Ask Mr Brain...all will be explained

PUBLISHED : Wednesday, 02 May, 2001, 12:00am
UPDATED : Wednesday, 02 May, 2001, 12:00am

If chlorine in drinking water kills bacteria, why does it not kill cells in our body?

Although chlorine is known to be very effective at killing bacteria, scientists do not fully understand how the process works. It is thought that chlorine reacts with chemicals on the surface of bacteria cells, destroying the cell wall. The cell's vital functions stop and the bacteria dies.

Chlorine kills a wide variety of bacteria found in water, including those that cause typhoid fever, dysentery and cholera.

Fortunately, when we drink water from the tap, it only contains low levels of chlorine. The concentration is just enough to kill bacteria present in the water and to keep it safe as it travels along pipes to the tap. Also food in our stomachs and the materials normally present in the intestine quickly neutralise the chlorine and stop it from damaging our cells. So the concentration of chlorine along cell membranes in the intestine is probably too low to cause injury. However, at much higher concentrations, chlorine could damage the cells in our body.

Adding chlorine to public drinking water supplies has been hailed as the probably the most significant contribution to public health during the 20th century. Since the United States started treating public drinking water with chlorine in 1905, it has virtually wiped out typhoid fever.

Why is there a frozen layer on top of a milk shake after a while?

A milk shake is made up of small ice particles which are suspended in a sugar, fat and protein solution (the sweetened milk). This fluid has a lower melting point than the ice because of the solutes it contains, so the ice particles are free to creep slowly upwards through the slushy mixture. Once all the crystals have got to the top, they will amalgamate to form an icy plug.

Why is it that we can build a sandcastle with damp sand whereas if we use dry sand, it just crumbles away?

When water is added to sand, it forms 'pendular' bridges between the sand grains, which hold them together. The force is stronger than gravity and prevents the walls of the sandcastle from falling down.

The pendular bridges work through the water's surface tension. The surface of the liquid bridges are concave, generating a 'capilllary action' which holds the sand grains firmly together.

If you keep adding water to the sand, the concave liquid surfaces continue to generate a capillary action which holds the sand grains together.

But if you add even more water, the surface of the liquid bridges become convex and the capillary action disappears. The water no longer creates any attractive force between the particles and the walls of the sandcastle will begin to slump.