Ahad, 27 Oktober 2013

Amali Sains: Perbandingan Ketumpatan Air, Ais dan Minyak

Air lebih tumpat daripada minyak. Minyak lebih tumpat daripada ais. Maka, ais akan terapung di atas minyak dan air. Bagaimanapun, apabila ais melebur, ia akan membentuk titisan yang akan tenggelam ke dasar bekas.

Mengapa ais lebih tumpat daripada air?
Mengapa leburan ais tenggelam ke dasar bekas?

Oil floats on top of water and ice floats on top of oil. What happens? As the ice begins to melt, you will see a drop of water hanging from the bottom of the ice cube. As the drop grows, the ice cube will float lower, as it is being weighted down by the denser water. Finally, the drop gets large enough to pull free of the ice and it slowly sinks to the bottom of the glass.

The ice floats because it's less dense than the water and the oil. But melted ice is more dense than oil, so it falls through the oil and mixes with the water below.

Water is a strange chemical. Most liquids get smaller when they freeze, which means the solid form is denser. When water freezes, it gets larger and the volume increse. It still weighs the same, but it takes up more space, which means it is less dense.

Why does the volume increase?

The normal pattern for most compounds is that as the temperature of the liquid increases, the density decreases as the molecules spread out from each other. As the temperature decreases, the density increases as the molecules become more closely packed. This pattern does not hold true for ice as the exact opposite occurs.

A water molecule is made from one oxygen atom and two hydrogen atoms, strongly joined to each other with covalent bonds. Water molecules are also attracted to each other by weaker chemical bonds (hydrogen bonds) between the positively-charged hydrogen atoms and the negatively-charged oxygen atoms of neighboring water molecules. As water cools below 4°C, the hydrogen bonds adjust to hold the negatively charged oxygen atoms apart. This produces a crystal lattice, which is commonly known as 'ice'. The crystal lattice allow the molecules organize themselves into a special six-sided arrangement that produces a perceived hollow crystal interior.

Compare the structures of Liquid Water and Solid Ice. Notice the empty spaces within the ice structure, as this translates to a more open or expanded structure. The ice structure takes up more volume than the liquid water molecules, hence ice is less dense than liquid water.

The increase in volume of ice is about 9%. Therefore, the density of ice is 9% less liquid water.