid. Melting occurs when the molecules of a solid speed up enough that the motion overcomes the attractions so that the molecules can move past each other as a liquid.

Section 2.1 (Conduction, Heat, Temperature) Energy can be added or removed from a substance through a process called conduction. In conduction, faster-moving molecules contact slower-moving molecules and transfer energy to them. Temperature is a measure of the average kinetic energy of the atoms or molecules of a substance. Heat is the transfer of energy from a substance at a higher temperature to a substance at a lower temperature. Some materials are better conductors of heat than others.

Section 3.1 (Density defined) Density is a characteristic property of a substance. The density of a substance is the relationship between the mass of the substance and how much space it takes up (volume). The mass of atoms, their size, and how they are arranged determine the density of a substance. Density equals the mass of the substance divided by its volume; D = m/v. Objects with the same volume but different mass have different densities.

Section 3.2 (calculating volume through displacement) A submerged object displaces a volume of liquid equal to the volume of the object. One milliliter (1 mL) of water has a volume of 1 cubic centimeter (1 cm3). Different atoms have different sizes and masses. Atoms on the periodic table are arranged in order according to the number of protons in the nucleus. Even though an atom may be smaller than another atom, it might have more mass.

Section 3. 3 (Density in Liquids) Section 3. 4 and 3 Section 3.3 (Density in Liquids) Section 3.4 and 3.5 (sinking and floating) Section 3.6 (Temperature-density) Just like solids, liquids also have their own characteristic density. The mass and size of the molecules in a liquid and how closely they are packed together determine the density of the liquid. The density of an object determines whether it will float or sink in another substance. Heating a substance causes molecules to speed up and spread slightly further apart, occupying a larger volume that results in a decrease in density.