Density, surface tension, properties of liquids

Kindergarten No 240

Warsaw, Poland

Floating, sinking paper clip

Make sure you’re in a place where it’s ok if water spills, prepare some paper towels.You will need:A soup plate, a paperclip, liquid soap or dishwashing soap, tap water

Procedure

Fill up the soup plate with water. Hold your paperclip flat against the edge of the cup and carefully slide it toward the middle of the cup. The paperclip should be floating on top of the water. Put a drop of liquid soap into the water. Your paperclip will sink.

ConclusionThe surface tension of water is strong enough to even support a paperclip. When you put the soap into the water, it disrupts the order of the water molecules. The surface tension isn’t strong enough to support the weight of the paperclip, so the paperclip falls into the water just as quickly as if you had dropped it into the soup plate.

Why the boat is made of steel float on water when a bar of steel sinks?

You will need:A soup plate, plasticine (a type of modeling clay) , tap water

Procedure: Fill the soup plate with cold tap water. Separate two walnut-size pieces of clay. The pieces should be of equal size. Roll one of the clay pieces into a ball. Carefully place the clay ball on the surface of the water in the bowl. Remove the clay ball from the water and set it aside. Take the second piece of clay and form it into a boat. Gently place the clay boat on the surface of the water.

Conclusion:The clay ball sinks in the water. The same amount of clay, when formed into the shape of a boat, floats on the surface of the water. Different phisical phenomena, including density, buoyancy, and surface tension, explain the floating and sinking of objects. The average density of a boat -- the combination of the steel and the air -- is very light compared to the average density of water. So very little of the boat actually has to submerge into the water before it has displaced the weight of the boat. In other words the boat floats because it takes up more space than the clay ball of equal weight; thus, it has a lower density. Buoyancy is this upward water pressure pushing on the bottom of the boat that is causing the boat to float. Objects with a density lesser than water will float. The weight of the water displaced by an object is equal to the buoyant force exerted by the water on the object. The larger-size clay boat displaces more water, making it more buoyant than the smaller clay ball of equal weight.

A festival of color

You will need:A soup plate, whole milk, food dyes, dishwashing soap, cotton swabs.ProcedurePour milk in the soup plate. Add a few drops of food colorings of your choice. Now submerge a cotton swab in dishwashing soap. Touch the soapy end of the cotton swab in the middle of the dyes in milk and hold it there for 10 to 15 seconds.

ConclusionMilk contains fat. Dishwashing soap repels fat molecules (the same is happening when we wash the dishes). Fat is "pushed aside". Along with fat molecules food coloring dyes also move.

How to make oil ran away?

You will needA soup plate, water, cotton swabs, dishwashing soap, cooking oil.ProcedurePour water into a plate. Add a few drops of cooking oil. Submerge a cotton swab into dishwashing soap and touch a surface of the water near the cooking oil. At first oil will rush to the outer edges of the plate. After some time it will stop moving.

ConclusionWhen you add detergent to water the surface tension of the water is lowered. Detergent (also water, oil and everything else) is build from small particles called molecules . Detergent is attracted to both water and oil helping them all join together. In other words dish soap like water and oil. Imagine a molecule of detergent as a piece of string. One end like water, other end like fat. The end of the detergent molecule which attaches to fat tries to avoid being around water molecules (water fearing). By attempting to move away from the water molecules, the part of molecule which "like" fat ends is pushed up to the surface. The result is a break in the surface tension of the water. It is very handy when we wash the dishes with dish soap.