Question

Suggest a way to determine if the colorless liquid in a beaker is water. How could you discover if there is salt dissolved in the water?

Short answer

Evaporate the liquid to see if salt remains or perform conductivity and chemical tests to confirm salt presence.

Step-by-step solution

Evaporation Test

To determine if the liquid consists of water and potentially dissolved salt, you can start with an evaporation test. Place a small amount of the liquid on a shallow dish or a piece of glass and let it sit at room temperature or apply gentle heat. If the liquid evaporates and leaves behind white crystalline residues, it indicates the presence of dissolved salt in the water.

Taste Test (Caution Advised)

Perform a taste test if the safety of the liquid has been confirmed and it's sure that no harmful substances are dissolved. Put a small drop of the liquid on your tongue to test for a salty taste, which would indicate the presence of salt.

Conductivity Test

To further confirm the presence of dissolved salt, perform a conductivity test. Insert electrodes into the liquid and connect them to a conductivity meter. Water with salt dissolved in it will conduct electricity and show higher conductivity compared to pure water.

Chemical Test with Silver Nitrate

For a more definitive chemical confirmation, add a few drops of silver nitrate (\(AgNO_3\)) solution to a sample of the liquid. If a white precipitate (silver chloride) forms, it confirms the presence of chloride ions, indicating dissolved salt (sodium chloride, \(NaCl\)).

Key concepts

Evaporation Test

An evaporation test is a straightforward method to check if a colorless liquid is pure water or if it contains dissolved substances such as salt. Here's how it works.
You need to take a small sample of the liquid and place it in a shallow dish or on a piece of glass.
Allow the liquid to sit at room temperature and evaporate naturally. Alternatively, you can gently heat it to speed up the process.
If the liquid is pure water, it will leave no residue.
However, if there is dissolved salt, you'll notice a white, crystalline residue once all the water has evaporated.
This residue is typically salt that was unable to evaporate along with the water.
The evaporation test is simple and doesn't require any special equipment other than a heat source, making it an accessible first step in water purity testing.

Conductivity Test

A conductivity test helps determine if a solution conducts electricity, which in turn can indicate the presence of dissolved salts.
This is because salt dissolved in water dissociates into ions, and ions are charged particles that carry electric current through the solution.
To perform the test, you need a conductivity meter and electrodes.
Begin by inserting the electrodes into the liquid. The electrodes will detect if the solution allows electric current to pass through.
The conductivity meter provides a reading that indicates the conductivity level of the liquid.
Pure water has very low conductivity because it has few ions.
If the reading on the meter is high, it indicates that the water contains dissolved salts that are increasing its ability to conduct electricity.
This test is a reliable and easily interpretable method for confirming the presence of ionic substances in water.

Chemical Test with Silver Nitrate

A chemical test using silver nitrate is a definitive way to confirm the presence of chloride ions in a liquid solution, indicating dissolved salt such as sodium chloride.
Here's how you can perform this test.
Prepare a small sample of the liquid and add a few drops of silver nitrate (\(AgNO_3\)
) solution to it. Watch closely for any changes.
If chloride ions are present, you will see a white precipitate forming in the solution.
This white precipitate is silver chloride (\(AgCl\)), created when the silver ions from the silver nitrate combine with the chloride ions from the dissolved salt.
This reaction serves as clear evidence of dissolved sodium chloride in the liquid.
This chemical test is especially useful because it provides visual confirmation of salt presence through the formation of the precipitate.

Safety in Chemical Testing

When conducting chemical testing, safety should always be a top priority.
This is crucial because even seemingly innocuous tests might have hidden risks.
To begin with, always wear protective gear such as gloves, goggles, and aprons to protect against splashes or accidental contact with reagents.
When using heat, be cautious of hot surfaces and use heat-resistant containers to prevent burns and breakage.
Moreover, ensure you are working in a well-ventilated area to avoid inhaling any fumes that might be released during tests.
It's also essential to understand the properties and potential hazards of the chemicals you use, such as silver nitrate, which can cause stains and needs careful handling.
Lastly, follow all disposal regulations for chemical waste to prevent environmental harm.
By adhering to these safety guidelines, you can ensure a safe testing environment for yourself and others.