A gallon of milk is heavier than a gallon of water because milk contains other substances that are heavier than water.
Also Sand is heavier than water.
For example a 5 gallon bucket of sand would weigh slightly more than a 5 gallon bucket of water.
Sand has a higher density than water so 5 gallons (not a unit usually used to measure solids) is heavier than 5 gallons of water.
If you have ever been to a beach it should be obvious since the sand sinks.
Or if you filled a gallon jug with water and a gallon jug with sand the gallon jug of sand would weight a bit more than the gallon jug of water.
Also wet sand weighs more than dry sand does.
So equal volumes of wet and dry sand would not weigh the same; the wet sand would weigh more because it has more mass, the mass of the water in-between the sand and the mass of the sand itself.
The dry sand has only the mass of the sand and the air between the grains of sand.
Sand is much more heavier than an equal volume of water, as its density is higher than water.
It's estimated that dry sand weighs approximately 100 pounds (45 kg) per cubic foot.
Wet sand is naturally heavier and weighs between 120 and 130 pounds (54 to 58 kg) per cubic foot.