# IGCSE Physics Paper-1: Specimen Questions with Answers 136 - 138 of 149

## Question 136

### Question

MCQ▾

What is the melting point of pure water?

Choice (4)

a.

b.

c.

d.

b.

### Explanation

• The temperature at which the substance or solid will get is known as melting point.
• The melting point of pure water is
• And at the temperature the liquid gets evaporate is known as boiling point.
• The boiling point of pure water is

## Question 137

### Question

MCQ▾

Sodium-24 has half-life of 15 hours. If there are 1600g of Na-24 initially, how will it take for 1400g of Na-24 to decay?

Choice (4)

a.

45 hrs

b.

50 hrs

c.

1 day

d.

40 hrs

a.

### Explanation

For finding half-life the formula is

In () =-kt

Here,

K =

K = constant and = half life

K =

In () =-kt

In ()

In

t =

t = 45 hrs

(OR)

3 half-lives for sample to decay from 1600g to 200g

i.e..

45hours for 1400g of sodium-24 to decay

## Question 138

### Question

MCQ▾

The circuit shown contains three lamps and 4 switches

Which switches must be closed to light up lamp 1 and lamp 3?

### Choices

Choice (4)

a.

switch 1,2, 3 and 4 only

b.

switch 5 only

c.

switch 1, switch 2 and switch 5 only

d.

switch 1 and switch 2 only