Question

Someone gave you a colorless liquid. Describe three chemical tests you would perform on the liquid to show that it is water.

Short answer

Three tests that can be performed are boiling point test where we expect the liquid to boil at 100 degrees Celsius, freezing point test where we expect it to freeze at 0 degrees Celsius, and a reaction test with Sodium where it would form sodium hydroxide and hydrogen gas.

Step-by-step solution

Test 1: Boiling Point Test

One could perform a boiling point test to determine if the sample is water. The liquid is heated, it should start to boil at 100 degrees Celsius under 1 atmospheric pressure and transform into vapor. If the sample begins to boils around 100 °C, it could be water.

Test 2: Freezing Point Test

The freezing point test can also be used. Water solidifies at 0 degrees Celsius under 1 atmosphere of pressure, forming ice. If the liquid freezes into solid state around 0 °C, this is an indicator that it could be water.

Test 3: Reaction with Sodium

A reaction test with Sodium metal can be used as well. When water (H2O) reacts with sodium (Na), it forms sodium hydroxide (NaOH) and hydrogen gas (H2) which can be identified by the gas being evolved. If this reaction occurs, the sample may be water.

Key concepts

Boiling Point Test

The boiling point test is a simple, yet effective method to help identify a colorless liquid as water. Every substance has a specific temperature at which it changes from a liquid to a gas, known as its boiling point. For water, this temperature is precisely 100 degrees Celsius or 212 degrees Fahrenheit, when at standard atmospheric pressure (1 atmosphere).

To perform this test, heat the liquid carefully and observe when it begins to boil, transforming from liquid to vapor.

• If it boils at around 100 degrees Celsius, the liquid is likely water.
• If it boils at a significantly different temperature, it is probably not water.

It is important to ensure that external factors, such as atmospheric pressure, remain constant at 1 atmosphere, as fluctuations can alter the boiling point.

Freezing Point Test

Another way to identify water is through the freezing point test. The freezing point is the temperature at which a liquid turns into a solid. For water, this occurs at 0 degrees Celsius or 32 degrees Fahrenheit under standard atmospheric conditions (1 atmosphere).

To conduct this test, cool the liquid and watch for the formation of ice.

• If the liquid solidifies into ice near 0 degrees Celsius, it supports the hypothesis that it is water.
• If the freezing temperature deviates significantly, it might not be water.

Similar to the boiling point test, maintaining a consistent atmospheric pressure is crucial, as pressure changes can affect the freezing point.

Reaction with Sodium

The reaction with sodium test is a classic chemical approach to determining whether a liquid is water. When sodium metal comes into contact with water, a vigorous chemical reaction ensues, producing hydrogen gas and sodium hydroxide. You may recognize the reaction by observing bubbling or fizzing as hydrogen gas is released.

Here's how you can spot this reaction:

• Place a small piece of sodium on a watch glass and cautiously add the liquid.
• Look for the rapid evolution of gas, indicative of hydrogen being produced.
• If visible fizzing or bubbling is noted, the reaction occurred, suggesting the liquid is water.

This test should be performed with caution, as sodium reacts exothermically and may produce heat. Protective equipment and safe practices are essential.