Feature

Accidents can happen with the most innocuous of items, such as plastic bottles and even batteries. Lester Kok gets NTU professors to explain the science behind some freak mishaps seen in the mass media and on social media

Hotpot blast

When having hotpot for dinner, the last thing you’d expect is an explosion. But this happened at a popular hotpot restaurant in Bugis last April, hurting five diners. Assoc Prof David Butler says the culprit in this case is most likely the gas-powered stove, since cooking gas is highly flammable. “A gas leak can be caused by a number of things, such as a damaged pipe or an improperly installed gas canister. Depending on the environment, it is possible for the gas to build up in the surrounding air, where it can quickly catch fire when exposed to a flame or spark.” This is what causes an explosion, explains the professor from the School of Mechanical & Aerospace Engineering.

Shattering light bulbs

Most bulbs are made of glass and they can get very hot when their filaments are lit. Dr Leek Meng Lee from the School of Physical & Mathematical Sciences warns that light bulbs are best kept away from water. “If a drop of water lands on the hot surface of the bulb, the droplet cools that part of the glass and it suddenly contracts,” explains Dr Leek. “This creates stress in the thin glass, causing it to shatter.”

Exploding plastic bottles

Putting dry ice into an empty plastic bottle is a bad idea. It could blow up and hurt you, as a primary school boy found out in Singapore.

Dry ice is carbon dioxide in a solid form. When it “melts”, it turns into gas, which occupies about 600 to 800 times its volume as a solid, according to Asst Prof Zhang Baile, a physicist. “When dry ice is placed inside an enclosed space such as a plastic bottle, the pressure created by the expanding melting gas will be so huge that the bottle will be blown apart.”

Petrol fire

In a viral video clip, a car suddenly catches fire when the driver refuels at a petrol kiosk in Penang. Asst Prof Zhang says the likely culprit is static electricity. Static electricity is “an electric charge accumulated from friction, and it can reach over 10,000 volts,” he explains. “Such high voltages can easily cause an electric spark to jump from the driver’s hand to the car, so if any petrol is nearby, the spark will ignite it.”

Thankfully, with the high humidity in Singapore, the chances of such an incident happening here are quite low. This is because electric charges don’t easily build up in moist environments, unlike in countries with a drier climate.

Too hot for comfort

Hoverboards are all the rage, but some of them recently caught fire in the United States. Poorly made or defective lithium-ion batteries may be to blame.

Batteries have a positive and negative side, kept apart by a plastic separator. As batteries undergo discharge and recharge cycles, their components age and degrade, causing small “hot spots” in them, says battery pioneer Prof Rachid Yazami. “When a battery gets too hot, the separator melts, causing both ends to touch and short-circuit. This is how a battery can catch fire.”

Since your mobile phone uses the same kind of batteries, it’s always best to use the original battery. If you notice your phone becoming unusually hot, especially during charging, change the battery or get it checked out at an authorised service centre.