1. Water, a Liquid:  Investigation 4

# What does a drop of water weigh?

## 2. Develop a strategy

All Class 15 Mins
Now that we've improved this question, how can we go about answering it?

Strategy 1: Use the scale.

It's likely someone will suggest putting one or more drops on a scale. Explain that if we had more accurate scales, this strategy could work.

Demonstrate placing a single drop of water on the scale. The scale will not register any weight. Even when placing multiple drops of water on the scale, one drop at a time, it's unlikely these scales will register weight. If they did, the weight would be rounded to the nearest gram, providing inaccurate results.

Encourage students to come up with another strategy.

Strategy 2: Use the information that 1 cubic centimeter of water weighs 1 gram.

• There is a way to use something we discovered in the last investigation-that one cubic centimeter of water weighs one gram-to help us answer the investigation question. How might counting the drops in 1cc of water help us?
• If we could make just 2 drops of water from one whole cubic centimeter of water, what would each of those drops weigh?
• If we could make 10 drops of water from one whole cubic centimeter of water, what would each of those drops weigh?

### Fill and weigh a dropper

Place one empty dropper on a classroom scale and read the weight (approximately 3g).

If I add exactly one cubic centimeter of water to this dropper, what do you predict it will weigh?

Point out that the dropper has marks on it that allow us to add exactly one cubic centimeter of water. Demonstrate a standard procedure for accurately filling the dropper with 1cc of water:

• Push the plunger as far as it will go into the barrel of the dropper.
• Put the tip of the dropper in a cup of water and slowly pull on the plunger to completely fill the dropper.
• Point the tip of the dropper towards the ceiling and tap the barrel with a finger until all air bubbles have risen out of the water.
• Carefully depress the plunger until the black ring of the seal aligns with the 1cc mark.

Place the dropper on the scale again. It should weigh one gram more than its weight when empty.

### Demonstrate making drops

To answer the question, What does a drop of water from our classroom dropper weigh?, it's important that everyone use the same tool in the same way in order to get accurate results. Demonstrate a standard procedure to make drops:

• Hold the dropper vertically, tip down, about 1 inch above a desk top.
• Gently push the plunger to release one drop of water.
• Move the dropper sideways about 2 inches and release another drop.
• Form a line of 5 or 6 drops.

Keeping the drops separate from one another makes it possible to count them after the dropper is empty, in the event someone loses count while making the drops.