Question

Which of the following statements is true?

A. The reaction of baking soda and vinegar to produce carbon dioxide gas is an example of a precipitation reaction.

B. Bubbles in water that appear during boiling show that a chemical reaction is occurring.

C. When balancing a chemical equation, you may alter the coefficients but not the subscripts in the equation.

D. The formation of a new solid or gas is not an evidence of a chemical reaction.

Which of the following statement is true?

A. Precipitation reactions always occur when two aqueous solutions are mixed.
B. Mixing two aqueous solutions will always result in formation of a precipitate.
C. A precipitation reaction occurs when water is formed as a product.

D. The key to predicting a precipitation reaction is the formation of an insoluble compound from soluble compounds.


Which is following statement is NOT true?

A. We remove the spectator ions from the complete ionic equation to form the net ionic equation.

B. A spectator ion is one that does not actively participate in a chemical reaction.

C. One of the characteristics of an acid-base reaction is that this type of reaction forms water.

D. An oxidation-reduction reaction must involve reaction with oxygen.

E. In an oxidation-reduction reaction, if one substance loses electrons, then another substance must gain electrons.

Which statement concerning the mole is not correct?

A) The molar mass of any compound is equal to its molecular weight in grams.

B) A mole of metal contains NA atoms of that metal.

C) A mole of any compound contains one mole of each kind of atom found in that compound.

D) A mole of a diatomic element contains 2 × NA moles of atoms of the element.

E) All of these statements are correct

Which of the following statement is FALSE?

A. The properties of a compound are an average of the properties of the individual elements.

B. Although some substances we encounter in our routine lives are elements, most occur in the combined state.

C. Ionization energy and electron affinity generally decrease from top to bottom in the group of periodic table.

D. The ratio in a chemical formula is a ratio of atoms, not a ratio of masses.

Answer 1

1- Correct statement- Option- C

C. When balancing a chemical equation, you may alter the coefficients but not the subscripts in the equation.

Explanation-

A chemical reaction lets say is of the format

H₂O + O₂ -----------------> H₂O₂  

Balanced form

2 H₂O + O₂ -----------------> 2H₂O₂  

Now here the numbers in subscripts are 2 (in H2O), 2 in(O2), 2,2 (in H2O2). Now these numbers are a part of their molecular formula. So during balancing the equation, we can only increase the total number of molecules by multiplying numbers (coefficients) before them so that balancing the number of individual atoms on both sides. But we can never alter the numbers in subscript, because then the complete molecular formula will change and t will not be the same chemical reaction again.

Option a- No precipitate is formed in this reaction. So it is not a precipitation reaction.

Option b- It is a physical change where only the form of the liquid changes from liquid to gas. But no change in its chemical properties. So not a chemical change.

Option c- It can be either chemical change (i.e formation of a completely new substance) or a physical change (i.e only the physical appearance of the reactant change)

2-

1- Correct statement- Option- D

D. The key to predicting a precipitation reaction is the formation of an insoluble compound from soluble compounds.

Explanation-

Ex- Mixing silver nitrate ( water soluble) and sodium chloride (also water soluble) in water will cause silver chloride to precipitate out of solution as a solid. In this example, the precipitate is silver chloride.

AgNO_3(aq) + NaCl_(aq) --------------> AgCl_(s) + NaNO3_(aq)

Here the formed solid AgCl is the precipitate. But it does not mean that every time we mix two aqueous solution it will result in precipitate. It depends upon the products formed. If any one of the products formed is water insoluble then only precipitation will be formed. Otherwise to precipitation.