# Liquid Density Lab - Salty or Sweet

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## Objective

Students will create a density column with salt water, sugar water, and tap water.

#### Big Idea

A a hands-on activity using three different mixtures (salt water, sugar water, and tap water) that are stacked by the students in a graduated cylinder to create a density column.

## NGSS Background

This lesson is based on California's Middle School Integrated Model of NGSS.

MS-PS1: Matter and Its Interactions

PE: MS-PS1-2 - Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred. This lesson is specifically designed to assess the property of density of a substance.

DCI: PS1.A: Structure and Properties of Matter - Each pure substance has characteristic physical and chemical properties.

SEP: (4) Analyzing and Interpreting Data - Students will make predictions about the possible densities of salt water, sugar water, and tap water before the actually build a density column. The students will later analyze their predictions and compare them to actual results.

CCC: (1) Patterns - Macroscopic patterns are related to the nature of microscopic and atomic-level structure. Intrinsic particle movement, often described as temperature, causes water to achieve different levels of density. The students will be able to recognize that these different densities form a predictable patten in nature and can be used to identify specific types of matter.

The purpose of this lab is to give students hands-on experience with density and see for themselves that denser substance will displace less dense substances and sink to the bottom of a graduated cylinder. Buoyancy can be different for objects placed in the same material (water) with varying levels of density (salt water, sugar water, and tap water).

This lab is often run as part of two other labs:

## Prelab (Set-up)

25 minutes

For this activity, students build a density column with salt water, sugar water, and tap water.

Materials

1. 2 empty baby food jars
2. Small beaker
4. Spoon
5. Pipet
6. Red food coloring
7. Blue food coloring
8. Green food coloring
9. Raisins
10. Salt
11. Sugar

Student Directions

1. Fill each baby food jar and the small beaker half full with water.
2. Add 2 teaspoon of salt to the beaker and stir until dissolved. Add several drops of red food coloring and stir.
3. Add 2 teaspoon of sugar to the baby food jar and stir until dissolved. Add several drops of blue food coloring and stir.
4. Add several drops of green food coloring to the last baby food jar and stir.
5. Gently place a raisin in each container Does it float or sink. Remove with a spoon.
6. Pour the salt water (red) into the graduated cylinder until it is about ¼ full.
7. Using the pipet, slowly add the sugar water (blue) one drop at a time to the graduated cylinder. Continue until the graduated cylinder is ½ full.
8. Using the pipet, slowly add the pure water (green) one drop at a time to the graduated cylinder. Continue until the graduated cylinder is ¾ full.

TIP: The secret to building a clear density table is slowly adding the different density waters with a pipet. If the different water is added to quickly the colors will mix and spoil the effect. Patience is the key word.

## Lab (Student Activity)

15 minutes

Pass out Liquid Density Lab - Salty or Sweet to each student.

The students must first mix the salt and sugar water, while adding the correct food coloring.

Students must carefully add the different waters to ensure a clean density column.

On completed the salt water should be at the bottom, with the sugar water in the middle and finally the tap water on top. Note the color change.

Question

1. Which is denser sugar water or salt water?
2. Prediction (what do you think is going to happen).
3. Were your predictions correct? Explain.
4. Did adding salt and sugar to the water make the water more dense or less dense? Explain.
5. Which was more dense, the salt water or the sugar water?

Student Work Sample

## Extension

45 minutes

I created two Powerpoints that teaches density and buoyancy.

Density is mass per unit volume

The buoyant force acts on objects