Which is a disadvantage of using chemical indicators?
they are expensive to use.
they measure ph directly.
--> must be selected for a narrow ph <
they do not use electricity to operate.

Answer : The correct option is, They must be selected for a narrow pH range.

Explanation :

Chemical indicators or pH indicators : It is used to determine the solution is acidic or basic. It gives the visible sign by the color change of a solution.

As there are many indicators which are phenolphthalein, litmus paper, methyl orange, methyl red, etc.

pH : It is defined as the negative logarithm of the hydrogen ion or hydronium ion concentration.

pH measures the acidity and basicity of a substance.

The pH range for acidic solution is, 0.1 to 6.9

The pH range for basic solution is, 7.1 to 14

For neutral solution the pH is, 7

Some of the most widely-used pH indicators and their ranges are :

Phenolphthalein (range pH 8.2 to 10.0)

Bromthymol blue (range pH 6.0 to 7.6)

Litmus (range pH 4.5 to 8.3; red to blue)

The disadvantages of chemical indicators is that they must be selected for a narrow pH range. They can not determine the exact value of pH of the solution.

Hence, the correct option is, They must be selected for a narrow pH range.

the transfer of heat by electromagnetic radiation is called thermal radiation . it is usually carried out by electromagnetic waves in an energy range just below the visible spectrum, which is the infrared range.

so the answer would be radiation.

: )