Unit 4- AP CHEM

physical change

A change in a substance that does not involve a change in the identity of the substance (separation of mixture, going from solid->liquid->gas->)

chemical change

A change in matter that produces one or more new substances (production of heat or light, formation of a gas, formation of precipitate, and/or color change provide possible evidence that a chemical change has occured)

Which of the following describes the changes in forces of attraction that occur as H2O changes phase from a liquid to a vapor? A) H-O bonds break as H-H bonds and O-O bonds B) Hydrogen bonds between H2O molecules are broken C) Covalent bonds between H2O are broken D) Ionic bonds between H+ ions and OH- ions are broken E) Covalent Bonds between H+ ions and H2O molecules become more effective

B) Hydrogen bonds between H2O molecules are broken liquid to vapor--->phase change. Hydrogen bonds are forces, not bonds! The intermolecular interactions between molecules are overcome. This is a physical process, and no chemical bonds are being made

ANother student is asked to formulate a hypothesis about what would happen to sodium metal when it is placed in water. Which of the following hypotheses indicates that the student thinks a chemical change will occur? A) If sodium is added to water, it will dissolve B) If sodium is added to water, heat and light will be released C) If sodium is added to water, the total mass will be the same D) If sodium is added to water, the water will remain clear, colorless liquid

B) If sodium is added to water: heat and light will be released. Remember for a chemical change to occur it needs to be a new composition

A student places a sample of a pure metal in a crucible and heats it strongly in air. Data from the experiment are given in the table above. The final mass was determined after the sample was cooled to room temperature. Which of the following statements related to the experiment is correct? Data: Mass of empty crucible 12.0g Mass of crucible and sample before heating 14.4g Mass of crucible and sample after heating 16.0g Answer choices: A )The mass of the sample decreased, so physical changes occurred as the metal first melted and then boiled out of the crucible. B) The mass of the sample increased, so a chemical change occurred when bonds formed between the metal and another substance. C )There was nothing for the metal to react with, so only a physical change could have occurred. D )The sample was only heated, so neither a physical nor a chemical change occurred.

The mass of the sample increased, so a chemical change occurred when bonds formed between the metal and another substance.````

A student was studying physical and chemical changes. The student carried out some procedures in the laboratory and recorded observations. For one of the procedures, the student concluded that a physical change took place, but not a chemical change. Which of the following could have been the results of the procedure? A) A cube of metal was changed into a flat sheet of metal. B) When two liquids at room temperature were combined in a beaker, the beaker became hot. C) When two clear liquids were combined, the resulting mixture was cloudy. D) When a colorless liquid was added to a blue liquid, the resulting solution was yellow.

A) A cube of metal was changed into a flat sheet of metal.

The table above summarizes data given to a student to evaluate the type of change that took place when substance X was mixed with water. The student claimed that the data did not provide enough evidence to determine whether a chemical or physical change took place and that additional tests were needed. Which of the following identifies the best way to gather evidence to support the type of change that occurred when water and X were mixed? Data Table: Water: Clear, colorless liquid BP= 100.0°C1.00g/mL X: White, crystalline solid BP 1935.5°C2.15g/mL Mixture of water+X: Clear, colorless liquid BP=101.6°C1.08g/mL A) Measuring the melting point of the mixture of water and X B) Adding another substance to the mixture of water and X to see whether a solid forms C) Measuring and comparing the masses of the water, X, and the mixture of water and D) Measuring the electrical conductivities of X and the mixture of water and X

D) Measuring the electrical conductivities of X and the mixture of water and X

A student mixes 20.0g of white KCl crystals with distilled water in a beaker. After the mixture was stirred, no crystals are visible and the solution is clear. After several days, all of the water evaporates and white crystals are found in the beaker. Which of the following pieces of experimental evidence would best help the student to confirm that a new compound had not been made and that only a physical change occurred? A)The solution does not change color after stirring. B) The KCl crystals are no longer visible after mixing with water. C) There is a temperature change in the solution during the dissolving process. D)After the water has evaporated, the white crystals in the beaker have a mass of 20.0g.

D)After the water has evaporated, the white crystals in the beaker have a mass of 20.0g ^confirming evidence would show that the crystals are white and has a mass of 20.0g

A student is carrying out various experiments in a laboratory. In one experiment, the student mixes two clear, colorless solutions together, and a yellow precipitate is formed. The student determines that this is evidence of a chemical reaction. Do you agree or disagree? Justify your answer.

I agree with the student. A chemical change is when it undergoes a reaction. It is when a new composition forms, and the yellow precipitate is just that. (a new product made up of diff chemical properties than the orig solution.)

A student was asked to formulate a hypothesis about what would happen if 100.mL of 0.1M NaOH(aq) at 25°C was combined with 100.mL of 0.1M MgCl2(aq) at 25°C. Which of the following hypotheses indicates that the student thought a chemical change would occur? A)The volume of the resulting solution will be equal to the sum of the volumes of the original solutions. B)The mass of the resulting solution will be equal to the sum of the masses of the original solutions. C)The resulting solution would contain a precipitate. D)The resulting solution will be clear.

The resulting solution would contain a precipitate. The production of a precipitate is evidence that a new substance was formed and a chemical change has occurred.

A student had two dilute, colorless solutions, HCl(aq) and NaOH(aq), which were at the same temperature. The student combined the solutions, and the reaction represented above occurred. Which of the following results would be evidence that a chemical reaction took place? HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l) A)The resulting solution is colorless. B)The temperature of the reaction mixture increases. C)The total volume of the mixture is approximately equal to the sum of the initial volumes. D)The resulting solution conducts electricity.

The temperature of the reaction mixture increases. This is an exothermic reaction, and the temperature will increase. Production of heat is possible evidence that a chemical change has occurred.

A student is given a sample of baking soda and a sample of vinegar. The student is to predict what will happen when the two are mixed together, as well as whether a chemical or physical change will occur. What prediction will the student make, and how could she provide evidence to justify her answer?

When baking soda and vinegar react together, bubbles are forming, a gas formation. It takes energy to break apart the vinegar and baking soda, the production of heat and the gas formation is the evidence of chemical change.