Convert the given lengths between cm and mm.

$$
4.5 \text{ cm} \times \frac{10 \text{ mm}}{1 \text{ cm}} = 45 \text{ mm}
$$
The length is $\boxed{\text{45 mm}}$.

$$
60 \text{ cm} \times \frac{10 \text{ mm}}{1 \text{ cm}} = 600 \text{ mm}
$$
The length is $\boxed{\text{600 mm}}$.

$$
50 \text{ mm} \times \frac{0.1 \text{ cm}}{1 \text{ mm}} = 5 \text{ cm}
$$
The length is $\boxed{\text{5 cm}}$.

$$
25 \text{ mm} \times \frac{0.1 \text{ cm}}{1 \text{ mm}} = 2.5 \text{ cm}
$$
The length is $\boxed{\text{2.5 cm}}$.

$$
5000 \text{ cm} \times \frac{10 \text{ mm}}{1 \text{ cm}} = 50000 \text{ mm}
$$
The length is $\boxed{\text{50000 mm}}$.

$$
8 \text{ km} \times \frac{1000 \text{ m}}{1 \text{ km}} \times \frac{1000 \text{ mm}}{1 \text{ m}} = 8000000 \text{ mm}
$$
The length is $\boxed{\text{8000000 mm}}$.

$$
0.65 \text{ km} \times \frac{1000 \text{ m}}{1 \text{ km}} \times \frac{1000 \text{ mm}}{1 \text{ m}} = 650000 \text{ mm}
$$
The length is $\boxed{\text{650000 mm}}$.

$$
2 \text{ kg} \times \frac{1000 \text{ g}}{1 \text{ kg}} = 2000 \text{ g}
$$
The mass is $\boxed{\text{2000 g}}$.

$$
500 \text{ g} \times \frac{0.001 \text{ kg}}{1 \text{ g}} = 0.5 \text{ kg}
$$
The mass is $\boxed{\text{0.5 kg}}$.

$$
2 \text{ Tonne} \times \frac{1000 \text{ kg}}{1 \text{ Tonne}} = 2000 \text{ kg}
$$
The mass is $\boxed{\text{2000 kg}}$.

$$
500 \text{ kg} \times \frac{0.001 \text{ Tonne}}{1 \text{ kg}} = 0.5 \text{ Tonne}
$$
The mass is $\boxed{\text{0.5 Tonne}}$.

$$
0.005 \text{ Tonne} \times \frac{1000 \text{ kg}}{1 \text{ Tonne}} \times \frac{1000 \text{ g}}{1 \text{ kg}} = 5000 \text{ g}
$$
The mass is $\boxed{\text{5000 g}}$.

$$
\frac{3}{4} \text{ hrs} \times \frac{60 \text{ min}}{1 \text{ hr}} = 45 \text{ min}
$$
The time is $\boxed{\text{45 min}}$.

$$
5 \text{ hrs} \times \frac{60 \text{ min}}{1 \text{ hr}} = 300 \text{ min}
$$
The time is $\boxed{\text{300 min}}$.

$$
30 \text{ min} \times \frac{1 \text{ hr}}{60 \text{ min}} = 0.5 \text{ hrs}
$$
The time is $\boxed{\text{0.5 hrs}}$.

$$
1800 \text{ min} \times \frac{1 \text{ hr}}{60 \text{ min}} = 30 \text{ hrs}
$$
The time is $\boxed{\text{30 hrs}}$.

$$
3 \text{ min} \times \frac{60 \text{ sec}}{1 \text{ min}} = 180 \text{ sec}
$$
The time is $\boxed{\text{180 sec}}$.

$$
0.75 \text{ min} \times \frac{60 \text{ sec}}{1 \text{ min}} = 45 \text{ sec}
$$
The time is $\boxed{\text{45 sec}}$.

$$
0.005 \text{ hrs} \times \frac{60 \text{ min}}{1 \text{ hr}} \times \frac{60 \text{ sec}}{1 \text{ min}} = 18 \text{ sec}
$$
The time is $\boxed{\text{18 sec}}$.

$$
6 \text{ L} \times \frac{1000 \text{ ml}}{1 \text{ L}} = 6000 \text{ ml}
$$
The capacity is $\boxed{\text{6000 ml}}$.

$$
0.25 \text{ L} \times \frac{1000 \text{ ml}}{1 \text{ L}} = 250 \text{ ml}
$$
The capacity is $\boxed{\text{250 ml}}$.

$$
300 \text{ ml} \times \frac{0.001 \text{ L}}{1 \text{ ml}} = 0.3 \text{ L}
$$
The capacity is $\boxed{\text{0.3 L}}$.

$$
4000 \text{ ml} \times \frac{0.001 \text{ L}}{1 \text{ ml}} = 4 \text{ L}
$$
The capacity is $\boxed{\text{4 L}}$.

$$
2 \text{ kL} \times \frac{1000 \text{ L}}{1 \text{ kL}} = 2000 \text{ L}
$$
The capacity is $\boxed{\text{2000 L}}$.

$$
0.0025 \text{ kL} \times \frac{1000 \text{ L}}{1 \text{ kL}} = 2.5 \text{ L}
$$
The capacity is $\boxed{\text{2.5 L}}$.