When placed in a freezer, liquid water turns into solid ice. why is this considered a physical change?

This is considered a physical change because the water does not change its chemical properties - it is still water. It is simply just moving its shape such as gas form, liquid form, or solid form. 

Because you can physically see the object melting when it comes to the melting point. The objects texture, color, temperature, shape, and state of matter (solid, liquid, gas) are possibly changing. Especially the bolded changes are happening.

For example, in your question, your talking about ice. It changes its shape, state of matter, temperature, and texture.

The answer is b carbon-14. the radioactive isotope of carbon used in carbon dating has a half-life of 5730 years. potassium-40 is used to date 1.25 billion years