The purpose of an indicator is to a. test and determine the ph values of unknown solutions. b. acids and bases dissociate when placed in water. c. make a solution more neutral. d. conduct electricity.

Answer is: A. test and determine the pH values of unknown solutions.

Acid-base indicators are usually weak acids or bases and they are chemical detectors for hydrogen or hydronium cations (pH or pOH values).

Example for acid-base indicator is phenolphthalein (molecular formula C₂₀H₁₄O₄). Phenolphthalein is colorless in acidic solutions and pink in basic solutions.

Another example is methyl orange. It is red colour in acidic solutions and yellow colour in basic solutions.

The correct answer is option A.

Explanation:

An indicator is a substance which changes color to show any chemical change happening in the reaction. It gives the visible sign of any change.

Hence, main role of indicators from the given options is to determine the pH values of unknown solutions.

pH is the scale which shows the acidity and basicity of the solutions. It is the negative logarithmic value of hydrogen ion concentration of the solution. More the hydrogen ion concentration, less will be the pH and more acidic will be the solution.

The scale ranges from 0 to 14.

The value ranging from 0 to 6.9 are acidic solutions.

The value ranging from 7.1 to 14 are basic solutions.

The value having pH = 7 is neutral solution.

Indicator used to determine the pH of the solution is pH-paper which is shown in the image.

assuming that your periodic table has eighteen columns, you should highlight   the four elements in the third column from the left of the table.

neutral atoms of elements after calcium contain d electrons. the energy of d orbitals of a particular main shell lies between the energy of the s and p orbitals of the next main shell. for example, electrons in the have energy higher than those in the orbital while lower than those of the orbital.

by the aufbau principle, electrons fill orbitals one at a time, starting from those of the lowest energy. the orbital is therefore filled right after orbitals and before orbitals.  

potassium has electron configuration and calcium . the s orbital of a main shell holds a maximum of two electrons. the next element would thus have the electron configuration and satisfy the requirements.

a neutral atom of the element contains three electrons on top of the most recent noble gas element. it should thus be placed in the third column of the periodic table. both scandium and yttrium would satisfy the conditions. an aufbau diagram indicates that the 4f orbital is filled before the 5d. however, it appears that exceptions exists for both lanthanum and actinium. neutral atoms of the two d-block elements also demonstrate the and should be highlighted as well.