Which one of the following is a chemical change?
- A.Dissolving sugar in water
- B.Melting of ice
- C.Crystallization
- D.Milk turning sour
Correct answer
Explanation
The correct answer is D, Milk turning sour. Bacteria turn the lactose in milk into lactic acid, a new substance, so souring is a chemical change that cannot be reversed.
A chemical change produces new substances with new properties and is usually permanent; the signs include a change of taste or smell, the release of gas or heat, or a change of colour. Rusting of iron, burning of paper, cooking food and curdling of milk are other examples. A physical change alters only the state, shape or size, and the substance itself stays the same.
A is wrong: dissolving sugar in water is a physical change, since the sugar can be recovered by evaporating the water.
B is wrong: melting of ice changes only the state, and the water can be frozen back into ice.
C is wrong: crystallization only separates a pure solid from its solution, without forming a new substance.
Exam tip: a new substance and no way back = chemical change; a change of state or dissolving = physical change.
Practice Questions
View allA snowboard pulled up by a tow rope travels at the rate of 5 m/s up a mountain. If 3000 watt of power is used, what force was applied to it ?
- A.50 N
- B.100 N
- C.600 N
- D.15000 N
Show answer
Power equals force times velocity, so the force is 3,000 W divided by 5 m/s, which is 600 N. Power is the rate of doing work: P = W/t. Work is force times distance, W = F × d, so P = F × d/t, and since d/t is the…
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- A.material of the medium only
- B.energy only
- C.both material of the medium and energy
- D.none of the above
Show answer
A wave carries energy from one place to another, while the particles of the medium only vibrate about their fixed positions.
Read the full explanation →Which one of the following conservation laws is a consequence of the Newton's third law of motion?
- A.Conservation of energy
- B.Conservation of momentum
- C.Conservation of charge
- D.Conservation of mass
Show answer
Equal and opposite forces acting for the same time give equal and opposite changes in momentum, so the total stays the same.
Read the full explanation →Which one of the following properties decreases across the periodic table from left to right and increases from top to bottom?
- A.Ionization energy
- B.Electron affinity
- C.Electronegativity
- D.Atomic radius
Show answer
Atoms get smaller along a period and larger down a group. Moving from left to right in a period, electrons are added to the same shell while the nuclear charge rises by one at each step.
Read the full explanation →A sound wave of frequency of 2 kHz has a wavelength of 35 cm in a given medium. How long will it take to travel a distance of 2.1 km through the medium?
- A.30 s
- B.2.1 s
- C.3.0 s
- D.4.1 s
Show answer
The speed of the wave is frequency times wavelength, 700 m/s, and 2,100 m at 700 m/s takes 3 seconds.
Read the full explanation →