Question

You are given a colorless liquid. Describe three chemical tests you would perform on the liquid to show it is water.

Short answer

Perform pH test, measure boiling point, and check density to confirm the liquid is water.

Step-by-step solution

Test for pH

Use litmus paper to test the pH of the liquid. Water should be neutral, so if you dip blue or red litmus paper into the liquid, it should not change color. This indicates that the pH of the liquid is around 7, which is a characteristic pH for pure water.

Test for Boiling Point

Measure the boiling point of the liquid by heating it. Pure water boils at 100°C (212°F) under normal atmospheric pressure. Use a thermometer to ensure the boiling point is as expected. A boiling point of 100°C suggests the liquid is likely water.

Test for Density

Determine the density of the liquid by measuring its mass and volume. Use a balance to find the mass and a graduated cylinder for the volume. Calculate the density using the formula, density = mass/volume. The known density of water at 4°C is approximately 1 g/mL. If the density aligns closely with this value, it suggests the liquid is water.

Key concepts

pH Testing

One essential method to identify water is by testing its pH level. The pH scale ranges from 0 to 14, with 7 being neutral, which is typical for pure water. To determine the pH of a liquid, you can use litmus paper. This is a simple paper strip that is sensitive to acidic and basic environments. If you dip red or blue litmus paper into the liquid and neither changes color, it indicates that the liquid is neutral. A pH value of around 7 confirms the neutrality expected of pure water. This simple test can quickly identify whether a liquid lacks acidic or alkaline properties.

Why is this important? The neutrality of water is crucial for many biological processes and chemical reactions. Many organisms thrive in environments where the pH is close to that of pure water. Thus, knowing if a liquid has a neutral pH can offer clues about its composition.

Boiling Point Measurement

Another reliable way to verify if a liquid is water is by measuring its boiling point. Pure water boils at 100°C (212°F) under standard atmospheric pressure. Heating the liquid while measuring its temperature can reveal its boiling point.

• Use a thermometer to accurately monitor the temperature as the liquid heats up.
• Observe when the liquid reaches a constant temperature while boiling. This is its boiling point.

If your liquid boils consistently at 100°C, it strongly indicates that the liquid is water.

Understanding boiling points is vital because every substance has a unique boiling point depending on its molecular structure and intermolecular forces. Thus, this property can differentiate water from other similar-looking liquids.

Density Calculation

Finally, to ascertain whether a liquid is water, one can calculate its density. The density of a substance is its mass per unit volume, commonly expressed in grams per milliliter (g/mL).

To find this:

• Measure the mass of the liquid using a balance.
• Determine its volume using a graduated cylinder.
• Calculate density using the formula: \( \text{density} = \frac{\text{mass}}{\text{volume}} \).

For water, the density is approximately 1 g/mL at 4°C. If your calculated density is close to this, it's a strong indicator that the liquid in question is water. Calculating density can be a powerful tool, as it provides a straightforward numerical value that identifies substances by comparing against known densities.