Got Salt? Comparing Salt Content During High and Low Tides

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Updated on Aug 24, 2011

Comparing salt content in back bay water during high tides and low tides.

The gravitational pull of the Moon and the Sun creates a daily flow of water toward and away from sea coasts (high tide and low tide). As water flows toward the coast, the water level along the shore can be seen to rise, and water flows through inlets, filling back bay areas. Hours later, an ebb tide occurs, when the water recedes out of the bays and away from the shoreline.

Does this tidal change affect the salt content of the water that accumulates in the back bays? If a significant difference exists between the salt content at high tide and low tide, plants and animals living there would have to be tolerant of these changes.

Hypothesize that a noticeable difference will occur in the salt content in back bay water depending on the cycle of the tide (high tide, low tide).

  • Access to an inlet and bay areas that experience tidal changes, fed by an ocean or a large body of salt water
  • Four wide-mouth jars (peanut butter, pickle, or other food containers) of equal size
  • Masking tape
  • Pen or marker
  • About two weeks of waiting time
  • Several small twigs
  • A sunny window
  • Tide chart helpful, but optional
  • Possible adult supervision needed

The amount of water gathered for each sample and the location the samples are taken from remain constant. The tide cycle is the variable.

For this project, you must have access to an inlet and a back bay that receives tidal flows from an ocean or a large body of salt water. When you work around water, make safety your number one concern. Know how to swim, wear a life preserver, and always have a friend or an adult accompany you.

Gather four clear glass or plastic jars that have wide mouths. Jars of this type include 16- or 18-ounce peanut butter, pickle, or sauce containers. All four jars must be identical.

Place a strip of masking tape on each jar and label each one as to the location and tidal status that identifies the water sample they will contain.

You need to determine the time of high and low tides. Tide tables are often found in local marinas, newspapers, or by listening to a National Oceanic and Atmospheric Administration (NOAA) weather station (weather radios can be purchased at many consumer electronic stores). If you do not have access to a tide table, you can spend a day making note of where the high- and low-tide levels are along bulkheads or other land markings. Throwing a small twig in the water at an inlet and watching the direction it floats tells you whether the tide is flowing in or receding out.

The figure on the top of the next page shows two points where you should collect water samples: one is located in a back bay area, and the other is at the mouth of the inlet, where the bay meets the ocean.

When the tide is just beginning to flow in (just past the time of low tide), fill a jar with water from Point A and one from Point B. Secure lids on the jars to keep the water from spilling as you transport them home.

Later, when the tide is just beginning to recede (just past the time of high tide), fill a jar with water from Point A and one from Point B.

[Optional: If you have access to two inlets that feed back bay areas, you can enhance your project by collecting additional samples at points shown in the figure below.]

When you get home, place the jars in a warm, sunny window and remove the lids. It takes about two weeks for all the water to evaporate. You can decrease evaporation time by placing them in an area of increased heat, such as near a heat duct or in an oven at a low temperature. Do not place the jars directly on a stove burner, as the jars are not designed to be exposed to high temperatures.

When all the water has evaporated, screw the lids on again to prevent any further contamination and to keep the contents intact.

Do you see chunks of salt in the jars? Salts are crystals, and one of the characteristics of crystals is their unique shapes. Do the chunks have shapes characteristic of crystals? You may also want to examine the salt chunks under a magnifying glass or microscope.

Write down the results of your experiment. Document all observations and data collected.

Come to a conclusion as to whether or not your hypothesis was correct.