Question

1. during a lab, a student mixes two clear solutions. the mixture becomes cloudy and warm. what can the student most reasonably infer?

a. a physical change occurred because light was not produced.
b. a chemical reaction occurred because new substances formed.
c. a physical change occurred because only the temperature changed.
d. no change occurred since the starting substances were both liquids.

1. in the chemical equation (2h_{2}+o_{2}\to2h_{2}o) what does the arrow ((\to)) represent?

a. equal sign showing both sides are identical
b. the mass of the products is greater than the reactants
c. the direction in which the reactants yield or react to form products
d. the physical change that separates reactants from products

1. which statement best explains what happens during a chemical reaction?

a. atoms are destroyed and new atoms are created. c. molecules shrink but remain chemically the same.
b. atoms are rearranged to form new substances. d. energy is neither absorbed nor released.

1. a scientist notes that an unknown substance reacts with acid to produce bubbles, but dissolving it in water causes no visible change. what can be concluded?

a. dissolving is a chemical change; reaction with acid is physical. c. both are physical changes.
b. reaction with acid is a chemical change; dissolving is physical. d. both are chemical changes.

1. which situation best demonstrates the law of conservation of mass?

a. a burning candle becomes smaller because matter is destroyed.
b. water freezes and its mass decreases as it cools.
c. baking soda and vinegar form gas, but the total mass stays the same when sealed.
d. sugar dissolves in water and the total mass increases.

1. the equation below shows photosynthesis: (6co_{2}+6h_{2}oxrightarrow{hght}c_{6}h_{12}o_{6}+6o_{2}). what does the coefficient \6\

in front of (co_{2}) indicate?
a. each molecule of carbon dioxide has 6 atoms of carbon.
b. there are 6 total atoms of oxygen in the entire reaction.
c. six molecules of carbon dioxide participate as reactants.
d. the reaction happens six times faster than normal.

1. which statement correctly describes the difference between reactants and products?

a. reactants and products have identical chemical formulas.
b. reactants are formed during a reaction, while products start it.
c. reactants are substances that start a reaction, and products are new substances formed.
d. reactants are always solids, while products are always gases.

1. a student crushes a piece of chalk into powder. which statement best explains why this is not a chemical reaction?

a. the chalks color changes slightly.
b. the chalk is now in smaller pieces but has the same composition.
c. the chalk reacts with air to produce a gas.
d. the chalk becomes warmer as its crushed.

1. a graph on the right shows the energy changes during a chemical reaction. which statement best explains what the graph shows about the enzymes role?

a. the enzyme increases the total energy released by the reaction.
b. the enzyme lowers the activation energy, allowing the reaction to occur faster.
c. the enzyme provides extra energy to the reactants to start the reaction.
d. the enzyme changes the reactants into different products than normal.

1. in a reaction energy diagram, the activation energy is represented by:

a. the total amount of energy released by the reaction.
b. the difference in energy between the reactants and the products.
c. the energy peak between reactants and products showing the energy required to start the reaction.
d. the energy stored in the chemical bonds of the products.

Explanation:

1.

A chemical reaction is indicated by the formation of a cloudy mixture (a new substance - precipitate) and the release of heat. Physical changes do not form new substances.

In a chemical equation, the arrow shows the direction of the reaction, from reactants to products. It is not an equal sign (as in physical equations) and does not compare masses directly in that symbol.

In a chemical reaction, atoms are rearranged (not destroyed or created - law of conservation of mass). Molecules change chemically, and energy is either absorbed or released.

Answer:

C. A chemical reaction occurred because new substances formed.

2.