CHANGES IN MATTER
Physical and Chemical Changes
In this investigation, you observed a number of situations that involved changes in matter, both physical and chemical. A physical change involves changes in the appearance of the material but does not involve creation of new materials. A change of a solid to a liquid is a physical change. When the candle wax melted it may have appeared different, but it was still wax. After it solidified, it had a similar appearance to the initial product. When the ice cube melted, it may have appeared different, but it was still water. Dissolving is also a physical change. When you added the salt to the water, the salt crystals seemed to disappear as they dissolved in the water. However, they had only spread out into a solution. A solution is a homogeneous mixture of at least two different materials. The materials forming a solution are called the solute and solvent. The material usually present in the largest amount is called the solvent. When the solvent (water) evaporated away, the solute (salt crystals) remained the same as it was originally.
A chemical change involves the formation of new materials. The new materials are called products, and the starting materials are called reactants. The process that brings about a chemical change is called a chemical reaction. Chemical reactions are characterized by a number of changes, including color changes, the formation of a precipitate or gas, and in many cases a release of thermal energy or light. Chemical changes are usually not easy to reverse. When you burned the wooden splint you could not put the charcoal and gases back together to form the original splint as you could when you simply broke the splint into pieces.
physical change: a change that involves changes in the state or form of a substance but does not cause any change in chemical composition.
solution: a homogeneous mixture consisting of two or more substances and at least one solute.
homogeneous: a word used to describe a substance which appears to contain only one type of matter.
solute: the substance that interacts with a solvent to form a solution.
solvent: a substance present in a larger amount that interacts with the solute to make a solution.
chemical change: a change that converts the chemical composition of a substance into different substance(s) with different chemical composition.
product: a substance formed by a chemical reaction.
reactant: a starting substance in a chemical reaction.
chemical reaction: a process in which new substance(s) are formed from starting substance(s).
precipitate: an insoluble solid formed in a liquid solution as a result of a chemical reaction.
Saturated and Supersaturated Solutions
The solution your teacher used in the demonstration was a supersaturated solution of sodium acetate. You probably recognize the term “saturated.” When something is saturated, it is full. A saturated sponge is full of water; it can’t hold any more. A saturated solution is one in which no more solute will dissolve in the solvent at the given temperature and pressure. To say that the sodium acetate solution is supersaturated is to say that it is “over-full.” A supersaturated solution contains more solute particles than it normally would under the given conditions. Not all solutes will form such solutions, but if sodium acetate is added to hot water until it is saturated and allowed to cool slowly, all the solute will remain in solution at the lower temperature. However, this supersaturated solution is unstable. By introducing a “seed” crystal, the excess solute particles begin to crystallize and come out of the solution as a solid.
Concentrations of Solutions
One way to quantify the amount of solute in solution is by measuring its concentration. Concentration is a measure of how much solute is contained in a certain amount of solution, usually a liter. This way of describing solutions has the advantage of being independent of the amount of solution involved. In chemistry, concentration is measured in units of molarity, which is abbreviated as M. Molarity stands for the number of moles of solute that are dissolved in one liter of solution.
For example, when 82.0 g of sodium acetate (NaC2H3O2, formula weight = 82.0 g/mol) is dissolved in water to make 1.00 L of solution, this is a 1.00 molar solution. In a similar manner, 5.00 g of sodium acetate (0.0610 moles) dissolved in water to give 100 mL of solution is 0.610 M.
Molarity and Dilution
At the end of this investigation, 150 mL of liquid glass (sodium silicate solution) was added to 400 mL of water. This is called dilution. Chemists often have to dilute a more concentrated solution with a solvent, such as water, to make a less concentrated solution. For example, if 100 mL of 1.00 M hydrochloric acid, HCl, is diluted to 1.00 L with about 900 mL of water, its molarity would be decreased by a factor of 10. It would now measure at 0.100 M HCl.
supersaturated solution: a solution that contains more solute particles than it normally would under the given conditions.
saturated solution: a solution that has the maximum amount of solute that can be dissolved at a given temperature and pressure.
concentration: a measure of the composition of a solution, often given in terms of moles of solute per liter of solution.
molarity: a measure of the concentration of a solution in moles of solute per liter of solution.
mole: the number equal to the number of carbon atoms in exactly 12 g of pure 12C. It is represented by Avogadro’s number, 6.022 × 1023.
dilution: lowering the concentration of a solute by adding more solvent.
There is a simple relationship between molarity and volume, which is used to calculate dilutions.
M1V1 = M2V2
M1 and V1 are the initial molarity and volume of Solution 1, the more concentrated solution. M2 and V2 represent the final molarity and volume of Solution 2, the less concentrated solution. The dilution of 1.00 M HCl above can be stated as “What volume of 1.00 M HCl is needed to make 1.00 L of 0.100 M HCl?” Using the equation,
1.00 M × V1 = 0.100 M × 1.00 L
Solving for V1, V1 = 0.100 L or 100 mL.
The chemical material that you were working with when you investigated the absorbency of the diaper was sodium polyacrylate. It is a chemical compound called a polymer. It is made up of many (poly) repeating units of a smaller group of elements (the monomer called acrylate).This particular polymer has a unique property. It will absorb more than 800 times its own mass in distilled water. The fascinating ability of this polymer (sodium polyacrylate) to absorb large amounts of water has led to its use in a number of commercial endeavors.
polymer: a substance that is a macromolecule consisting of m